99 99 99 99 99 ---------------------------------------- ------------------- ,transcriber's ,note3 ,inconsistent hyphenation in the original document has been preserved4 ,in the equations1 superscripted characters are marked with 's1 and subscripted characters are marked with 7underscore7 in front4 ,obvious typographical errors have been corrected4 ,for a complete list1 please see the end of this document4 ---------------------------------------- ------------------- 99 99 99 99 99 ,a ,,study ,,of ,,recent ,,earthquakes4 ,,by ,,charles ,,davison1 ,sc4,d41 ,f4,g4,s4 ,,author ,,of ,,the ,,hereford ,,earthquake ,,of ,,december #ag,,th1 #ahif4 ,,with #hj ,,illustrations ,london and ,newcastle-on-,tyne3 ,,the ,,walter ,,scott ,,publishing ,,co41 ,,ltd4 #aije ,,preface4 ,the present volume differs from a text-book of seismology in giving brief1 though detailed1 accounts of individual earthquakes rather than a discussion of the phenomena and distribution of earthquakes in general4 ,at the close of his ,les ,tremblements de ,terre1 ,professor ,fouqu has devoted a few chapters to some of the principal earthquakes between #ahed and #ahhg2 and there are also the well-known chapters in ,lyell's ,principles of ,geology dealing with earthquakes of a still earlier date4 ,with these exceptions1 there is no other work covering the same ground2 and he who wishes to study any particular earthquake can only do so by reading long reports or series of papers written perhaps in several different languages4 ,the object of this volume is to save him this trouble1 and to present to him the facts that seem most worthy of his attention4 ,the chapter on the ,japanese earthquake is reprinted1 with a few slight additions1 from a paper published in the ,geographical ,journal1 and ,i am indebted to the editor1 not only for the necessary permission1 but also for his courtesy in furnishing me with clichs of the blocks which illustrated the original paper4 ,the editor of ,knowledge has also allowed me to use a paper which appeared four years ago as the foundation of the ninth chapter in this book4 ,,charles ,,davison4 ,,birmingham1 ,january1 #aije4 ,,contents4 ,,chapter ,i4 ,,page ,,introduction #a ,,chapter ,,ii4 ,,the ,,neapolitan ,,earthquake ,,of ,,december #af,,th1 #aheg #g ,,chapter ,,iii4 ,,the ,,ischian ,,earthquakes ,,of ,,march #d,,th1 #ahha1 ,,and ,,july #bh,,th1 #ahhc #de ,,chapter ,,iv4 ,,the ,,andalusian ,,earthquake ,,of ,,december #be,,th1 #ahhd #ge ,,chapter ,v4 ,,the ,,charleston ,,earthquake ,,of ,,august #ca,,st1 #ahhf #ajb ,,chapter ,,vi4 ,,the ,,riviera ,,earthquake ,,of ,,february #bc,,rd1 #ahhg #ach ,,chapter ,,vii4 ,,the ,,japanese ,,earthquake ,,of ,,october #bh,,th1 #ahia #agg ,,chapter ,,viii4 ,,the ,,hereford ,,earthquake ,,of ,,december #ag,,th1 #ahif1 ,,and ,,the ,,inverness ,,earthquake ,,of ,,september #ah,,th1 #aija #bae ,,chapter ,,ix4 ,,the ,,indian ,,earthquake ,,of ,,june #ab,,th1 #ahig #bfb ,,chapter ,x4 ,,conclusion #cba ,,index #cdi ,,list ,,of ,,illustrations4 ,,fig4 ,,page #a4 ,diagram to illustrate ,simple ,harmonic ,motion #d #b4 ,isoseismal ,lines of the ,neapolitan ,earthquake #i #c4 ,diagram to illustrate ,wave-path and ,angle of ,emergence #ab #d4 ,diagram to illustrate ,mallet's ,method of determining ,position of ,epicentre #ad #e4 ,plan of ,cathedral ,church at ,potenza #af #f4 ,fallen ,gate-pillars near ,saponara #ag #g4 ,model to illustrate the ,motion of an ,earth-particle during an ,earthquake #ai #h4 ,plan of ,directions of ,fall of ,overturned ,stone-lamps at ,tokio during the ,earthquake of #ahid #ai #i4 ,meizoseismal ,area of ,neapolitan ,earthquake #bb #aj4 ,distribution of ,death-rate within ,meizoseismal ,area of ,neapolitan ,earthquake #bd #aa4 ,diagram to illustrate ,mallet's ,method of determining ,depth of ,seismic ,focus #bf #ab4 ,vertical ,section of ,cathedral ,church at ,potenza #bg #ac4 ,diagram of ,wave-paths at ,seismic ,vertical of ,neapolitan ,earthquake #bi #ad4 ,geological ,sketch-map of ,ischia #dg #ae4 ,isoseismal ,lines of ,ischian ,earthquake of #ahha #ea #af4 ,isoseismal ,lines of ,ischian ,earthquake of #ahhc #ei #ag4 ,diagram of ,wave-paths at ,seismic ,vertical of ,ischian ,earthquake of #ahhc #fb #ah4 ,diagram showing connection between ,depth of ,focus and ,rate of ,decline in ,intensity #fh #ai4 ,isoseismal ,lines of ,andalusian ,earthquake according to ,taramelli and ,mercalli #gi #bj4 ,isoseismal ,lines of ,andalusian ,earthquake according to ,fouqu1 etc4 #ha #ba4 ,magnetograph ,records of ,andalusian ,earthquake at ,lisbon #hc #bb4 ,nature of ,shock of ,andalusian ,earthquake #hh #bc4 ,diagram to illustrate ,variation in ,nature of ,shock #hi #bd4 ,structure of ,meizoseismal ,area of ,andalusian ,earthquake #ajj #be4 ,isoseismal ,lines of ,charleston ,earthquake #ajf #bf4 ,curve of ,intensity at ,charleston #aaj #bg4 ,flexure of ,rails at ,jedburgh #aac #bh4 ,epicentral ,isoseismal ,lines of ,charleston ,earthquake according to ,sloan #aah #bi4 ,epicentral ,isoseismal ,lines of ,charleston ,earthquake according to ,dutton #aai #cj4 ,planes of ,oscillation of ,stopped ,pendulum ,clocks at ,charleston #aba #ca4 ,diagram to illustrate ,dutton's ,method of determining ,depth of ,seismic ,focus #abd #cb4 ,diagram to explain ,origin of ,regions of ,defective ,intensity #acf #cc4 ,isoseismal ,lines of ,riviera ,earthquake #add #cd4 ,meizoseismal ,area of ,riviera ,earthquake #adh #ce4 ,nature of ,shock of ,riviera ,earthquake #aeb #cf4 ,seismographic ,record at ,moncalieri #aee #cg4 ,distribution of ,observatories at which ,magnetographs were disturbed by ,riviera ,earthquake #aeh #ch4 ,record of ,tide-gauge at ,nice #afc #ci4 ,record of ,tide-gauge at ,genoa #afd #dj4 ,distribution of ,seismic ,activity in the ,riviera #agb #da4 ,isoseismal ,lines of ,japanese ,earthquake #agh #db4 ,structure of ,meizoseismal ,area of ,japanese ,earthquake #ahj #dc4 ,plan of ,directions of ,fall of ,overturned ,bodies at ,nagoya #ahg #dd4 ,map of ,mean ,directions of ,shock and ,isoseismal ,lines in ,central ,district of ,japanese ,earthquake #ahh #de4 ,meizoseismal ,area of ,japanese ,earthquake #aij #df4 ,fault-scarp near ,fujitani #aia #dg4 ,fault-scarp at ,midori #aib #dh4 ,displacement of ,field-divisions by the ,fault near ,nishi-,katabira #aic #di4 ,map of ,swamp formed by ,stoppage of ,river ,toba by ,fault-scarp #aid #ej4 ,shifting of ,trees by ,fault at ,umhara #aie #ea4 ,daily ,frequency of ,after-shocks at ,gifu and ,nagoya #aif #eb4 ,monthly ,frequency of ,after-shocks at ,gifu #aig #ec4 ,distribution of ,preliminary ,shocks in ,space #bjb #ed4 ,distribution of ,after-shocks in ,space1 ,nov4-,dec4 #ahia #bjc #ee4 ,distribution of ,after-shocks in ,space1 ,jan4-,feb4 #ahib #bjd #ef4 ,distribution of ,after-shocks in ,space1 ,march-,april #ahib #bje #eg4 ,distribution of ,after-shocks in ,space1 ,may-,june #ahib #bjf #eh4 ,distribution of ,audible ,after-shocks in ,space1 ,nov4 #ahia-;,dec4 #ahib #bjh #ei4 ,map of ,adjoining ,regions in which ,seismic ,activity was affected by ,japanese ,earthquake #baj #fj4 ,isoseismal and ,isacoustic ,lines of ,hereford ,earthquake #baf #fa4 ,nature of ,shock of ,hereford ,earthquake #bbb #fb4 ,coseismal ,lines of ,hereford ,earthquake #bbh #fc4 ,map of ,minor ,shocks of ,hereford ,earthquake #bci #fd4 ,geology of ,meizoseismal ,area of ,hereford ,earthquake #bda #fe4 ,isoseismal ,lines of ,inverness ,earthquake #bdh #ff4 ,diagram to illustrate supposed ,fault-displacement causing ,inverness ,earthquake #bef #fg4 ,map of ,epicentres of ,after-shocks of ,inverness ,earthquake #beh #fh4 ,isoseismal ,lines of ,indian ,earthquake #bfc #fi4 ,section of ,tombs in ,cemetery at ,cherrapunji #bgj #gj4 ,time-curve of ,indian ,earthquake #bgh #ga4 ,seismographic ,record of ,indian ,earthquake at ,rocca di ,papa #bhb #gb4 ,seismographic ,record of ,indian ,earthquake at ,edinburgh #bhc #gc4 ,displacement of ,alluvium at ,foot of a ,hill #bhg #gd4 ,twisting of ,monument at ,chhatak #bid #ge4 ,epicentral ,area of ,indian ,earthquake #cjc #gf4 ,plan of ,chedrang ,fault #cje #gg4 ,re-triangulation of ,khasi ,hills #cac #gh4 ,diagram of ,thrust-planes #cah #gi4 ,seismographic ,record of ,tokio ,earthquake of #ahid #cbi #hj4 ,time-curves of ,principal ,epochs of ,earthquake-waves of ,distant ,origin #cch ,a ,,study ,,of ,,recent ,,earthquakes4 ,,chapter ,i4 ,,introduction4 ,i propose in this book to describe a few of the more important earthquakes that have occurred during the last half century4 ,in judging of importance1 the standard which ,i have adopted is not that of intensity only1 but rather of the scientific value of the results that have been achieved by the study of the shocks4 ,even with this reservation1 the number of earthquakes that might be included is considerable2 and ,i have therefore selected those which seem to illustrate best the different methods of investigation employed by seismologists1 or which are of special interest owing to the unusual character of their phenomena or to the light cast by them on the nature and origin of earthquakes in general4 ,thus1 the ,neapolitan earthquake possesses interest from a historical point of view2 it is the first earthquake in the study of which modern scientific methods were employed4 ,the ,ischian earthquakes are described as examples of those connected with volcanic action2 the ,andalusian earthquake is chiefly remarkable for the recognition of the unfelt earth-waves2 that of ,charleston for the detection of the double epicentre and the calculation of the velocity with which the vibrations travelled4 ,in the ,riviera earthquake are combined the principal features of the last two shocks with several phenomena of miscellaneous interest1 especially those connected with its submarine foci4 ,the ,japanese earthquake is distinguished from others by its extraordinary fault-scarp and the very numerous shocks that followed it4 ,the ,hereford earthquake is a typical example of a twin earthquake1 and provided many observations on the sound phenomena2 while the ,inverness earthquakes are important on account of their connection with the growth of a well-known fault4 ,the great ,indian earthquake owns few1 if any1 rivals within historical times1 whether we consider the intensity of the disturbance or the diversity and interest of the phenomena displayed by it--the widespread changes in the earth's crust1 both superficial and deep-seated1 and the tracking of the unfelt pulsations completely round the globe4 ,,terms ,,and ,,definitions4 ,some terms are of such frequent use in describing earthquakes that it will be convenient to group them here for reference1 others more rarely employed being introduced as they are required4 ,an earthquake is caused by a sudden displacement of the material which composes the earth's interior4 ,the displacement gives rise to series of waves1 which are propagated outwards in all directions1 and which1 when they reach the surface1 produce the sensations known to us as those of an earthquake4 ,the region within which the displacement occurs is sometimes called the hypocentre1 but more frequently the seismic focus or simply the focus4 ,the portion of the earth's surface which is vertically above the seismic focus is called the epicentre4 ,the focus and epicentre are often spoken of for convenience as if they were points1 and they may then be regarded as the centres of the region and area in which the intensity was greatest4 ,this is not quite accurate1 but to attempt a more exact definition would at present be out of place4 ,an isoseismal line is a curve which passes through all points at which the intensity of the shock was the same4 ,it is but rarely that the absolute intensity at any point of an isoseismal line can be ascertained1 and only one example is given in this volume4 ,as a rule1 the intensity of a shock is determined by reference to the degrees of different arbitrary scales4 ,these will be quoted when required4 ,in every strong earthquake there is a central district which differs in a marked manner from that outside in the far greater strength and complexity of the phenomena4 ,as this district includes the epicentre1 it is sometimes referred to as the epicentral area1 but the term meizoseismal area is more appropriate1 and will be employed accordingly4 ,the district over which an earthquake is perceptible to human beings without instrumental aid is its disturbed area4 ,in like manner1 that over which the earthquake-sound is heard is the sound-area4 ,a great earthquake never occurs alone4 ,it is merely the most prominent member of a group of shocks of greater or less intensity1 and is known as the principal shock or earthquake1 while the others are called minor or accessory shocks1 and fore-shocks or after-shocks according as they occur before or after the principal earthquake4 ,when the sound only is heard1 without an accompanying tremor being anywhere perceptible1 it is more accurately called an earth-sound1 but is frequently for convenience numbered among the minor shocks4 ,illustration3 ,,fig4 #a4--;,diagram to illustrate simple harmonic motion4 ,the movement of the ground during a vibration of the simplest character 7known as simple harmonic motion7 is represented in ,fig4 #a4 ,the pointer of the recording seismograph is here supposed to oscillate along a line at right angles to ,,ab1 and the smoked paper or glass on which the record is made to travel to the left4 ,the distance ,,mp of the crest ,p of any wave from the line ,,ab represents the amplitude of the vibration1 the sum of the distances ,,mp and ,,nq its range1 and the length ,,ab the period of the vibration4 ,from the amplitude and period we can calculate1 in the case of simple harmonic motion1 both the maximum velocity and maximum acceleration of the vibrating particles of the ground4#a ,a few terms describing the nature of the shock are also in common use among ,italians and ,spaniards4 ,an undulatory shock consists of one or several waves1 the movement to and fro being along a nearly horizontal line2 a subsultory shock of movements in a nearly vertical direction2 while a vorticose shock consists of undulatory or subsultory movements crossing one another in different directions4 ,,origin ,,of ,,earthquakes4 ,earthquakes are grouped1 according to their origin1 into three classes4 ,the first consists of slight local shocks1 caused by the fall of rock in underground passages2 the second of volcanic earthquakes1 also local in character1 but often of considerable intensity near the centre of the disturbed area2 while in the third class we have tectonic earthquakes1 or those directly connected with the shaping of the earth's crust1 which vary in strength from the weakest perceptible tremor to the most destructive and widely felt shock4 ,of the earthquakes described in this volume1 the ,ischian earthquakes belong to the second class1 and all the others to the third4 ,that tectonic earthquakes are closely connected with the formation of faults seems now established beyond doubt4 ,they occur far from all traces of recent volcanic action4 ,their isoseismal lines are elongated in directions parallel to known faults1 and this is sometimes the case in one and the same district with faults that occur at right angles to one another4 ,indeed1 when several isoseismals are carefully drawn1 it is possible from their form and relative position to predict the position of the originating fault4#b ,the initial formation and further spreading of the rent may be the cause of a few earthquakes1 but by far the larger number are due to the subsequent growth of the fault4 ,the relative displacement of the rocks adjoining the fault1 which may amount to thousands of feet1 occasionally even to miles1 is the result1 not of one great movement1 but of innumerable slips taking place in different parts of the fault and spread over vast ages of time4 ,with every fault-slip1 intense friction is suddenly brought into action by the rubbing of one mass of rock against the other2 and1 according to the modern view1 it is this friction that gives rise to the earthquake waves4 ,in most earthquakes1 the slip takes place at a considerable depth1 perhaps not less than one or several miles1 and the vertical slip is so small that it dies out before reaching the surface4 ,but1 in a few violent earthquakes1 such as the ,japanese and ,indian earthquakes described in this volume1 the slip is continued up to the surface and is left visible there as a small cliff or fault-scarp4 ,in these cases1 the sudden spring of the crust may increase and complicate the effects of the vibratory shock4 ,,footnotes3 #a ,if a is the amplitude of the vibration and ,t its period1 the maximum velocity is #b99pi99a_/,t and the maximum acceleration #d99pi#b;a_/,t#b #b ,see ,chapter ,,viii41 on the ,hereford and ,inverness earthquakes4 ,,chapter ,,ii4 ,,the ,,neapolitan ,,earthquake ,,of ,,december #af,,th1 #aheg4 ,half a century ago1 seismology was in its infancy4 ,on the ,continent1 ,alexis ,perrey of ,dijon was compiling his earthquake catalogues with unfailing enthusiasm and industry4 ,in #ahdf1 ,robert ,mallet applied the laws of wave-motion in solids1 as they were then known1 to the phenomena of earthquakes2 and his memoir on the ,dynamics of ,earthquakes#c may be regarded as the foundation-stone of the new science4 ,during the next twelve years he contributed his well-known ,reports to the ,british ,association1#d and prepared a series of instructions for the observation and study of earthquake-shocks4#e ,the latter1 it is worth noting1 contains an outline1 but hardly more than an outline1 of the methods of investigation which he developed and employed eight years afterwards in studying the ,neapolitan earthquake4 ,the history of ,mallet's preparation for his great work is somewhat strange4 ,no one else at that time possessed so full a knowledge of earthquake phenomena4 ,it was1 however1 a knowledge that had little1 if any1 foundation in actual experience2 for1 when he was awakened by the ,british earthquake of ,november #ith1 #aheb1 he failed to recognise its seismic character4 ,although this shock disturbed an area of about #ge1jjj square miles and was felt in all four parts of the kingdom1 the paucity of observations and the absence of durable records combined in preventing the successful application of his new modes of study4#f ,nevertheless1 with confidence unshaken in their power1 he awaited the occurrence of a more violent shock1 but five years had to pass before his opportunity came towards the close of #aheg4 ,so destructive was the ,neapolitan earthquake of this year 7,mallet ranks it third among ,european earthquakes in extent and severity71 that nearly a week elapsed before any news of it reached the outer world4 ,without further loss of time1 he applied for and obtained a grant of money from the ,council of the ,royal ,society1 and proceeded early in the following ,february to what was then the kingdom of ,naples4 ,armed with letters of authority to different officials1 he visited the chief towns and villages in the meizoseismal area2 and1 in spite of unfavourable weather and the difficulties of travelling in a country so recently devastated1 he completed his examination in little more than two months4 ,it was a task1 surely1 that would have baffled any but the most enthusiastic investigator or one unspurred by the feeling that he possessed the key to one of the most obscure of ,nature's problems4 ,mallet's confidence in the accuracy of his methods was almost unbounded4 ,his great report was published four years later2 but he seems to have regarded it almost as a text-book of observational seismology and the results of his ,neapolitan work as mere illustrations4 ,his successors1 however1 have transposed the order of importance1 and rank his two large volumes as the model1 if not the inspirer1 of many of our more recent earthquake monographs4 ,illustration3 ,,fig4 #b4--;,isoseismal ,lines of the ,neapolitan ,earthquake of #aheg4 7,mallet47 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,the position of the meizoseismal area1 to which ,mallet devoted most of his time1 is indicated by the small oval area marked #a in ,fig4 #b1 represented on a larger scale in ,fig4 #i4 ,it is #dj miles long and #bc miles wide1#g and contains #iej square miles4 ,within this area1 the loss of life was great and most of the towns were absolutely prostrated4 ,the next isoseismal1 ,no4 #b1 which is also shown more clearly in ,fig4 #i1 bounds the area in which the loss of life was still great and many persons were wounded1 while large portions of the towns within it were thrown down4 ,its length is #fe miles1 width #dg miles1 and area #b1bdj square miles4 ,the third isoseismal includes a district in which buildings were only occasionally thrown down1 though none escaped some slight damage1 and in which practically no loss of life occurred4 ,this curve is #ajc miles long1 #hb miles wide1 and includes #f1fae square miles4 ,lastly1 the fourth isoseismal marks the boundary of the disturbed area1 which is #bej miles long1 #baj miles wide1 and contains not more than #ci1bjj square miles2 an amount that must be regarded as strangely small1 and hardly justifying ,mallet's estimate of the ,neapolitan earthquake as the third among ,european earthquakes in extent as well as in seventy4 ,,damage ,,caused ,,by ,,the ,,earthquake4 ,as regards destruction to life and property1 however1 the ,neapolitan earthquake owns but few ,european rivals4 ,less favourable conditions for withstanding a great shock are seldom1 indeed1 to be found than those possessed by the medival towns and villages of the meizoseismal area4 ,in buildings of every class1 the walls are very thick and consist as a rule of a coarse1 short-bedded1 ill-laid rubble masonry1 without thorough bonding and connected by mortar of slender cohesion4 ,the floors are made of planks coated with a layer of concrete from six to eight inches thick1 the whole weighing from sixty to a hundred pounds per square foot4 ,only a little less heavy are the roofs1 which are covered with thick tiles secured1 except at the ridges1 by their own weight alone4 ,thus1 for the most part1 the walls1 floors1 and roofs are extremely massive1 while the connections of all to themselves and to each other are loose and imperfect4 ,again1 the towns1 for greater security from attacks in early times1 are generally perched upon the summits and steep flanks of hills1 especially of the lower spurs that skirt the great mountain ranges2 and the rocking of the hill-sites1 in ,mallet's opinion1 greatly aggravated the natural effects of the shock4 ,the streets1 moreover1 are steep and narrow1 sometimes only five feet1 and not often more than fifteen feet1 in width2 and the houses1 when shaken down1 fell against one another and upon those beneath them4 ,as ,dolomieu said of the great earthquake in #aghc1 the ground was shaken down like ashes or sand laid upon a table4 ,of the total amount of damage1 not even the roughest estimate can be made4 ,the official returns are clearly1 and no doubt purposely1 deficient1 and obstacles were placed in ,mallet's way when he endeavoured to ascertain the numbers of persons killed and wounded4 ,taking only the towns into account1 he calculated that1 out of a total population of #bjg1jjj1 the number of persons killed was #i1ehi1 and of wounded #a1cdc4h ,a few towns were marked by an excessively high death-rate4 ,thus1 at ,montemurro1 #ejjj out of #gjjb persons were killed and #ejj wounded2 at ,saponara1 #bjjj out of #djaj were killed2 and1 at ,polla1 more than #bjjj out of a population of less than #gjjj4 ,,general ,,objects ,,of ,,investigation4 ,the principal objects of ,mallet's investigation were to determine the position of the epicentre and the depth of the seismic focus4 ,if1 in ,fig4 #c1 ,f represents the seismic focus 7here1 for convenience1 supposed to be a point71 the vertical line ,,fe will cut the surface of the earth in the epicentre ,e4#i ,the dotted lines represent circles drawn on the surface of the earth with ,e as centre and passing through the places ,p and ,q4 ,illustration3 ,,fig4 #c4--;,diagram to illustrate wave-path and angle of emergence4 ,when the impulse causing the earthquake takes place at the focus1 two elastic waves spread outwards from it in all directions through the earth's crust4 ,the first wave which reaches a point ,p consists of longitudinal vibrations1 that is1 the particle of rock at ,p moves in a closed curve with its longer axis in the direction ,,fp4 ,mallet supposes this curve to be so elongated that it is practically a straight line coincident in direction with ,,fp4 ,in the second or transversal wave1 the vibration of the particle at ,p takes place in a plane at right angles to ,,fp4 ,these vibrations ,mallet1 for his main purpose1 neglects4 ,returning to the longitudinal wave1 ,mallet calls the line ,,fp the wave-path at ,p4 ,the direction ,,ep gives the azimuth of the wave-path1 or its direction along the surface of the earth4 ,the angle ,,lpa1 or ,,epf1 he defines as the angle of emergence at the point ,p4 ,if ,q be farther from ,e than ,p1 the angle ,,eqf is less than the angle ,,epf1 or the angle of emergence diminishes as the distance from the epicentre increases4 ,at the epicentre1 the angle of emergence is a right-angle2 at a great distance from the epicentre1 it is nearly zero4 ,mallet argued that the direction of the wave-path ,,fpa1 or its equivalents1 the horizontal direction ,,epl and the angle of emergence ,,epf1 should be discoverable from the effects of the shock at ,p4 ,the cracks in damaged buildings1 he urged1 would be at right angles to the wave-path ,,fpa2 overturned monuments or gate-pillars should fall along the line ,,epl1 either towards or from the epicentre according to their conditions of support2 loose or slightly attached bodies1 such as the stone balls surmounting gate-pillars1 should be projected nearly in the direction of the wave-path ,,fpa1 and their subsequent positions1 supposing the balls not to have rolled1 should give the horizontal direction ,,epl of the wave-path1 and might1 in some circumstances1 determine the angle of emergence and the velocity with which they were projected4 ,i shall return to details later on4 ,for the present1 it is clear that1 in the destruction wrought by the earthquake1 ,mallet expected to find the materials most valuable for his purpose4 ,indeed1 so obvious did this mode of examination appear to him1 that he could not conceal his surprise at the blindness of his predecessors4 ,they seem1 he says1 to have been perfectly unconscious that in the fractured walls and overthrown objects scattered in all directions beneath their eyes1 they had the most precious data for determining the velocities and directions of the shocks that produced them4 ,,position ,,of ,,the ,,epicentre4 ,mallet's ,method of ,determining the ,position of the ,epicentre4--,in many cases the examination of a damaged building or of an overthrown body served more than one purpose1 providing materials for ascertaining the depth of the seismic focus as well as the position of the epicentre4 ,for the present1 however1 it will be convenient to consider alone the method by which the latter object was to be attained4 ,illustration3 ,,fig4 #d4--;,diagram to illustrate ,mallet's method of determining position of epicentre4 ,nothing could be simpler than the principle of the method proposed4 ,the horizontal direction ,,pl of the wave-path at any place ,p 7,fig4 #d71 when produced backwards1 must pass through the epicentre ,e2 and the intersection of the directions at two places1 ,p and ,q1 must therefore give the position of the epicentre4 ,in practice1 it is of course impossible to determine the direction with very great accuracy1 and ,mallet therefore found it necessary to make several measurements in every place1 and to visit all the more important towns within and near the meizoseismal area4 ,in a ruined town there are many objects from which the direction may be ascertained1 the most important of all1 according to ,mallet1 being fissures in walls that are fractured but not overthrown4 ,he regarded such fissures1 indeed1 as the sheet-anchor1 as respects direction of wave-path1 to the seismologist in the field1 and at least three out of every four of his determinations of the direction were made by their means4 ,if the buildings are detached and large1 simple and symmetrical in form1 well built and not too much injured1 the fissures in the walls should1 he argued1 occur along lines at right angles to the wave-path1 whether that path be parallel or inclined to the principal axis of the building4 ,cracks in the floors and ceilings should also be similarly directed1 and provide evidence which ,mallet regarded as only second in value to that given by the walls4 ,illustration3 ,,fig4 #e4--,plan of ,cathedral ,church at ,potenza4 7,mallet47 ,no building showed the different kinds of evidence on which ,mallet relied as clearly as the cathedral church at ,potenza1 the plan of which is given in ,fig4 #e1 and the vertical section along its axis in ,fig4 #ab4 ,this is a modern work1 nearly #bjj feet long1 with its axis directed east and west4 ,the walls are composed of fairly good rubble masonry and brick2 and the arches in the nave and transepts1 the semi-cylindrical roof and the central dome are made of brick4 ,the fissures represented in both diagrams were drawn to scale by the cathedral architect before ,mallet's arrival1 and1 as the work of an unbiassed observer1 are of special value4 ,most of those in the roof1 it will be seen1 were transverse to the axial line of the church2 but there were others parallel to this line1 one in particular running right along the soffit of the nave and chancel4 ,there were also numerous small fissures in the dome1 due to local structural causes and therefore of varying direction1 and a large portion of the dome slipped westward1 leaving open fissures of seven to eight inches in width4 ,the mean direction of the wave-path1 as deduced from nine sets of fissures1 none of which differs more than four degrees from the mean1 is ,w4 #b-a_/b ,s4 and ,e4 #b-a_/b ,n41 which corresponds precisely with the direction of throw on the displaced portion of the dome4 ,the great east and west fissures in the arch of the nave and chancel ,mallet attributed to a second shock1 of the existence of which there is ample evidence4 ,illustration3 ,,fig4 #f4--;,fallen gate-pillars near ,saponara4 7,mallet47 ,next to fissures1 ,mallet made most use of overthrown objects1 such as the two gate piers near ,saponara1 represented in ,fig4 #f4 ,they were made of rubble ashlar masonry1 three feet square and seven feet in height4 ,both were fractured clean off at the level of the ground1 the mortar being poor1 and fell in directions that were accurately parallel1 indicating a wave-path towards ,s4 #ci-a_/b;,e4 ,a few observations were also made on projected stones1 fissures in nearly level ground1 and the swinging of lamps and chandeliers2 but their value was small1 except as corroboration of the more important evidence afforded by fissures in the walls and roofs of buildings4 ,remarks on ,mallets ,method4--,it would have been more difficult in ,mallet's day than it is now1 to offer objections to his method of determining the position of the epicentre4 ,the focus1 as he was well aware1 could not be a point1 and1 at places near the epicentre 7the very places where most of his observations were made71 there must be rapid changes of direction due to the arrival of vibrations from different parts of the focus4 ,he records the occurrence of the so-called vorticose shocks at several places1 though he attributes them to another cause4 ,perhaps the best known example of such a shock is that which has been so well illustrated by the late ,professor ,sekiya's model of the motion of an earth-particle during the ,japanese earthquake of ,january #aeth1 #ahhg4 ,the motion in this case was so complicated that the model was1 for simplicity1 made in three parts1 the first of which alone is represented in ,fig4 #g4aj ,it is clear that in such an earthquake1 ,mallet's method would utterly fail in giving definite results4 ,while this shock was one of great complexity1 another ,japanese earthquake1 that of ,june #bjth1 #ahid1 was unusually simple in character4 ,the movement at ,tokio consisted of one very prominent oscillation with a total range of #gc mm4 or #b4i inches in the direction ,s4 #gj ,w42 the vibrations which preceded and followed it being comparatively small4 ,most1 if not all1 of the damage caused by the earthquake must have been due to this great oscillation2 and yet the cylindrical stone-lamps so common in ,japanese gardens were found by ,professor ,omori to have fallen in many different directions4 ,taking only those which had circular bases1 twenty-nine were overthrown in directions between north and east1 sixteen between east and south1 eighty-one between south and west1 and fourteen between west and north4#aa ,fig4 #h represents ,professor ,omori's results graphically1 the line drawn from ,o to any point being proportional to the number of lamps which fell in directions between #g-a_/b on either side of the line4 ,illustration3 ,,fig4 #g4--,model to illustrate the motion of an earth-particle during an earthquake4 7,sekiya47 ,illustration3 ,,fig4 #h4--,plan of directions of fall of overturned stone-lamps at ,tokio during the earthquake of #ahid4 ,it will be seen from this figure that most of the stone lamps fell in directions between west and south-west1 and it is remarkable that the mean direction of fall is ,s4 #gj ,w41#ab which is exactly the same as that of the great oscillation4 ,somewhat similar results were obtained by this able seismologist at different places affected by the great ,japanese earthquake of #ahia 7,figs4 #dc and #dd71 and the study of the apparent directions observed during the ,hereford earthquake of #ahif leads to the same conclusion4 ,it thus appears that an isolated observation may give a result very different from the true direction4 ,indeed1 if we may judge from ,professor ,omori's measurements in #ahid1 the chance that a single direction may be within five degrees of the mean direction is about #a in #i4 ,but1 on the other hand1 it is equally clear from these and other observations that the mean of a large number of measurements will give a result that agrees very closely with the true direction4 ,one other point may be alluded to before leaving ,professor ,omori's interesting observations4 ,it would seem1 from the list that he gives1 that he exercised no selection in his measurements1 but continued measuring the direction of every fallen lamp indifferently until he had obtained sufficient records for his purpose4 ,now1 if the number of fallen lamps at his disposal had been small1 say #ab instead of #add1 the mean observed direction would probably have differed from the direction given from the seismograph4#ac ,but1 on the other hand1 a preliminary survey without any actual measurements would have revealed at once the predominant direction of overthrow1 and a fairly accurate result might have been obtained by neglecting discordant directions and taking the mean of those only which appeared to agree with the mentally determined average4 ,this1 indeed1 appears to have been the course followed1 more or less unconsciously1 by ,mallet in his ,neapolitan work4 ,when the observer1 he says1 first enters upon one of those earthquake-shaken towns1 he finds himself in the midst of utter confusion4 ,the eye is bewildered by 'a city become an heap4' ,he wanders over masses of dislocated stone and mortar1 with timbers half buried1 prostrate1 or standing stark up against the light1 and is appalled by spectacles of desolation4444 ,houses seem to have been precipitated to the ground in every direction of azimuth4 ,there seems no governing law1 nor any indication of a prevailing direction of overturning force4 ,it is only by first gaining some commanding point1 whence a general view over the whole field of ruin can be had1 and observing its places of greatest and least destruction1 and then by patient examination1 compass in hand1 of many details of overthrow1 house by house and street by street1 analysing each detail and comparing the results1 as to the direction of force1 that must have produced each particular fall1 with those previously observed and compared1 that we at length perceive1 once for all1 that this apparent confusion is but superficial4 ,illustration3 ,,fig4 #i4--,meizoseismal area of ,neapolitan earthquake4 7,mallet47 ,mallet's ,determination of the ,epicentre4--,within the third isoseismal line ,mallet made altogether #agg measurements of the direction of the wave-path at #gh places4 ,these are plotted on his great map of the earthquake2 but1 owing to the small scale of ,fig4 #i1 it is only possible to represent1 by means of short lines1 the mean or most trustworthy direction at each place4#ad ,producing these directions backwards1 he found that those at sixteen places passed within five hundred yards of a point which is practically coincident with the village of ,caggiano2 those at sixteen other places passed within one geographical mile 7#a4aec statute miles7 of this point2 the directions at sixteen more places within two and a half geographical miles2 while those at twelve places passed through points not more than five geographical miles from ,caggiano4 ,as the direction of the shock at places near the epicentre must have been influenced by the mere size of the focus1 this approximate coincidence is certainly remarkable1 and there can be little doubt1 ,i think1 that the epicentre1 or1 at any rate1 an epicentre must have been situated not far from the position assigned to it by ,mallet's laborious observations4 ,existence of ,two ,epicentres4--,it is difficult1 however1 to realise that the impulse at the focus corresponding to ,mallet's epicentre was the origin of all the destruction of life and property that occurred4 ,the position of the epicentre close to the north-west boundary of the meizoseismal area1 the extraordinary extension of that area towards the south-east1 and especially the great loss of life at ,montemurro and the adjoining towns1 can hardly be accounted for in this manner4 ,mallet himself recognised that these facts required explanation1 and he suggested that the situation and character of the towns were in part responsible for their ruin1 and the physical structure of the country for the course of the isoseismal lines4 ,but the comparative escape of places much nearer ,caggiano1 and the wide extent of the meizoseismal area1 embracing many towns and villages of varied character and site and many different surface-features1 point unmistakably to a different explanation4 ,illustration3 ,,fig4 #aj4--;,distribution of death-rate within meizoseismal area of ,neapolitan earthquake4 ,one clue to the solution of the problem is afforded by the seismic death-rate of the damaged towns4 ,from a table given by ,mallet 7vol4 ii4 pp4 #afb-afc71 we know the population before the earthquake of the different communes in the province of ,basilicata1 and the loss of life in each due to the shock2 and from these figures we can find the percentage of deaths at nearly every place of importance4 ,as will be seen from ,fig4 #aj1 it varies from seventy-one at ,montemurro and fifty at ,saponara down to less than one at all the places marked to which figures are not attached4 ,there is thus a group of places1 with its centre near ,montemurro1 where the loss of life far exceeded that in the surrounding country2 and also a slightly less-marked group1 with its centre near ,polla1 in the north-west of the meizoseismal area2 while in the intermediate region the death-rate was invariably small4 ,too much stress should not be laid upon the exact figures1 for there were no doubt local conditions that affected the death-roll4 ,but it seems clear that one focus was situated not far from ,montemurro2 while the north-westerly group of places1 combined with ,mallet's observations on the direction1 point to a second focus near ,polla1 about twenty-four miles to the north-west4 ,it will be seen in a later section that the observations on the nature of the shock also imply the existence of a double focus4 ,,depth ,,of ,,the ,,seismic ,,focus4 ,mallet's ,method of ,determining the ,depth of the ,focus4--,in ascertaining the position of the epicentre1 ,mallet's work was remarkable only for the novelty of the method employed by him2 but1 in his attempt to calculate the depth of the seismic focus1 he was breaking new ground4 ,that the depth must be comparatively small had already been recognised1 and was indeed obvious from the limited area disturbed by nearly every earthquake4 ,no one1 however1 had tried to estimate the depth in miles2 and it is impossible not to sympathise with ,mallet while he accumulated his observations with feverish activity and subjected them to the first rough examination even if one cannot share his confidence that he had succeeded in measuring the depth in miles and yards with the certainty that belongs to an ordinary geodetic operation4 ,the method employed by him for the purpose is no less simple theoretically than that used for locating the epicentre4 ,if the position of the latter 7,e7 is known1 one accurate measurement of the angle of emergence ,,epf1 at any other point ,p would be sufficient to fix the depth of some point within the focus ,f 7,fig4 #aa74 ,here1 again1 ,mallet relied chiefly on fissures in walls that were fractured but not overthrown4 ,in detail1 these fissures are nearly always jagged or serrated1 for they tend to follow the lines of joints rather than break through the solid stone1 though they sometimes traverse bricks and mortar alike4 ,but the general course of the fissures1 he urged1 would be at right angles to the wave-path1 and their inclination to the vertical should be equal to the angle of emergence4 ,illustration3 ,,fig4 #aa4--;,diagram to illustrate ,mallet's method of determining depth of seismic focus4 ,in obtaining measurements of this angle1 the buildings to be chosen are those of large size1 with few windows or other apertures1 and with walls made of brick or small short-bedded stones4 ,the cathedral-church at ,potenza perhaps satisfies these conditions more closely than any other structure examined by ,mallet4 ,the plan of the fissures in the walls and roof has been given in ,fig4 #e1 and ,fig4 #ab represents the fissures ,in the vertical section along the axial line and looking north1 as drawn by the cathedral architect4 ,from these fissures ,mallet calculated the mean angle of emergence at ,potenza to be #bc #g'4 ,the distance of ,potenza from ,caggiano being seventeen miles1 and the height of the former being #b1ehj feet1 the depth of the focus resulting from this observation alone would be #f-c_/d miles below the level of the sea4 ,illustration3 ,,fig4 #ab4--,vertical section of ,cathedral ,church at ,potenza4 7,mallet47 ,objection to ,mallet's ,method4--,the weakest point in ,mallet's method is probably his assumption that the wave-paths are straight lines extending outward from the focus4 ,even if the depth of the focus is not more than a few miles1 the waves must traverse rocks of varying density and elasticity1 and1 at every bounding surface1 they must undergo refraction4 ,if the rocks are so constituted that the velocity of the earth-waves in them increases with the depth1 then the wave-paths must be bent continually outwards from the vertical1 so that the angle of emergence at the surface may be considerably less than it would have been with a constant velocity throughout4 ,in this case1 the actual depth will be greater1 perhaps much greater1 than the calculated depth4 ,for instance1 if the angle of emergence at ,potenza were diminished only #e by refraction1 the calculated depth of the focus would be too small by #a-c_/d miles4 ,mallet's ,estimate of the ,depth of the ,focus4--,mallet measured the angle of emergence at twenty-six places1 the mean angle 7i4e4 the mean of the greatest and least observed angles7 varying from #gb at ,vietri di ,potenza and #gj at ,pertosa1 which are about two miles from the calculated epicentre1 to #aa-a_/b at ,salerno1 distant about #dj miles4 ,fig4 #ac reproduces part of the diagram on which he plotted the mean angle of emergence at different places4 ,the horizontal line represents the level of the sea1 and the vertical line one passing through the epicentre and focus1 called by ,mallet the seismic vertical4 ,the lines on the left-hand side represent the commencing wave-paths 7assumed straight7 to the observing stations situated to the westward of the meridian through the epicentre1 those on the right-hand side corresponding to places to the eastward of the same meridian4 ,small horizontal marks are added to indicate the depth in miles below the level of the sea4 ,illustration3 ,,fig4 #ac4--;,diagram of wave-paths at seismic vertical of ,neopolitan earthquake4 7,mallet47 ,it will be seen1 from this diagram1 that all the wave-paths start from the seismic vertical at depths between three and nine miles2 but the points of departure are clustered thickly within a portion1 the length of which is about #c-a_/b miles and the mean depth about #f-a_/b miles4 ,so great was ,mallet's confidence in these calculations that he assigns the diverging origin of the wave-paths to different points of the focus1 and thus concludes that1 while the mean depth of the focus was about #f-a_/b miles1 its dimensions in a vertical direction did not exceed #c-a_/b miles4 ,how far ,mallet's results should be accepted as correct1 it is difficult to say in our ignorance of the constitution of the earth's interior4 ,there can be no doubt that the focus was of considerable size1 and that1 in consequence1 the wave-paths would diverge from different points of it4 ,but that each wave-path should actually intersect the focus1 and so enable its magnitude to be determined1 would surely involve an approach to some law connecting the direction of a wave-path with the depth of its own origin1 and no such law seems to be ascertainable4 ,nor can the limitation of these apparent origins between certain depths be held to argue that the focus1 or any part of it1 was equally confined1 for the wave-paths would to a great extent be similarly refracted4 ,i fear that the only conclusions that we can with safety draw from ,mallet's admirable work are that his figures indicate the order of magnitude both of the vertical dimensions and of the mean depth of the focus4 ,,nature ,,of ,,the ,,shock4 ,it is not easy to form any precise image of the earthquake as it appeared to the terrified witnesses within the meizoseismal area4 ,to minds unbalanced by the suddenness of the shock and by the crash of falling houses1 actuated too by the intense need of safety1 the mere succession of events must have presented but little interest4 ,the interval of two months that elapsed between the occurrence of the earthquake and its investigation was also unfavourable to the collection of accurate accounts from a wonder-loving people4 ,only one feature1 therefore1 stands out clearly in the few records given by ,mallet--namely1 the division of the shock into two distinct parts4 ,in the central district1 this division is perhaps less apparent than elsewhere4 ,at ,polla1 for instance1 which lies close to the north-west epicentre1 the first warning was given by a rushing sound2 almost instantly1 and while it was yet heard1 came a strong subsultory or up-and-down movement1 succeeded after a few seconds1 but without any interval1 by an undulatory motion4 ,at ,potenza1 which is not far from the same epicentre but a few miles outside the meizoseismal area1 the separation was more pronounced4 ,according to one observer1 the first movement was from west to east2 and1 within a second or two afterwards1 there was a less violent shock in a transverse direction1 followed immediately by a shaking in all directions1 called by the ,italians vorticose4 ,naples lies sixty-nine miles from the north-west epicentre1 and here more accurate observations could be made4 ,dr4 ,lardner1 well known fifty years ago as a writer of scientific works1 describes the first movement felt there as a short1 jarring1 horizontal oscillation1 that made all doors and windows rattle1 and the floors and furniture creak4 ,this ceased1 and after an interval that seemed but a few seconds was renewed with greater violence1 and1 he thought1 with a distinctly undulatory movement1 'like that in the cabin of a small vessel in a very short chopping sea4' ,in five other earthquakes studied in this volume1 the separation of the shock into two parts was a well-marked phenomenon4 ,in the ,neapolitan earthquake1 the separation was so distinct that ,mallet took some pains to account for its origin4 ,he regarded it in every case as due to the reflection or refraction of the earth-waves by underlying rocks1 though he does not explain why the reflected or refracted wave should be more intense than that transmitted directly4 ,i shall refer to the subject in greater detail when describing the ,andalusian1 ,charleston1 ,riviera1 and ,hereford earthquakes4 ,for the present1 it may be sufficient to urge that the double shock cannot have been due to the separation of the original waves by underground reflection or refraction1 for then the second part should have been generally the weaker2 nor to the succession of longitudinal and transverse waves1 for1 in that case1 every earthquake-shock should be duplicated4 ,the only remaining supposition is that there was a second impulse occurring either in the same or in a different focus4 ,which alternative should be adopted1 the evidence on the nature of the shock is too scanty to determine4 ,the defect is1 however1 supplemented by ,mallet's observations on the direction of motion2 for1 at many places within and near the meizoseismal area1 he met with the clearest signs of a double direction4 ,sometimes this was apparent to the senses of the observer2 in other cases1 damaged buildings presented two sets of fissures4 ,at ,la ,sala and near ,padula1 the first movement was roughly east and west1 the second north and south4 ,at ,moliterno1 there was evidence of a subordinate shock at right angles to the chief one2 in the neighbourhood of ,tramutola1 its direction was from about ,e4 #cj ,s4 ,in these and other cases1 ,mallet saw the effects of earthquake-echoes2 but the underground reflection of earth-waves would give rise to the second part of the shock1 not the first as at ,la ,sala and ,padula4 ,moreover1 the secondary directions1 though they are seldom recorded accurately1 point nearly to an epicentre not far from ,montemurro4 ,the observations on the nature and direction of the double shock thus confirm the conclusion1 derived from the distribution of the seismic death-rate1 that there were two detached foci1 one near ,polla and the other near ,montemurro4 ,this seems to be the best explanation of the facts recorded by ,mallet4 ,there is1 however1 a possible difficulty that should not be overlooked--namely1 the apparently slight influence of the ,montemurro focus on the mean direction of the shock 7,fig4 #i74 ,at a few places1 of course1 the mean direction passes through both epicentres2 at some others1 as we have seen1 one of the two observed directions points towards the ,montemurro epicentre4 ,it is not impossible1 also1 that ,mallet1 after the first few days' work1 may occasionally have quite unconsciously selected and measured those fissures from the maze presented to him which agreed most closely with his early impressions obtained from the neighbourhood of ,polla4 ,but1 for places nearer ,polla than ,montemurro 7and these form the majority of those visited by ,mallet71 the probable explanation of the difficulty is that the ,montemurro focus was not so deep as the ,polla focus4 ,this1 as will appear more fully in the next chapter1 would account for the comparatively great intensity in the immediate neighbourhood of ,montemurro and for its rapid decline outwards2 and it receives some support from an isolated reference by ,mallet to two angles of emergence at ,padula1 one of #be from the north1 and the other of #h or #aj in the perpendicular walls4 ,,elements ,,of ,,the ,,wave-,,motion4 ,the elements of the wave-motion1 as mentioned in the introductory chapter1 are four in number1 namely1 the period1 amplitude1 maximum velocity1 and maximum acceleration4 ,if any two of these are known for each vibration--and the first two are now given by every accurately constructed seismograph--the others can be determined if the vibrations follow the law of simple harmonic motion4#ae ,amplitude4--,to ascertain the amplitude1 ,mallet had to rely chiefly on the fissures made in very inelastic walls4 ,if the parts into which such a wall are fractured are free to move1 and yet1 being inelastic1 obliged to remain in the farthest position to which they are carried by the wave1 the distance traversed by the centre of gravity of one of the displaced parts should give a rude approximate measure of the horizontal amplitude of the earth-wave4 ,at ,certosa1 near ,padula1 he thus found the amplitude to be about #d inches1 at ,sarconi about #d-c_/d inches1 and at ,tramutola about #d-a_/b inches4 ,from somewhat similar evidence1 the amplitude at ,polla appears to have been about #b-a_/b or #c inches2 and1 from the oscillation of a suspended clock or watch on a rough wall1 about #c-a_/b inches at ,la ,sala and #a-c_/d inches at ,barielle4 ,with the exception of ,barielle1 these places lie nearly on a straight line passing through ,mallet's epicentre1 and he gives the following table1 showing an increase in amplitude with the distance from the epicentre3-- ,polla4 ,la ,sala4 ,certosa4 ,tramutola4 ,sarconi4 ,distance in miles #d4j #ac4d #ai4j #bc4h #cj4h ,amplitude in inches #b-a_/b #c-a_/b #d #d-a_/b #d-c_/d ,the existence of the ,montemurro focus must1 however1 complicate any relation that may connect these two quantities4 ,maximum ,velocity4--,the means at ,mallet's disposal for determining the maximum velocity were more numerous than those available for the amplitude4 ,from the dimensions of a fallen column of regular form we should be able1 he remarks1 to find an inferior limit to the value of the maximum velocity2 while a superior limit at the same place may be obtained from some other regular solid which escaped being overthrown4 ,if a loose body is projected by the shock at a place where the angle of emergence is known1 the horizontal and vertical distances traversed by the centre of gravity will give the velocity of projection4 ,or1 if two such bodies are projected at one place1 the same measures for each will as a rule give both the angle of emergence and the velocity of projection4 ,a third method depends on the fissuring of walls1 supposing that we know the force per unit surface which1 when suddenly applied1 is just sufficient to produce fracture4 ,sometimes more than one method must be applied to the same object4 ,the two gate-pillars near ,saponara 7illustrated in ,fig4 #f7 for example required a horizontal velocity of #e4dh feet per second to fracture them1 and an additional velocity of #e4ad feet per second to overthrow them4 ,the well-known seismologist1 ,professor ,milne1 urges very forcibly that measurements obtained from the projection or fall of columns are unreliable1 for the earlier tremors might cause the columns to rock1 and their overthrow need not therefore measure accurately the maximum velocity of the critical vibration4#af ,there can be no doubt that ,mallet was alive to this difficulty1 though he may not have appreciated it at its full value4 ,thus1 at the ,certosa de ,st4 ,lorenzo1 a monastery near ,padula1 a vase projected from the summit of a slender gate-pier implied a velocity of #ba-c_/d feet per second2 and the excess of about #h-a_/d feet per second above the velocity determined by other means is attributed by him to the oscillation of the pier itself4 ,how far this source of error enters into other observations it is impossible to say2 but it is worth noticing how closely the velocities obtained by different methods agree with one another4 ,thus1 from projection only1 we have velocities of #aa4e feet per second at the ,certosa1 #aa4h at ,moliterno and ,monticchio1 #ad4h at ,tramutola1 and #i4h feet per second at ,sarconi2 from overthrow alone1 #aa4j feet per second at ,viscolione1 near ,saponara1 and #aa4f at ,barielle2 from overthrow and projection1 #ac4b feet per second at ,polla and #ab4i at ,padula2 from fracture and overthrow1 #ab4c feet per second at ,potenza and #ae4f at ,saponara4 ,the comparatively high values at ,tramutola and ,saponara1 ,mallet imagined might be due to the oscillation of the hills on which these towns are built4 ,he therefore omits them in calculating the mean maximum velocity1 which he finds to be twelve feet per second1 a velocity less than that with which a man reaches the ground when he jumps off a table4 ,with the same omissions1 ,mallet gives the following table1 showing a general decrease in the maximum velocity as the distance from his epicentre increases3-- ,polla4 ,padula4 ,certosa4 ,moliterno4 ,viscolione4 ,sarconi4 ,distance in miles #d4j #ai4j #ai4j #bi4d #cj4j #cj4h ,max4 vel4 in ft4 per sec4 #ac4b #ab4i #aa4e #aa4h #aa4j #i4h ,on the north side of the epicentre we have3-- ,potenza4 ,monticchio4 ,barielle4 ,distance in miles #ag4c #bg4a #bh4b ,max4 vel4 in ft4 per sec4 #ab4c #aa4h #aa4f ,it is not impossible that the high calculated velocities at ,tramutola and ,saponara were partly or entirely due to the impulse from the ,montemurro focus4 ,if we take #d inches for the amplitude of the largest variation1 and #ab feet per second for the maximum velocity1 and assume the motion to have been of a simple harmonic character1 the period of a complete vibration would be less than one-fifth of a second4#ag ,now1 we know from seismographic records that this is roughly the period of the small tremors that form the commencement of an earthquake-shock1 while the period of the largest vibrations may amount to as much as one or two seconds4 ,we may therefore conclude either that the assumption of simple harmonic motion is incorrect1 or that the maximum velocity is too great1 or more probably perhaps that the amplitude is too small4#ah ,,sound-,,phenomena4 ,mallet was one of the first seismologists to realise the significance of the earthquake-sound2 and he attended closely to the subject1 though finding the sound even more elusive of precise observation than the shock4 ,the chief result obtained by him was the comparative smallness of the area over which the sound was heard4 ,he estimates it at little more than #c1cjj square miles1 or about one-twelfth of that over which the shock was felt4 ,it extends north and south from ,melfi to ,lagonegro1 and east and west from ,monte ,peloso to ,duchessa and ,senerchia4 ,the sound was thus confined to the region in which the shock attained its most destructive character4 ,towards the north and south ends of the sound-area all observers described the sound as a low1 grating1 heavy1 sighing rush1 lasting from twenty to sixty seconds1 some adding that it was also of a rumbling nature4 ,near the centre and the east and west boundaries1 the sound was distinctly more rumbling2 it was shorter in duration1 and began and ended more abruptly4 ,the earthquake1 ,mallet remarks1 began everywhere with tremors2 the sounds generally arrived at the same time2 the apparent direction of movement of the tremulous oscillations appeared rapidly to change1 and still more rapidly to increase in amplitude2 then the great shove of the destructive shock arrived1 in some places rather before1 in some a little after1 the moment of loudest sound1 and it died away suddenly 7i4e41 with extreme rapidity7 into tremors again1 but differing in direction from that of the great shock itself4#ai ,the earthquake-sound will be described more fully in the chapter dealing with the ,hereford earthquake of #ahif1 in which it will be found that the phenomena recorded by ,mallet are equally characteristic of the slighter shocks felt in this country4 ,,velocity ,,of ,,the ,,earth-,,waves4 ,in #aheg little was known about the velocity of earthquake-waves4 ,experiments had been made by ,mallet himself in #ahdi in the neighbourhood of ,dublin4 ,these gave #hbe feet per second for the velocity in dense wet sand1 #a1cjf feet per second in discontinuous granite1 and #a1ffe feet per second in more solid granite4#bj ,the only earthquake for which the velocity had been calculated was the ,rhenish earthquake of #ahdf1 the value ascertained by ,schmidt being #a1cgf ,french feet1 or #a1dff ,english feet1 per second4 ,the accurate public measurement of time1 which1 as ,mallet remarks1 is one of the surest indications of advancing civilisation1 was1 however1 unknown in the kingdom of ,naples2 and his attempt was therefore fettered by the rarity of precise estimates of the time of occurrence4 ,throughout the whole disturbed area only six good records could be obtained1 and three of these 7at ,vietri di ,potenza1 ,atella1 and ,naples7 were derived from stopped clocks1 witnesses of rather doubtful value4 ,at ,montefermo and ,barielle the time was at once read from a watch1 and at ,melfi from an accurate pocket chronometer4 ,the times given vary from #i;h4 #eim4 #afs4 ,p4,m4 7,naples mean time7 at ,vietri di ,potenza to #aj;h4 #gm4 #dds4 at ,naples4 ,allowing for the supposed change of direction by refraction at the ,monte ,st4 ,angelo range on the way to ,naples1 ,mallet finds the mean surface velocity to be #ghg feet per second4 ,omitting the ,naples record1 and taking account of the calculated depth of the focus1 the mean velocity becomes #hjd feet per second4 ,,minor ,,shocks4 ,a great earthquake rarely1 if ever1 occurs without some preparation in the form of a marked increase of seismic activity4 ,perrey records several shocks during the two years #ahef-eg that were felt at places as far apart as ,naples1 ,melfi1 and ,cosenza4 ,on ,december #gth1 #aheg1 a slight shock1 with a report from beneath like the explosion of a mine1 was felt at ,potenza4 ,then came the great earthquake on ,december #afth1 at about #aj ,p4,m4 ,this was followed by numerous after-shocks--how numerous it is impossible to say1 for the records are of the scantiest description4 ,for some hours the ground within the meizoseismal area is said to have trembled almost incessantly4 ,at ,potenza many slight shocks1 both vertical and horizontal1 were felt during the night1 and for a month or more they were so frequent as to render enumeration difficult4 ,mallet's last record is dated ,march #bcrd1 #aheh1 when four slight shocks were felt at ,la ,sala and ,potenza1 but occasional tremors were reported to him until ,may #ahei4 ,the most important of all these after-shocks was one felt about an hour after the principal earthquake4 ,everywhere far less powerful1 it was yet strong enough to shake down many buildings at ,polla that had been shattered by the great shock4 ,towards the south at ,moliterno1 and towards the north at ,oliveto and ,barielle1 it evidently attracted very little attention4 ,so far as can be judged from the evidence given by ,mallet1 the disturbed area seems to have been approximately of the same form and dimensions as the meizoseismal area1 and elongated in the same direction1 but concentric with the north-west focus4 ,on the other hand1 if we may rely on too brief evidence1 several after-shocks recorded only at ,montemurro1 ,saponara1 ,viggiano1 or ,lagonegro1 were probably connected with the south-east or ,montemurro focus4 ,,origin ,,of ,,the ,,earthquake4 ,mallet's theories have suffered perhaps more than any other part of his work from the recent growth of our knowledge4 ,from a historical point of view1 some reference to his explanation of the origin of the ,neapolitan earthquake seems desirable1 and his own conscientious work demands it4 ,on the other hand1 his conclusions are1 for the present at any rate1 superseded1 and it will therefore be sufficient to describe them briefly4 ,most of the wave-paths1 as we have seen1 pass within three miles of a point almost coincident with the village of ,caggiano4 ,of the remainder1 six traverse a spot about two miles farther to the south-west1 and three cross another about two miles farther to the north-east4 ,neglecting other points of intersection1 but taking account of the observed emergences at ,vietri di ,potenza1 ,auletta1 ,polla1 etc41 ,mallet infers that the horizontal section of the focus was a curve 7indicated by the dotted line in ,fig4 #i7 not less than ten miles in length1 and passing from near ,balvano on the north1 close to ,vietri di ,potenza1 ,caggiano1 and ,pertosa1 to a point about two and a half miles west of ,polla4 ,again1 he remarks1 the observed emergences at places near the epicentre indicate that the vertical section of the seismic focus was either more or less curved1 or more probably a surface inclined towards the south-east4 ,he concludes1 therefore1 that the seismic focus was a curved fissure1 #aj miles long and #c-a_/b miles in height1 and with its centre at a depth of #f-a_/b miles below the level of the sea4 ,the production of this great fissure1 accompanied1 perhaps by the injection into it of steam at high pressure1 was regarded by ,mallet as the cause of the principal earthquake4 ,he imagines that the rent would start at or near the central point of the focus and then extend rapidly outwards in all directions4 ,in the initial stage1 vibrations of very small amplitude would alone be transmitted1 and these would give rise to the early sounds and tremors4 ,as the rending proceeded1 the vibrations would increase in strength up to a certain point when they produced the shock itself4 ,after this1 they would decrease2 and1 in the final stage1 would give place to the small vibrations corresponding to the sounds and tremors that marked the close of the earthquake4 ,the rush of steam at high pressure into the focus ,mallet does not seem to have considered essential1 though he evidently regarded it as possible1 indeed probable2 and he suggests that it may have been in part the cause of the earthquake which occurred an hour later4 ,though feeling sceptical as to the existence of any general law of increase of underground temperature1 he assumes it1 for the sake of illustration1 to be #a ,f4 for every #fj feet of descent4 ,this would give a temperature of #cci ,f4 at the upper limit of the focus1 #fdc ,f4 at its central point1 and #hhd ,f4 at its lower margin4 ,if the focus were filled with steam at each of these temperatures1 the corresponding pressures on its walls would be #h1 #adi1 and #fhd atmospheres1 respectively4 ,as the steam may be supposed to be admitted suddenly and to be unlimited in supply1 ,mallet infers that it might exist at the tension due to the highest of these temperatures1 in which case it would be capable of lifting a column of limestone #h1eej feet in height 7or about one-half the depth of the upper margin of the focus71 and would exert a pressure on the walls of the focus of #d4eh tons per square inch1 or of more than #fdj1ebh millions of tons upon its whole surface4 ,so many pages have already been given to this interesting earthquake that ,i must sketch still more briefly my own view as to its origin4 ,there were1 ,i believe1 two distinct foci with their centres about twenty-four miles apart along a north-west and south-east line1 and it was to this arrangement that the elongation of the meizoseismal area was chiefly1 though not entirely1 due4 ,the evidence is insufficient to determine whether the earthquake was caused by fault-slipping2 it is in no way opposed to this view1 but if the ,neapolitan earthquake stood alone1 we should hardly be justified in drawing any further inference4 ,relying1 however1 on knowledge obtained from the study of more recent shocks1 it seems to me probable that the two foci formed parts of one fault with a general north-west and south-east direction4 ,the slip causing the first part of the double shock apparently took place within the south-east focus1 and was followed after a few seconds by one within the north-west focus1 greater in amount as well as more deeply seated4 ,in consequence of these displacements there were local increases of stress1 causing numerous small slips within or near both principal foci2 and1 if we may judge from some slight shocks felt at ,la ,sala1 accompanied also by other minor slips in the intermediate region of the fault4 ,,reference4 ,,mallet1 ,r4--,the ,great ,neapolitan ,earthquake of #aheg3 ,the ,first ,principles of ,observational ,seismology1 etc4 #b vols #ahfb4 ,,footnotes3 #c ,irish ,acad4 ,trans41 vol4 xxi41 #ahdh1 pp4 #ea-aje 7read ,feb4 #i1 #ahdf74 #d ,brit4 ,assoc4 ,reports1 #ahej1 pp4 #a-hg2 #ahea1 pp4 #bgb-ccj2 #aheb1 pp4 #a-agf2 #ahec1 pp4 #aag-bab2 #ahed1 pp4 #a-cbf2 #aheh1 pp4 #a-acf4 #e ,a ,manual of ,scientific ,enquiry1 edited by ,sir ,j4,f4,w4 ,herschel1 #ahdi1 pp4 #aif-bbc4 #f ,irish ,acad4 ,trans41 vol4 xxii41 #ahee1 pp4 #cig-daj4 #g ,the linear dimensions of the isoseismal lines are obtained by measurements from ,mallet's maps4 ,the areas are given by him in geographical square miles4 #h ,mallet1 by some accident1 omitted the losses at ,polla and neighbouring towns from this estimate4 ,mercalli 7,geologia d',italia1 pte4 #c1 p4 #cbd7 gives the number of killed as more than #ab1cjj4 #i ,mallet does not make use of the term epicentre2 he speaks of the line ,,fe as the seismic vertical4 ,the modern and accepted terms are used above #aj ,japan ,seismol4 ,soc4 ,trans41 vol4 xi41 #ahhg1 pp4 #age-agg4 #aa ,ital4 ,seismol4 ,soc4 ,boll41 vol4 ii41 #ahif1 pp4 #ahj-ahh4 #ab ,professor ,omori gives the mean direction as ,s4 #ga ,w41 but this was obtained from observation on lamps with square1 as well as with circular bases4 #ac ,twelve measurements chosen at random from ,professor ,omori's list gave a mean direction of ,s4 #gh ,w4 #ad ,when the accuracy of all the observations seemed equally probable1 he adopted the mean of the two extremes as the true direction4 #ae ,if a be the amplitude of a simple harmonic vibration1 ,t its complete period1 v its maximum velocity1 and f its maximum acceleration1 we have v#b99pi99a_/,t and f#d99pi#b99;a_/,t#b #af ,earthquakes and other ,earth ,movements1 pp4 #ha-hb4 #ag ,obtained from the formula3 ,t#b99pi99a_/v#b99pi99x997#a_/c7_/ab #ah ,if we take the maximum velocity to be #ab feet per second1 and the period to be one second1 the amplitude would be about #aa-a_/b inches4 #ai ,vol4 ii41 p4 #bii4 ,the punctuation of the original is not followed in the above extract4 #bj ,british ,association ,report1 #ahea1 pp4 #bgb-cbj4 ,,chapter ,,iii4 ,,the ,,ischian ,,earthquakes ,,of ,,march #d,,th1 #ahha1 ,,and ,,july #bh,,th1 #ahhc4 ,separated from ,italy by a distance of not more than six miles1 ,ischia and the intermediate island of ,procida strictly form part of the ,phlegran ,fields1 the well-known volcanic district to the north of ,naples4 ,ischia1 the larger of the two islands1 is six miles long from east to west1 and five miles from north to south1 and contains an area of twenty-six square miles4 ,in #ahha1 the total population was #bb1agj1 that of ,casamicciola1 the largest town1 being #c1ifc4 ,,volcanic ,,history ,,of ,,ischia4 ,the central feature of ,ischia is the great crater of ,epomeo 7a1 ,fig4 #ad74 ,on the south side1 and partly also on the east1 the crater-wall has been broken down and removed2 the portion remaining is about #a-a_/b mile in diameter from east to west1 and reaches a height of #b1fjj feet above the sea-level4 ,all the upper part of the mountain is composed of a pumiceous tufa1 rich in sanidine and of a characteristic greenish colour4 ,at two points1 to the west near ,forio and to the north between ,lacco and ,casamicciola1 this tufa is seen reaching down to the sea2 but1 in all other parts1 it is covered by streams of trachitic lava1 by more recent tufas1 or by a deposit of marly appearance1 which is regarded by ,fuchs as resulting from the decomposition of the ,epomean tufa4 ,there are two distinct periods in the geological history of ,ischia4 ,the first1 a submarine period1 probably began with the dawn of the quaternary epoch1 for all the marine fossils of the island belong to existing species4 ,about this time1 ,epomeo seems to have originated in eruptions occurring in a sea at least #a1gjj feet in depth--eruptions that preceded the formation of ,monte ,somma and were either contemporaneous or alternating with those that gave rise to the oldest trachitic tufas of the ,phlegran ,fields4 ,the destruction of the south wall may have occurred much later through some great eruptive paroxysm1 but more probably1 as ,professor ,mercalli suggests1 through early marine erosion and subsequent subaerial denudation4 ,to the submarine period must also be assigned the formation of the trachitic masses which compose ,monti ,trippiti1 ,vetta1 and ,garofoli 7b1 c1 d1 ,fig4 #ad71 on the east side of ,epomeo2 and1 in part only1 those of ,monte ,campagnano and ,monte ,vezza 7f1 g74 ,at or near the close of the elevation1 many violent eruptions occurred on the south-west of ,epomeo1 during which was formed the south-west corner of the island1 including ,monte ,imperatore and ,capo ,sant' ,angelo 7h1 i74 ,in the second or terrestrial period1 when the island had practically attained its present altitude1 the eruptive activity was almost confined to the eastern and northern flanks of ,epomeo4 ,at the beginning ,monte ,lo ,toppo 7j7 was formed by a lateral eruption4 ,in the north-west corner of the island1 ,monte ,marecocco and ,monte ,zale 7k and l7 owe their origin to a gigantic flow of sanidinic trachite1 issuing probably from the depression which now exists between them4 ,lastly1 towards the north-east1 are the recent lateral craters of ,rotaro1 ,montagnone1 ,bagno1 and ,cremate 7m1 n1 p1 s71 the first two being the most regular and best preserved in the island4 ,illustration3 ,,fig4 #ad4--;,geological sketch-map of ,ischia4 7,mercalli47#ba ,the earliest eruption of the historic1 or rather human1 period appears to have taken place from ,montagnone1 and probably also at about the same time from the secondary crater of ,porto d',ischia 7u71 about the beginning of the eleventh century ,b4,c4 ,the eruptions of ,marecocco and ,zale are referred to about ,b4,c4 #dgj2 and those of ,rotaro and ,tabor 7q7 to between the years #djj and #ceb ,b4,c4 ,another eruption is said to have occurred in ,b4,c4 #hi1 but the site of it is unknown2 and three others are recorded on doubtful authority about the years ,a4,d4 #gi-ha1 #ach-afa1 and #bhd-cje4 ,the last outburst of all took place after the series of earthquakes in #acjb from a new crater1 that of ,cremate 7s71 which opened on the north-east flank of ,epomeo1 and from which a stream of lava1 called the ,arso 7t71 flowed down rapidly and1 after a course of two miles1 reached the sea4 ,after the first eruptions to which it owed its origin1 the central crater of ,epomeo apparently remained inactive4 ,all the later eruptions occurred either on the external flanks of the mountain or on radial fractures of the cone4#bb ,trippiti1 ,lo ,toppo1 ,montagnone and the ,lago del ,bagno 7b1 j1 n1 p7 lie in one line1 ,vetta and ,cremate 7c1 s7 on another1 and ,garofoli and ,vatoliere 7d1 e7 on a third1 all passing through a point near the town of ,fontana1 which occupies the centre of the old crater of ,epomeo4 ,professor ,mercalli points out that the lateral eruptions of ,epomeo differ in one respect from those of ,etna and ,vesuvius4 ,in these volcanoes the lava ascends to a considerable height in the central chimney1 and by its own weight rends open the flanks of the cone4 ,in ,epomeo1 it appears to traverse lateral passages at some depth1 perhaps far below the level of the sea1 and to rend the mountain by means of the elastic force of the aqueous vapour1 etc41 which it contains4 ,it will be seen how important is the bearing of this difference on the occurrence of the ,ischian earthquakes4 ,the eruptions that have taken place during the last three thousand years agree in several particulars4 ,they either occurred suddenly1 or1 at any rate1 were not preceded by a stage of moderate ,strombolian activity2 they were always accompanied by violent earthquakes2 and all succeeded intervals of long repose4 ,as the eruption of #acjb happened after at least a thousand years of rest1 the lapse of six more centuries does not justify us in concluding that ,epomeo is at last extinct4 ,we seem1 on the contrary1 to be drawing near another epoch of activity4 ,during the four and a half centuries that followed the eruption of #acjb1 we have no record of ,ischian earthquakes4#bc ,then1 suddenly1 on the night of ,july #bh-bi1 #agfb1 ,casamicciola was visited by sixty-two shocks1 some of which were very strong and damaged buildings4 ,on ,march #ahth1 #agif1 another severe shock took place1 but destructive only in the neighbourhood of ,casamicciola1 where seven persons were killed4 ,on ,february #bnd1 #ahbh1 the area of damage1 though concentric with the former1 enlarged its boundaries2 #cj persons were killed and #ej wounded4 ,on ,march #fth1 #ahda1 and during the night of ,august #ae-af1 #ahfg1 further shocks injured houses at ,casamicciola1 but without causing any loss of life4 ,slight tremors occurred at various dates in #ahgd1 #ahge1 #ahgi1 and #ahhj1 leading up to the disastrous earthquakes here described1 those of ,march #dth1 #ahha1 when #abg persons were killed1 and ,july #bhth1 #ahhc1 which resulted in the death of #b1cac persons and the wounding of many others4 ,,earthquake ,,of ,,march #d,,th1 #ahha4 ,the ,ischian earthquakes have been fortunate in their investigators4 ,in the spring of #ahha1 ,dr4 ,h4,j4 ,johnston-,lavis1 the chronicler for many years of ,vesuvian phenomena1 was residing in ,naples4 ,impressed by a recent perusal of ,mallet's report on the ,neapolitan earthquake1 and wishing to test the value of the methods explained in the last chapter1 he crossed over to ,ischia on ,march #eth2 and to his unwearied inquiries extending over more than three weeks and lasting from thirteen to sixteen hours a day1 we are indebted for most of what we know about the earthquake of #ahha4 ,on ,march #dth1 at #a4e ,p4,m41 the great shock occurred abruptly1 without any warning tremors4 ,its effects were aggravated by the faulty construction of the houses4 ,the walls are of great thickness1 loosely put together1 and connected by mortar of the poorest quality4 ,the chimneys and roofs also are massive1 and the rafters are so slightly inserted in the walls that they were drawn out with the rocking of the houses4 ,in such cases1 the destruction was often so complete that no fissures were left available for measurement4 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,the isoseismal lines as drawn by ,dr4 ,johnston-,lavis are represented by the curves in ,fig4 #ae4 ,the isoseismal marked #a bounds the area of complete destruction2 it is about #a mile long from east to west1 #b_/c of a mile broad1 and contains an area of not more than half a square mile4 ,the next isoseismal 7#b7 marks the area of partial1 but still serious1 destruction2 this is nearly #b miles long from east to west1 #a-a_/d miles broad1 and #b square miles in area4 ,within the isoseismal #c1 buildings were more or less slightly damaged4 ,the course of this curve is somewhat doubtful1 but1 as drawn1 it is about #c miles long1 #b miles wide1 and #e square miles in area4 ,illustration3 ,,fig4 #ae4--;,isoseismal lines of the ,ischian earthquake of #ahha4 7,johnston-,lavis47 ,outside the last curve1 the shock diminished rapidly in intensity4 ,at ,monte ,tabor and ,bagno1 it was very slight2 in the town of ,ischia1 only about half the people were conscious of any movement2 and at ,capella1 a small village to the south1 it was not felt at all4 ,again1 the shock was perceptible1 though only faintly1 in the neighbourhood of ,campagnano1 at ,serrara to the south of ,epomeo1 and at ,panza near the south-west corner of the island4 ,on the other hand1 at ,fontana1 which occupies approximately the centre of the crater of ,epomeo1 there were evidences of a distinctly stronger shock4 ,no house actually fell1 and side walls were but little injured2 but the roofs1 which are of great weight1 suffered considerable injury4 ,in the adjacent island of ,procida1 the shock was felt distinctly by many people1 and by some1 though slightly1 at ,monte di ,procida1 ,misenum1 and ,bacoli1 on the coast of ,italy4 ,no record whatever was given by the seismographs in the university of ,naples and the observatory on ,vesuvius4 ,we have of course no means of estimating the exact size of the disturbed area1 but in this respect1 disastrous as the earthquake was in the neighbourhood of ,casamicciola1 it was clearly inferior to all but the very weakest earthquakes felt in the ,british ,islands4 ,,position ,,of ,,the ,,epicentre4 ,in determining the position of the epicentre1 ,mallet's method was closely followed4 ,fissures in buildings were used for the most part1 in two out of every three cases2 and occasional measurements were made from objects overthrown1 projected1 or shifted1 and also from the personal experiences of observers4 ,the attempt to apply the method was1 however1 fraught with difficulties4 ,the heterogeneous structure of the island was no doubt responsible for many divergent azimuths2 the irregularity of the buildings both in form and material and their variety of site furnished other sources of error2 even the smallness of the area was a disadvantage in lessening the number of trustworthy records4 ,measurements were made at #ee places altogether1 but in most cases they were the results of isolated observations1 not the means of several at each place4 ,on this account1 ,i have not reproduced in ,fig4 #ae the azimuths shown in ,dr4 ,johnston-,lavis's map of the earthquake4 ,a large number of them clearly converge towards an area lying to the west of ,casamicciola2 and1 from their arrangement1 ,dr4 ,johnston-,lavis concludes1 though the evidence does not seem to me quite strong enough for the purpose1 that they emanated from a fracture running from a little west of north to a little east of south4 ,this conclusion is1 however1 justified by other evidence4 ,in the centre of the injured district1 ,dr4 ,johnston-,lavis has traced a meizoseismal band1 in which the shock must have been nearly or quite vertical4 ,the damage inflicted on buildings included within this band was1 he says1 very characteristic of the nature of the shock2 the walls having received but slight injury1 whilst almost every floor and ceiling had been totally destroyed4 ,in fact1 he adds1 many houses would have required no other repairs than the replacing of the divisions between the different storeys4 ,the shaded central area in ,fig4 #ae represents this band1 passing in a nearly north and south direction from a point midway between ,campo and the upper part of ,lacco on the north1 through the west part of ,casamenella and ,campo1 to a point near ,frasso on the south2 the length of the band being thus about two-thirds of a mile4 ,if the central line of this band is produced towards the south1 as indicated by the dotted line1 it grazes the west side of ,fontana1 where1 as we have seen1 there was a second meizoseismal area1 much smaller than the other and surrounded by a district in which houses were almost uninjured4 ,that the shock in this town was vertical or nearly so1 is shown by the nature of the damage 7p4 #eb7 and also by the testimony of the inhabitants4 ,i will give ,dr4 ,johnston-,lavis's explanation of this detached meizoseismal area when discussing the origin of the ,ischian earthquakes2 but the evidence seems to me to favour either the existence of two distinct foci or1 more probably perhaps1 the extension of the fissure to the south with an increased impulse beneath the centre of ,epomeo4 ,,depth ,,of ,,the ,,seismic ,,focus4 ,at nine places1 ,dr4 ,johnston-,lavis was able to make measurements of the angle of emergence1 in every case from fissures in buildings1 and therefore liable to sources of error already referred to4 ,on the other hand1 owing to the small depth of the focus1 there would probably be less general refraction of the wave-paths than in the ,neapolitan earthquake4 ,the depths indicated by these observations vary between about #fae and #b1hhe feet1 a difference that is no greater than might be expected1 as the size of the focus was no doubt comparable with that of the district in which observations were made4 ,the mean depth ,dr4 ,johnston-,lavis finds to be about #a1gjj feet1 or a little less than one-third of a mile4 ,,nature ,,of ,,the ,,shock4 ,the limited depth of the focus is also evident from the nature of the shock4 ,it was only within the actual meizoseismal band that the shock was subsultory or vertical throughout2 at a short distance from the epicentre1 the movement was both subsultory and undulatory2 while near the third isoseismal1 and in most of the region outside1 the movement was entirely undulatory or lateral4 ,an observer at ,perrone 7which lies #a-b_/c miles east of the epicentre7 gives the following account of the shock3--,i was standing on my balcony 7this faces ,casamicciola7 admiring the scene 444 when ,i felt the house rock1 feeling at the same time as if something was rolling along beneath the ground4 ,this movement was accompanied by a sound like this1 ,boob1 boob -- boob -- -- boob -- -- -- boob -- -- -- -- boob4 ,both noise and movement seemed to come from ,casamicciola4444 ,in a few seconds1 in the distance over the town arose a terrific cloud of white dust1 so that ,i imagined the town on fire4444 ,i felt hardly any1 if any1 subsultory movement1 but as ,i leant upon the balcony rails1 ,i was alternately pressed against them and then drawn away4 ,at ,fontana1 however1 the undulatory shock was replaced by a vertical one4 ,this was the universal experience1 though one or two persons felt a slight lateral movement immediately after4 ,at ,valle 7near ,barano7 and ,piejo1 both places about a mile from ,fontana1 the vertical component was also perceptible4 ,,after-,,shocks4 ,the after-shocks were few and of slight intensity4 ,dr4 ,johnston-,lavis gives the following dates3 ,march #gth1 #ab4e ,a4,m4 and midday2 ,march #aa-ab1 #ae-af1 #ag-ah1 #bg 7871 ,april #eth and #fth1 and ,july #ahth1 #h4cj ,p4,m4 ,the only shock of the series marked as strong occurred at midnight on ,march #ae-af at ,casamicciola4 ,the last of all1 that of ,july #ahth1 consisted of a rumble and slight shock1 and was most perceptible at ,fango4 ,,earthquake ,,of ,,july #bh,,th1 #ahhc4 ,undeterred by the experience of #ahha or by the warnings of seismologists1 ,casamicciola was rebuilt1 only to suffer more complete disaster4 ,on ,july #bhth1 #ahhc1 at #i4be ,p4,m41 occurred the most destructive earthquake of which we have any record in ,ischia4 ,the shock lasted about fifteen seconds1 and before it was over clouds of dust were rising above the ruins of ,casamicciola1 ,lacco1 and ,forio2 #a1bjj houses were destroyed1 #b1cac persons were killed1 nearly #a1hjj in ,casamicciola alone1 and more than #hjj seriously wounded4 ,no better idea1 says ,dr4 ,johnston-,lavis1 of the absolute destruction of buildings could be conceived than what was actually realised at ,casamicciola and ,campo4 ,looking1 on the following ,monday1 over the field of destruction1 ,i could discover 7with few exceptions7 the wall-stumps only remaining4 ,dr4 ,johnston-,lavis again spent about three weeks in the island1 examining the effects of the new shock with equal zeal and wider experience4 ,his monograph is now our chief work of reference on ,ischian earthquakes4 ,inquiries were also made by several ,italian seismologists1 among others by ,professor ,m4,s4 de ,rossi1 the organiser of earthquake-studies in the peninsula2 by ,professor ,l4 ,palmieri1 the founder of the ,vesuvian observatory2 and especially by ,professor ,g4 ,mercalli1 whose valuable memoir supplements the report of ,dr4 ,johnston-,lavis in some important particulars4 ,,preparatory ,,signs4 ,the interval between ,july #ahth1 #ahha1 when the last shock of that year was felt1 and ,july #bhth1 #ahhc1 was one of almost complete quiescence4 ,early in ,march #ahhb1 a few slight shocks were noticed at ,casamicciola4 ,on ,july #bdth1 #ahhc1 a watch hanging from a nail in a wall was seen to swing at #f ,a4,m4 and #i ,a4,m41 and1 on the same morning1 at about #h4cj1 a slight shock1 accompanied by a rumbling sound1 was felt at ,casamicciola4 ,again1 on the #bhth1 about a quarter of an hour before the great shock1 one observer at ,casamicciola states that an underground noise was heard1 and that some persons in consequence left their houses4 ,many assertions have been made with regard to variations witnessed a day or two before the shock in the hot springs1 such as an increase of flow or temperature and changes in their volume and purity4 ,fumaroles are alleged to have burst out with violence1 and even flames to have been seen4 ,the statements1 though widely quoted1 can hardly be said to rest on satisfactory evidence4 ,on the other hand1 ,dr4 ,johnston-,lavis arrived in the island within twenty-four hours after the shock1 and1 before another day had elapsed1 he had examined most of the places where the phenomena were said to have occurred1 but could find no remarkable change nor any signs of such having taken place4 ,it is also known1 as he remarks1 that the temperature of the ,ischian springs and fumaroles sometimes varies considerably without any earthquake following1 that of the water of ,gurgitello occasionally changing by as much as #cj or #dj4 ,we may therefore1 ,i think1 conclude that1 except for one or two shocks and underground noises too slight to cause general alarm1 there were no decisive heralds of the great earthquake4 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,the curves in ,fig4 #af represent the isoseismal lines as drawn by ,dr4 ,johnston-,lavis4 ,as in the earthquake of #ahha1 they bound respectively the areas of complete destruction1 partial destruction and slight damage to buildings1 the course of the outer line being to a great extent conjectural owing to the small extent of land traversed by it4 ,the first isoseismal is about #b-a_/b miles long1 #a-a_/b miles broad1 and #c square miles in area2 the second about #d miles long1 #c-a_/b miles broad1 and #aa square miles in area2 and the third about #f-a_/b miles long1 #f miles broad1 and #cj square miles in area4 ,the curve drawn by ,professor ,mercalli 7,fig4 #ad7 coincides nearly with the second of these lines4 ,at ,fontana1 the damage exceeded that in the surrounding country1 though the difference was of course less marked than on the previous occasion4 ,illustration3 ,,fig4 #af4--;,isoseismal lines of the ,ischian earthquake of #ahhc4 ,outside ,ischia1 the shock was felt distinctly in all the island of ,procida and in ,vivara2 on the mainland1 by some as far as ,pozzuoli and by several persons in ,naples1 which is twenty miles from ,casamicciola4 ,the seismograph at the university of this city registered two small shocks1 the first at #i4aj ,p4,m41 and the second and stronger at #i4be ,p4,m42 and ,de ,rossi states that at about #i4cj ,p4,m4 the seismographs at ,ceccano1 ,velletri1 and ,rome recorded a shock consisting of very slow undulations4 ,there are again no materials for estimating the size of the disturbed area1 but there can be no doubt that it was much less than that of a moderately strong ,british earthquake4 ,,position ,,of ,,the ,,epicentre4 ,owing to the limited size of the disturbed area1 time-observations1 even had they been available1 would not have sufficed to determine the position of the epicentre1 and both ,dr4 ,johnston-,lavis and ,professor ,mercalli therefore had recourse to ,mallet's method1 the former relying chiefly1 as before1 on fissures in damaged buildings1 and the latter on the overthrow or displacement of columns and other objects4 ,dr4 ,johnston-,lavis measured the azimuth of the wave-paths at sixty-five places1 and at about one-third of these was able to make two or more observations4 ,the azimuths converge towards the same region as in #ahha1 but the area covered by their intersections is larger4 ,the meizoseismal band of maximum vertical destruction indicated by shading in ,fig4 #af is also of the same form and slightly greater extent1 reaching from the upper part of ,lacco to a little south of ,frasso1 and being therefore nearly a mile in length4 ,the centre of maximum impulse was in the same position as in #ahha1 or possibly a little more to the south4 ,professor ,mercalli's observations were made at forty-eight places1 and in only six cases were they the same as those used by his predecessor4 ,he also notices that most of the azimuths converge towards ,casamenella1 and intersect within an elongated area4 ,this area runs in the same direction as ,dr4 ,johnston-,lavis's meizoseismal band1 but is less elongated1 and situated a short distance farther to the south1 though on the whole the agreement between the two areas is remarkably close4 ,there was again apparently a second epicentre at ,fontana4 ,in this town1 according to ,dr4 ,johnston-,lavis1 there were two distinct types of damage4 ,as in #ahha1 there was evidence of a vertical blow1 the only one that absolutely ruined houses2 but1 in addition1 there was another independent set of fissures1 quite as widely distributed as the others1 though evidently caused by a less violent movement4 ,these indicated a wave-path with a low angle of emergence coming from between north and north-north-west1 or almost exactly in the line of meizoseismal band4 ,to the south of ,fontana1 however1 there is a group of places1 including ,panza1 ,serrara1 ,barano1 etc41 where the azimuths diverged rather widely from the epicentre at ,casamenella4 ,these azimuths are twelve in number1 and it is worthy of notice that they all intersected the crater of ,epomeo1 while half of them passed within a few hundred yards of ,fontana4 ,,depth ,,of ,,the ,,seismic ,,focus4 ,measurements of the angle of emergence were made by ,dr4 ,johnston-,lavis at twenty-four places1 and in every case from fissured walls4 ,the greater part of the diagram on which his results are depicted is reproduced in ,fig4 #ag4 ,the horizontal line1 as in ,fig4 #ac1 represents the level of the sea1 the longer vertical line one passing through the epicentre1 and the shorter another through ,fontana4 ,the short lines on the left of the former show the incipient wave-paths to places lying east of the epicentre2 those on the right1 with one exception1 represent the wave-paths to places west of the same meridian4 ,small horizontal marks are inserted on the vertical lines to show the depth in tenths of a mile below the level of the sea4 ,illustration3 ,,fig4 #ag4--;,diagram of wave-paths at seismic vertical of ,ischian earthquake of #ahhc4 7,johnston-,lavis47 ,the six angles of emergence that would give the greatest depth below the epicentre were all measured at places in the south of the island close to the line joining ,panza and ,barano1 and it will be noticed that five of these apparent depths are much greater than those obtained from the other wave-paths4 ,excluding these observations1 the remaining eighteen give depths ranging from about #dej to about #c1cej feet1 and a mean depth of #a1gcj feet1#bd or nearly one-third of a mile1 that is1 almost exactly the same as the mean depth found from the earthquake of #ahha4 ,the six exceptional angles of emergence come from the district of divergent azimuths to the south of ,epomeo4 ,three of the corresponding azimuths pass within one-quarter of a mile from the centre of ,fontana1 and none of the other three more than three-quarters of a mile from the same point4 ,though disbelieving in a subsidiary focus below this town1 ,dr4 ,johnston-,lavis has calculated its mean depth1 supposing it to exist1 and found it to be about #a1efj feet below the sea level1 a result which is remarkably close to the calculated mean depth of the focus near ,casamenella4 ,,nature ,,of ,,the ,,shock4 ,in the meizoseismal band1 preliminary tremor and rumbling sound were alike absent4 ,so sudden1 indeed1 was the onset of the earthquake1 that the survivors generally found themselves beneath the ruins of their houses before they were conscious of any shock4 ,the destruction1 practically instantaneous1 was wrought by four or five vertical blows1 so powerful that1 according to some observers1 ,casamicciola seemed to jump into the air4 ,then followed undulations1 not noticed by all1 that appeared to come from every direction4 ,the shock lasted altogether fifteen seconds or more1#be and was accompanied by a rumbling noise1 in the midst of which were detonations as of thunder or of great blows given upon an empty barrel4 ,in the immediate neighbourhood of the meizoseismal area1 at ,perrone1 ,pennella1 and ,lower ,lacco1 the subsultory movement was still the more prominent2 but1 farther away1 as at ,panza1 ,testacchio1 ,barano1 ,ischia1 and ,bagno1 the subsultory motion was followed by distinctly horizontal undulations1 while outside the island of ,ischia only slow undulatory movements were perceptible4 ,,landslips4 ,the dotted areas in ,fig4 #af indicate the sites of the only landslips of importance that were precipitated by the earthquake of #ahhc4 ,two of these occurred on the north slope of ,epomeo1 and the third on the west flank of ,monte ,rotaro4 ,the materials of the ,epomean landslips had evidently been separated for some time by shallow fissures from the adjoining rock1 for the surfaces of the fissures were discoloured by fumarolic action4 ,immediately after the earthquake a cloud of dust was seen to rise from the spots2 the masses1 already detached laterally1 were merely set in motion by the shock2 and they continued to slide down during the following days either through the action of the after-shocks or of the heavy rains that followed4 ,all over the island1 however1 fissures and minor landslips occurred4 ,at two places on the north coast the steep cliffs of incoherent tufa were so much damaged that1 according to ,dr4 ,johnston-,lavis1 large quantities of their materials were thrown into the sea4 ,the water then sorted out the pieces of pumice1 which in many cases were of very large size1 and were seen floating about in the neighbourhood for some days1 giving rise to the supposition that a submarine eruption had taken place to the north of the island4 ,,after-,,shocks4 ,the after-shocks in #ahhc were much more numerous than in #ahha4 ,between #i4be ,p4,m4 on ,july #bhth and noon on ,august #crd1 twenty-one slight shocks were recorded at ,casamicciola4 ,at #b4ae ,p4,m4 on ,august #crd1 a violent shock occurred that caused further damage at ,forio1 and even at places so far from the epicentre as ,fiaiano1 ,barano1 and ,fontana1 and increased the displacements of the landslips on ,epomeo4 ,this shock was also registered at the observatory on ,vesuvius4 ,after this the shocks became less frequent and slighter1 twelve being felt at ,casamicciola during the remainder of the year1 and six in the first half of #ahhd4 ,several shocks and rumbling noises were also observed in other parts of the island4 ,among them may be mentioned noises heard at ,fontana on ,august #abth and #aeth1 and a slight shock at the same place on ,august #agth2 also on ,september #dth1 at #aj4cj and #aj4dj ,a4,m41 slight shocks at ,barano1 ,serrara1 and ,forio4 ,on ,march #bgth1 #ahhd1 at #b4g ,p4,m41 another strong shock occurred2 strongest at ,serrara1 where the shock was subsultory and accompanied by noise2 and less strong1 though still subsultory1 at ,ciglio1 ,panza1 ,forio1 ,fiaiano1 and ,casamicciola1 and very slight at ,ischia4 ,the series seems to have ended during the following summer1 with a slight shock at ,casamicciola on ,july #bast1 and a stronger one on ,july #bcrd1 felt from ,casamicciola on the north to ,serrara on the south4 ,most of the after-shocks must have originated in the neighbourhood of ,casamicciola1 but it is worthy of notice that more than one centre was in action4 ,several were recorded at ,ischia only4 ,others1 as mentioned above1 affected chiefly the south part of the island1 and especially the small towns of ,serrara and ,fontana4 ,,characteristics ,,of ,,ischian ,,earthquakes4 ,after the eruption of #acjb1 there succeeded a period of comparative repose in ,ischia4 ,the revival of activity dates from #agfb1 and1 since that year1 there have been four great earthquakes1 namely1 those of #agif1 #ahbh1 #ahha1 and #ahhc4 ,in every respect but that of increasing intensity1 these earthquakes were apparently identical2 each1 as ,professor ,mercalli says1 was merely a replica on a different scale of those that preceded it4 ,the principal features in which they resemble one another1 and differ from the average tectonic earthquake1 are the coincidence of the epicentres1 the small depth of the foci1 and the sudden onset of the principal shock4 #a4 ,coincidence of ,epicentres4--,in ,fig4 #ad1 which is copied from ,professor ,mercalli's map1 are shown the areas in which buildings were seriously damaged by these four earthquakes4 ,the curves for #agif1 #ahbh1 and #ahha are approximately concentric4 ,in #agif1 the shock was disastrous only to the west of ,casamicciola2 in #ahbh1 according to ,covelli1 the ground most injured was not precisely the region of ,casamicciola1 but that which lies between the district called ,fango and that known as ,casamenella1 situated to the west of ,casamicciola1 and a short distance from it4#bf ,the epicentres may have varied slightly in size1 but1 in position1 it is clear that all four were nearly or quite coincident4 ,the meizoseismal bands in #ahha and #ahhc were also similar in form and elongated in the same direction4 ,in the last two earthquakes there was1 as we have seen1 very distinct evidence of a secondary meizoseismal area surrounding ,fontana1 and it is remarkable that this was also noticeable in the earthquake of #ahbh4 ,besides the centre of vibration in the district of ,fango1 says ,covelli1 another less powerful centre showed itself in the locality of ,fontana2 this made itself felt more heavily than in surrounding localities2 as if another centre of movement had taken place from that part1 independent of the former4 #b4 ,small ,depth of the ,foci4--,mallet's method1 as noted above1 cannot be trusted to yield accurate estimates of the focal depth1 or to indicate more than its order of magnitude4 ,but it is remarkable that the depths calculated by ,dr4 ,johnston-,lavis for the last two earthquakes are both only a little less than a third of a mile1 and it is probable that the actual depth did not differ very greatly from this amount4 ,the nature of the shock1 vertical or nearly so close to the epicentre and horizontal at a short distance from it1 is merely personal testimony of the same character as fissures in masonry1 and of course points to the same result4 ,illustration3 ,,fig4 #ah4--;,diagram showing connection between depth of focus and rate of decline in intensity4 ,but the most conclusive evidence on which we have to rely is the extraordinary intensity of the shock at the centre of a very small distributed area4 ,in ,great ,britain1 an earthquake felt over a district of equal size would hardly at the centre exceed the trembling produced in a station platform by a passing train4 ,the curves in ,fig4 #ah show how the rate of decline in intensity depends on the depth of the focus4 ,they are drawn on the supposition that the intensity at any point on the surface varies inversely as the square of its distance from the focus2 the curves a1 b1 c corresponding to foci situated at depths of one-third of a mile1 one mile1 and two miles respectively1 and the figures below the horizontal line denoting the distance in miles from the epicentre4 ,thus1 the rapid decline of intensity from the epicentre outwards shows that1 in each of the four great ,ischian earthquakes1 the depths of the focus must have been very small4 #c4 ,suddenness of the ,shocks4--,in #agif1 we have no record of preparatory shocks1 but the evidence is scanty2 in #ahbh and #ahha1 none are mentioned2 in #ahhc1 one or two tremors and underground noises1 possibly of seismic origin1 gave warning to a few4 ,fore-shocks1 for all practical purposes1 were conspicuous by their absence4 ,still more remarkable is the sudden advent of the great shocks4 ,there were no preliminary tremors or rumbling sound1 no animals showed signs of uneasiness and no birds fluttered screaming from trees or ground4 ,the shock of #ahbh1 says ,covelli1 was announced by three powerful blows coming almost vertically1 from below upwards2 and the same words apply equally well to the earthquakes of #ahha and #ahhc4 ,the destruction of houses in every case was practically instantaneous1 and coincident with the first vibration4 ,in all respects1 tectonic earthquakes differ widely from the ,ischian shocks4 ,the epicentres of successive earthquakes are rarely coincident1 but show a distinct tendency to migration along certain lines2 the decline in intensity outwards from the epicentre is nearly always very gradual1 and therefore indicative of a comparatively deep-seated focus2 they are almost invariably preceded either by a series of slight shocks and rumbling sounds1 or1 in an unstable district1 by a marked increase in their frequency4 ,distinctions1 so great as these are1 evidently remove the ,ischian shocks from the category of tectonic earthquakes4 ,,origin ,,of ,,the ,,ischian ,,earthquakes4 ,on the other hand1 the ,ischian earthquakes possess several features which connect them closely with true volcanic earthquakes4 #a4 ,they originate beneath the northern slope of ,epomeo--a volcano that we have no reason to consider absolutely extinct1 but rather as one subject to eruptions at long intervals of time--in a region as yet unoccupied by parasitic craters1 but having the same relation to the central cone of ,epomeo as those in which the recent craters of ,monte ,rotaro1 ,montagnone and ,cremate are situated4 #b4 ,in both the earthquakes of #ahha and #ahhc1 the epicentre is an elongated band1 the axis of which1 if produced1 would pass through the centre of the old crater of ,epomeo4 ,along the line of this band1 occur the fumaroles of ,monte ,cito and ,ignazio ,verde and the thermal springs of the ,rita and ,capitello4 ,these facts1 as ,professor ,mercalli suggests1 lead us to believe that the foci of the earthquakes coincide with a radial fracture of the volcano1 the course of which1 as traced by him1 is represented by the continuous line in ,fig4 #ad4bg #c4 ,except in their relations with actual eruptions1 the ,ischian earthquakes resemble closely the true volcanic earthquakes which from time to time shake the flanks of ,etna4 ,these are marked by great intensity of the shock at the centre of a comparatively small disturbed area1 epicentres often elongated radially to the cone1 frequent repetition with similar characters in the same districts2 and as a rule they precede by a short interval1 but sometimes accompany or follow1 volcanic eruptions4#bh ,two other phenomena may be referred to as probably indicating some connection between ,ischian earthquakes and the structure and history of ,epomeo4 ,we have seen that1 in the three earthquakes of #ahbh1 #ahha1 and #ahhc1 there is distinct evidence of a second meizoseismal area at ,fontana1 within which the shock was mainly subsultory4 ,dr4 ,johnston-,lavis1 though recognising the possibility of the existence of two epicentres1 prefers another explanation4#bi ,but the wide extension of the southern boundary of the area of destruction in #ahhc1 and the limitation of several of the after-shocks to the south of the island1 seem to me to favour the existence of a second focus beneath the crater of ,epomeo1 though1 it may be1 not entirely detached from the chief focus beneath ,casamenella4 ,again1 as ,professor ,mercalli remarks1 all historic eruptions on the flanks of ,epomeo were accompanied by very violent earthquakes2 while1 previously to #acjb1 only one disastrous earthquake1 so far as known1 occurred in the island without being attended by an eruption4 ,it should be noticed also that the principal shocks during the recent revival of activity 7i4e41 since #agfb7 show a continual increase in intensity1 whether this be measured by the damage to buildings1 the loss of life1 or the extent of the area of destruction 7,fig4 #ad74 ,it therefore seems legitimate to conclude that1 in the recent ,ischian earthquakes1 we have merely so many unsuccessful attempts to force a new volcanic eruption4 ,the passages once existing through ,epomeo and its parasitic craters having become blocked1 the highly heated magma beneath is compelled to find a new outlet4 ,its tension slowly increasing1 the crust above is at last rent1 or an incipient rent is enlarged1 the fluid rock is injected almost instantaneously with great force into the open fissure1 and its sudden arrest by the containing walls is the ultimate cause of an earthquake4 ,with the expansion of the magma1 its tension is at once correspondingly reduced1 and some time must elapse before it can again reach the critical point at which a further rupture1 resulting in a second shock1 takes place4#cj ,thus1 with each great ,ischian earthquake1 we are1 ,i believe1 advancing a step nearer the time1 which may be close at hand or may be very remote1 when the fracture will at last reach the surface1 and above the site of ,casamenella a new parasitic cone will rise1 from which1 as from ,cremate in #acjb1 a stream of lava may flow down towards the sea4 ,,references4 #a4 ,,baldacci1 ,l4--,alcune osservazioni sul terremoto avvenuto all' ,isola d',ischia il #bh luglio #ahhc4 ,ital4 ,com4 ,geol4 ,boll41 vol4 xiv41 #ahhc1 pp4 #aeg-aff4 #b4 ,,daubr,e1 ,a4--,rapport sur le tremblement de terre ressenti ,ischia le #bh juillet1 #ahhc2 causes probables des tremblements de terre4 ,paris1 ,acad4 ,sci41 ,compt4 ,rend41 vol4 xcvii41 #ahhc1 pp4 #gfh-ggh4 #c4 ,,du ,,bois1 ,f4--,the ,earthquakes of ,ischia4 ,japan ,seism4 ,soc4 ,trans41 vol4 vii41 pt i41 #ahhc-hd1 pp4 #af-db4 #d4 ---- ,farther ,notes on the ,earthquakes of ,ischia4 ,ibid41 vol4 viii41 #ahhe1 pp4 #ie-ii4 #e4 ,,johnston-,,lavis1 ,h4,j4--,monograph of the ,earthquakes of ,ischia 7#ahhe74 #f4 ,,mercalli1 ,g4--,vulcani e fenomeni vulcanici in ,italia 7vol4 iii4 of ,geologia d',italia1 by ,g4 ,negri1 ,a4 ,stoppani1 and ,g4 ,mercalli71 #ahhc1 pp4 #df-ej1 #cca-ccb4 #g4 ---- ,l',isola d',ischia ed il terremoto del #bh luglio #ahhc 7,milano1 #ahhd74 #h4 ,,palmieri1 ,l41 ,e ,a4 ,,oglialoro4--,sul terremoto dell' ,isola d',ischia della sera del #bh luglio #ahhc4 ,napoli1 ,r4 ,accad4 ,atti1 vol4 i41 #ahhd1 pp4 #a-bh4 #i4 ,,rossi1 ,m4,s4 ,,de4--,il terremoto di ,casamicciola del #d marzo #ahha4 ,bull4 del ,vulc4 ,ital41 anno viii41 #ahha1 pp4 #e-ab4 7,in the same volume are brief notices by different writers on pp4 #bb1 #ch-db1 #eb-ec1 #fg-fh1 #gj-gd47 #aj4 ---- ,raccolta di fatti1 relazioni1 bibliografie sul terremoto di ,casamicciola del #bh luglio #ahhc1 con brevi osservazioni4 ,bull4 del ,vulc4 ,ital41 anno xi41 #ahhd1 pp4 #fe-agb4 #aa4 ---- ,intorno all' odierna fase dei terremoti in ,italia e segnatamente sul terremoto in ,casamicciola del #d ,marzo #ahha4 ,ital4 ,soc4 ,geogr4 ,boll41 #ahha4 #ab4 ,,serpieri1 ,a4--,sul terremoto d',ischia il #bh luglio #ahhc4 ,scritti di ,sismologia1 ,pte4 ii41 pp4 #bjg-baf4 #ac4 ---- ,sul terremoto dell' ,isola d',ischia il #bh luglio #ahhc4 ,ibid41 pp4 #bag-bcb4 ,,footnotes3 #ba ,the shaded areas indicate the principal trachytic masses1 the broken lines represent the boundaries of the craters that are still recognisable1 and the dotted lines the boundaries of the areas within which buildings were damaged by the earthquakes of #agif1 #ahbh1 #ahha1 and #ahhc 7according to ,mercalli74 ,the continuous curved line shows the position of the radial fracture with which the earthquakes were probably connected4 ,the trachytic masses and craters are denoted by the following tables3-- a4 ,epomeo4 b4 ,trippiti4 c4 ,vetta4 d4 ,garofoli4 e4 ,vatoliere4 f4 ,campagnano4 g4 ,vezza4 h4 ,imperatore4 i4 ,c4 ,st4 ,angelo4 j4 ,lo ,toppo4 k4 ,marecocco4 l4 ,zale4 m4 ,rotaro4 n4 ,montagnone4 p4 ,bagno4 q4 ,tabor4 r4 ,p4 ,castiglione4 s4 ,cremate4 t4 ,arso4 u4 ,porto d',ischia4 #bb ,it is possible that ,monte ,campagnano may form an exception to this statement4 #bc ,shocks were felt in the island in #aeei and #afei1 but one at least was of external origin4 #bd ,prof4 ,mercalli1 from the five estimates of the angle of emergence which he considered most reliable1 found the mean depth to be about #c1bhj feet4 #be ,professor de ,rossi estimated the mean duration as not much exceeding ten seconds4 ,dr4 ,johnston-,lavis1 on the other hand1 considers the general estimate of fifteen seconds as far too low4 ,in one case1 at ,casamicciola1 he ranks it as high as thirty-one seconds4 #bf ,quoted from the useful translation of ,covelli's memoir given by ,dr4 ,johnston-,lavis4 #bg ,baldacci supposes that the thermal springs and fumaroles of ,forio1 ,stennecchia1 ,montecito1 ,casamicciola1 and ,castiglione lie along a tangential fracture starting from ,forio and passing by ,casamicciola to near ,punta di ,castiglione4 ,mercalli1 however1 argues forcibly against this inference4 #bh ,professor ,mercalli adds1 as a fourth point of contact between ,ischian earthquakes and volcanic phenomena1 the changes in the fumaroles and hot springs which preceded or accompanied or followed the earthquakes of #ahbh1 #ahha1 and #ahhc4 #bi ,fontana1 he says1 occupies the centre of the great crater of ,epomeo4441 and therefore lies immediately over the ancient chimney1 which in all probability is filled by an old plug of consolidated trachyte1 which must descend to the igneous reservoir4 ,any mass of igneous matter1 that might determine the further rupture of a collateral fissure1 would result in the conduction of any changes of pressure or vibrations1 along the column of highly elastic trachyte2 whilst the same earth-waves would be annulled or absorbed by the inelastic tufas surrounding it1 so that the blow would be struck perpendicularly to the surface1 and in a small area with well defined limits4 ,the undulatory sensations1 after the principal local shock1 were those that arrived from the great centre of impulse beneath ,casamenella4 #cj ,the above paragraph is a summary of the reasoning stated with admirable clearness by ,dr4 ,johnston-,lavis4 ,it should be mentioned that the late ,professor ,palmieri1 relying on the extremely limited disturbed area1 dissented from this view2 but his difficulty is met by supposing the focus to be small as well as shallow1 a supposition that is supported by the shortness of the meizoseismal band1 as well as by the elongation of the isoseismal lines in the direction perpendicular to this band4 ,,chapter ,,iv4 ,,the ,,andalusian ,,earthquake ,,of ,,december #be,,th1 #ahhd4 ,in most countries the principal seismic districts are of limited extent4 ,thus1 in central ,japan1 the east coast is frequently visited by earthquakes1 while the west coast is relatively undisturbed4 ,of the earthquakes felt in the kingdom of ,greece during the years #ahic-ih1 #fc per cent4 were observed in ,zante1 and were for the most part confined to that island4 ,in the interior of the ,iberian peninsula--in ,leon and in ,new and ,old ,castile--destructive earthquakes are practically unknown2 while the littoral regions of central and southern ,portugal1 ,andalusia1 and ,catalonia are noted for their disastrous shocks4 ,during the eighteenth century seismic activity was chiefly concentrated in ,portugal1 and culminated in the great ,lisbon earthquake of #agee4 ,in the following century the seat of disturbance was transferred from the west to the south of the peninsula2 ,portugal remained throughout in comparative repose1 while ,almeria experienced destructive shocks in #ahjd1 #ahfj1 and #ahfc1 and ,murcia in #ahbh-bi and #ahfd1 leading up to the ,andalusian earthquakes of #ahhd-he1 described in the present chapter4 ,the preparation for the principal earthquake of ,december #beth1 #ahhd1 was unusually indistinct4 ,for a day or two before1 shocks were felt here and there in ,andalusia1 but so weak were they that they passed almost unperceived4 ,during the night of ,december #bd-be1 one slight shock was noticed at ,colmear 7,fig4 #ai7 and another at ,zafarraya4 ,on the #beth1 a faint movement of the ground was noticed at ,malaga1 and a few weak tremors at ,periana2 and shortly after came the great shock at about #h4ej ,p4,m4 mean time of ,malaga1 or about #i4h ,p4,m4 ,greenwich mean time4 ,this earthquake was investigated by no fewer than three official committees4 ,the first in the field was nominated by the ,spanish ,government on ,january #gth1 #ahhe1 and consisted of four members1 the ,president being ,seor ,m4,f4 de ,castro1 the director of the ,geological ,survey of ,spain4 ,the report of this commission was presented to the ,minister of ,agriculture1 etc41 on ,march #abth4 ,early in ,february a ,french ,commission1 appointed by the ,academy of ,sciences1 proceeded to the scene of the disaster4 ,with ,professor ,f4 ,fouqu as chief1 and ,,mm4 ,lvy1 ,bertrand1 ,barrois1 ,offret1 ,kilian1 ,bergeron1 and ,bron as members1 this committee resolved itself after a time into one for studying the geology of the central area2 and1 of their voluminous report of more than #gjj quarto pages 7published in #ahhi71 only #ee are immediately concerned with the earthquake4 ,at the beginning of ,april1 ,professors ,taramelli and ,mercalli1 sent by the ,italian ,government1 arrived in ,andalusia2 and their memoir1 read a few months later before the ,reale ,accademia dei ,lincei1 forms by far the most valuable contribution to our knowledge of the earthquake4 ,,damage ,,caused ,,by ,,the ,,earthquake4 ,the meizoseismal area 7see ,figs4 #ai and #bj7 lies in a mountainous district1 almost equidistant from the cities of ,malaga and ,granada4 ,in this area1 which contains nearly #ijj square miles1 the shock was disastrous to all but well-built houses4 ,whole villages were overthrown4 ,in the surrounding zone many buildings escaped serious damage1 and only a few were completely destroyed4 ,it is estimated by the ,spanish ,commission that1 in the province of ,granada1 #c1cdb houses were totally1 and #b1ach partially1 ruined2 in the province of ,malaga1 #a1jeg houses were totally1 and #d1agh partially1 ruined2 while in the two provinces together #f1dfc houses were damaged2 making a total of #ag1agh buildings more or less seriously injured4 ,as usual in the ,south of ,europe1 bad construction and narrow streets were largely responsible for the loss of property1 houses that were regularly built and made of good materials being only slightly injured4 ,but1 in this case1 the great slope of the ground1 the bad quality of the foundations1 and the nature of the underlying rocks were contributing factors4 ,many buildings also had been damaged by previous shocks1 and their ruin was only completed by the earthquake of #ahhd4 ,the total loss of life is variously estimated4 ,according to the ,spanish ,commission1 #fij persons were killed and #a1dbf wounded in the province of ,granada1 while #ee were killed and #ei wounded in that of ,malaga1 making a total of #gde persons killed and #a1dhe wounded4 ,the ,italian seismologists1 having additional materials at their disposal1 raise the total figures to #gej persons killed and #a1eed severely wounded4 ,careful inquiries were also made on this subject by the conductors of the newspaper ,el ,defensor de ,granada4 ,in ,granada alone1 they reckon that #hbh persons were killed and #a1afd wounded4 ,from the table given in the ,italian report1 it appears that #ccj persons were killed at ,alhama1 #aah at ,arenas del ,rey1 #ajb at ,albuuelas1 #gg at ,ventas de ,zafarraya1 and #dj at ,periana2 the percentage of mortality being #i at ,arenas del ,rey1 about the same at ,ventas de ,zafarraya1 and #c or #d at ,alhama1 ,albuuelas and ,periana4 ,comparing these latter figures with the death rates of #ga per cent4 at ,montemurro1 caused by the ,neapolitan earthquake1 and of about #de per cent4 at ,casamicciola1 by the ,ischian earthquake of #ahhc1 it will be seen that the loss of life during the ,andalusian earthquake was comparatively small--an exemption which is attributed by the ,italian commissioners to the absence of inhabited places from the immediate neighbourhood of the epicentre1 and to the fact that the destructive vibrations occurred towards the end of the shock1 thus allowing opportunity for escape4 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,fig4 #ai shows the principal isoseismal lines as drawn by the ,italian commissioners4 ,the meizoseismal area1 which included all places at which the shock was disastrous1 is bounded by an ellipse 7marked #a on the map7 #dj miles long from east to west1 #bh miles wide1 and about #hhf square miles in area4 ,the next isoseismal 7#b7 includes the places in which some buildings were ruined1 but not as a rule completely1 and in which there was no loss of life4 ,its bounding line is also elliptical1 the longer axis being about #ga miles long and running nearly east and west4 ,towards the south this zone is interrupted by the sea4 ,it will be noticed that these isoseismals are not concentric1 the second extending much farther to the west and south-west than in the opposite direction4 ,a third isoseismal 7not shown in the map7 encloses the district in which the shock was very strong1 or just capable of producing cracks in the walls of houses4 ,it is similar in form to the second isoseismal1 reaching as far as ,estepone to the south-west1 ,osuna1 ,cordova1 and ,seville to the west1 ,jaen to the north1 while towards the east it stops short of ,almeria4 ,illustration3 ,,fig4 #ai4--;,isoseismal lines of ,andalusian earthquake4 7,taramelli and ,mercalli47 ,the ,french ,commission have also published a map of the earthquake1 and1 though the work of an experienced seismologist like ,professor ,mercalli is probably more trustworthy1 it is interesting to compare his isoseismal lines with those obtained by his ,french colleagues1 which are reproduced in ,fig4 #bj4 ,the curves in this figure are drawn so as to include the places that were1 respectively1 ruined1 seriously damaged1 and slightly damaged1 by the shock4 ,they should therefore correspond with the lines in ,fig4 #ai4 ,it will be seen that they differ considerably in form1 but at the same time they present certain points of agreement1 such as the east and west elongation of the meizoseismal area1 and the great extension of the two outer isoseismals towards the west and south-west ,the greatest difference is to be found in the eastern portion of the third isoseismal1 which1 according to the ,italians1 extends beyond the limits included in ,fig4 #bj1 and1 according to the ,french1 is bayed back by the great masses of the ,sierra ,nevada4 ,outside ,andalusia the earthquake was sensibly felt to the north as far as ,madrid and ,segovia1 to the west at ,huelva1 ,crceres and ,lisbon1 and to the east at ,valencia and ,murcia4 ,towards the south1 the greater part of the disturbed area was cut off by the ,mediterranean1 and there are no records forthcoming from the opposite coast of ,africa4 ,the total area disturbed by the earthquake is roughly estimated by the ,french ,commission at about #aed1jjj square miles1 and by the ,italian ,commission at about #agd1jjj square miles2 but1 as the shock was strong enough to stop clocks and ring bells at ,madrid1 it is evident that even the greater of these values is too small4 ,illustration3 ,,fig4 #bj4--;,isoseismal lines of ,andalusian earthquake4 7,fouqu1 etc47 ,,the ,,unfelt ,,earthquake4 ,far beyond the limits of the disturbed area1 however1 the long slow waves sped over the surface1 disturbing magnetographs and other delicate instruments4 ,more than a century before1 the great ,lisbon earthquake of #agee had caused oscillations in ,scottish lakes1 and on other occasions the effects of remote earthquakes had been witnessed at isolated places4 ,but1 in #ahhd1 the concurrent registration of the ,andalusian earth-waves at distant observatories attracted general attention1 and in part suggested the world-wide network of seismological stations1 the foundation of which was laid before another decade had passed4 ,in ,italy1 probable records of the earthquake were obtained at two observatories1 but1 owing to the approximate times given1 their connection with it is not established4 ,at ,velletri1 near ,rome1 ,professor ,galli's seismodynamograph registered a very slight movement at #aj ,p4,m41 and at ,rome itself ,professor de ,rossi found a tromometer making unusual oscillations at #aj4ae ,p4,m4#ca ,the most interesting records1 however1 are those furnished by the magnetographs at ,lisbon1 ,parc ,saint-,maur 7near ,paris71 ,greenwich1 and ,wilhelmshaven4 ,at ,lisbon1 the records are extremely clear4 ,the curves of the declination1 horizontal force and vertical force magnets1 as seen in ,fig4 #ba1 are abruptly broken at #h4cc ,p4,m4 7,lisbon time1 or #i;h4 #im4 #des41 ,g4,m4,t474 ,the disturbances1 which are greatest on the declination curve and least on the vertical force curve1 lasted in all three for about #ab minutes1 and are quite distinct from the ordinary magnetic perturbations4 ,at ,parc ,saint-,maur1 the magnetographs seem to be ill-adapted to act as seismographs1 for only a slight mark was discovered on a re-examination of the curves1 beginning at #i4bd ,p4,m4 7,paris time1 or #i;h4 #adm4 #cis41 ,g4,m4,t47 ,at ,greenwich1 ,mr4 ,w4 ,ellis writes1 there was a small simultaneous disturbance of the declination and horizontal force magnets1 occurring at #i;h4 #aem4444 ,both magnets were at this time set into slight vibration1 the extent of vibration in the case of declination being about #b' of arc1 and in horizontal force equivalent to 4#jja of the whole horizontal force nearly4 ,of the three instruments at ,wilhelmshaven1 only one showed any movement at the time of the earthquake4 ,the declination magnet was undisturbed1 the horizontal force curve was accidentally interrupted1 but the vertical force curve indicated a very perceptible shock4 ,beginning at #i4eb ,p4,m4 7,wilhelmshaven mean time1 or #i;h4 #bim4 #bis41 ,g4,m4,t471 the curve was broken for four minutes1 for the rapid swinging of the needle could not be registered until the motion became fainter4 ,further disturbances also occurred at #i4ei1 #aj1 #aj4b1 and #aj4e ,p4,m4#cb ,illustration3 ,,fig4 #ba4--,magnetograph records of ,andalusian earthquake at ,lisbon4 7,fouqu1 etc47 ,,position ,,of ,,the ,,epicentre4 ,the innermost isoseismal being too large1 and the time-records too inaccurate1 to give the position of the epicentre1 both ,commissions resorted to observations of the direction1 ,professor ,fouqu and his colleagues depending chiefly on the oscillation of hanging lamps1 and ,professors ,taramelli and ,mercalli on the fall or displacement of statues and other objects1 and all avoiding as far as possible the evidence of fissures in buildings4 ,the ,italian observers point out that1 among the divergent directions visible at any place1 there is generally one more distinctly marked than the others1 and this1 they consider1 corresponds to the movement coming almost directly from the centre of disturbance4 ,plotting these directions 7#cf in number71 they find that they converge as a rule within the triangle formed by joining ,ventas de ,zafarraya1 ,alhama1 and ,jatar1 while a large number of them traverse the elliptical area1 whose boundary is represented by the dotted line in ,fig4 #ai4 ,this area is about #i miles long and #b-a_/b miles wide1 its longer axis runs nearly east and west1 and its centre coincides with the western focus of the ellipse which forms the boundary of the meizoseismal area4 ,it lies1 moreover1 close to ,ventas de ,zafarraya and ,arenas del ,rey1 the two places where the seismic death-rate was highest1 while its major axis almost coincides with the line joining them4 ,the evidence of hanging lamps collected by the ,french ,commission was more consistent than that of the fallen objects4 ,at every place1 the plane in which the lamps oscillated was nearly constant1 the deviations being generally attributable to irregularities in the mode of suspension4 ,the azimuths again intersect within an elliptical area1 which1 according to the ,commission1 differs little from the central region of the earthquake 7,fig4 #bj74 ,it ,is clear1 however1 from the map accompanying the ,french report1 that the majority converge towards a narrow band extending east and west from near ,arenas del ,rey to near ,ventas de ,zafarraya1 and therefore agreeing closely with the epicentral area as determined by ,professors ,taramelli and ,mercalli4#cc ,,depth ,,of ,,the ,,seismic ,,focus4 ,if the depth of the seismic focus amounts to several miles1 one of the most serious objections to ,mallet's method lies in the varying refractive power of the different strata traversed by the earth-waves 7p4 #bh74 ,at present we have no way of meeting this objection1 and all calculations of the depth of the focus are therefore more or less doubtful4 ,a difficulty in practice has also been urged1 depending on the widely differing inclinations of the fractures at any place2 but the ,italian observers found that the errors from this source were greatly reduced by avoiding all fissures in poorly-built houses1 or which start from windows or other apertures1 and selecting only those which occur in homogeneous walls directed towards the epicentre4 ,the best angles of emergence thus measured by them are thirteen in number1 all made at places lying within #e and #bc miles from the centre of the epicentral area1 and1 with two exceptions1 inside the meizoseismal zone 7,fig4 #ai74 ,the depths corresponding to the different wave-paths vary from #e4c to #bc4j miles1 the mean depth of the focus given by all thirteen observations being #g4f miles4 ,the only estimate made by the ,french ,commission--and it is one that they rightly regarded with considerable doubt--was based on a method devised by ,falb4 ,as the sound generally precedes the shock1 ,falb assumes that it travels with a greater velocity4 ,if the velocities of both series of waves are known1 and if they start at the same instant and from the same region1 the interval that elapses between the arrivals of the sound and shock should give the distance traversed by them and consequently the depth of the focus4 ,it is unnecessary to mention more than two of the serious objections to this method4 ,the duration of the preliminary sound should increase rapidly with the distance from the focus1 and of this there is not the slightest evidence4 ,moreover1 the sound-vibrations that are first heard do not necessarily come from the same part of the focus as those which cause the shock1 but1 as will be seen in ,chapter ,,viii41 probably from its nearer lateral margin4 ,the ,french ,commission1 finding the average duration of the fore-sound near the epicentre to be #e seconds1 estimate the depth of the focus at about #g miles--a result which agrees remarkably with that obtained from the angles of emergence1 but which is not1 on that account1 entitled to credit4 ,,nature ,,of ,,the ,,shock4 ,in the nature of the shock1 there was a singular uniformity throughout the whole disturbed area1 the chief variation noticed being evidently dependent on the observer's distance from the epicentre4 ,for instance1 in the meizoseismal area 7,fig4 #ai71 at ,ventas de ,zafarraya1 a loud sound like thunder was first heard1 and before it ceased there came a violent subsultory movement preceded by a very brief oscillation1 then a pause of one or two seconds1 and lastly a more intense and longer series of undulations1 the whole movement lasting #ab seconds4 ,at ,cacin1 three phases were distinguished1 the first a slight undulatory movement coincident with the sound1 followed immediately by the subsultory motion1 a pause1 and stronger undulations1 the total duration being #ae seconds4 ,the variations noticeable in this zone seem to have been apparent only1 sensitive observers perceiving a tremulous motion before the vertical vibrations1 and in the pause between them and the concluding undulations4 ,in both phases1 the intensity increased to a maximum and then gradually decreased4 ,the movement at ,ventas de ,zafarraya and ,cacin is represented by ,professors ,taramelli and ,mercalli by the curves a and b in ,fig4 #bb4 ,in the second zone 7,fig4 #ai71 the same two phases were universally observed1 but the subsultory movement was less pronounced or the movement was partly subsultory and partly undulatory1 and occasionally both phases are described as undulatory4 ,the motion near ,malaga is represented by the curve c in ,fig4 #bb4 ,illustration3 ,,fig4 #bb4--,nature of shock of ,andalusian earthquake4 7,taramelli and ,mercalli47 ,outside the ruinous zone1 the first phase rapidly lost what remained of its subsultory form1 and the pause between the two parts was noticeably longer than near the epicentre4 ,thus1 at ,seville and ,cordova1 two shocks were felt1 separated by an interval of some seconds2 the second according to some observers at ,seville1 terminating with vertical tremors4 ,at ,madrid1 also1 the two parts were perceived1 the interval between them being #c or #d seconds in length2 but1 as a rule1 outside ,andalusia1 only a single undulatory shock was felt1 without any preliminary sound4 ,that the changes observed in the shock were merely an effect of less or greater distance1 will be obvious from ,fig4 #bc1 in which the intensity at any moment is that represented by the distance of the corresponding point on the curve from the different base-lines1 the base-line a corresponding to a place near the epicentre1 and b1 c1 d1 etc41 to places at gradually increasing distances4 ,thus1 at a place corresponding to the base-line b1 the intensity of the tremors during the intervening pause 7represented by the short line ,,pn7 was so slight that they frequently escaped notice1 while the preliminary tremors observed by some near the epicentre were altogether imperceptible4 ,at the places corresponding to the base-lines c1 d1 e1 f1 the duration of the whole shock and of each part gradually diminished1 while the interval between the two parts increased owing to the gradual extinction of the final vibrations of the first part and of the initial vibrations of the second4 ,at the farthest of these places 7f7 the first part was so weak that it sometimes passed unobserved4 ,lastly1 at a place corresponding to the base-line g1 the first part was imperceptible to all observers1 and the shock consisted of a single series of horizontal undulations4 ,illustration3 ,,fig4 #bc4--;,diagram to illustrate variation in nature of shock of ,andalusian earthquake4 ,origin of the ,double ,shock4--,if the double shock were observed at only a few places1 we should naturally look for some local explanation of the peculiarity4 ,the second shock1 for instance1 might be a subterranean echo1 the earth-waves being reflected at the bounding surface of two different kinds of rock4 ,in the case of the ,andalusian earthquake1 such an explanation is precluded by the almost universal observation of the double shock1 the greater intensity of the second part1 and the longer period of its vibrations4 ,the ,italian observers1 who paid considerable attention to the double shock1 give a more general explanation4 ,they regard the two parts of the shock as corresponding in the main to longitudinal and transversal waves starting simultaneously from the same focus 7see p4 #ac74 ,the former vibrations would be vertical at the epicentre and would gradually become horizontal in spreading outwards2 the latter would be horizontal at the epicentre and at a distance from it 7e4g4 at ,seville7 nearly vertical4 ,also1 as the longitudinal waves travel more rapidly than others1 the interval between the two parts of the shock would increase with the distance from the origin4 ,owing again1 to the large size of the focus1 the first part of the shock would at no place be instantaneous1 and its later vibrations might coalesce with the earlier transverse vibrations1 so that1 within and near the meizoseismal area1 the second part of the shock might be stronger than the first4 ,a similar result might be produced in the same district if the transverse vibrations coincided with reflected longitudinal vibrations1 and ,professors ,taramelli and ,mercalli think that such reflection would occur from the old crystalline rocks of the ,sierra de ,almijara and possibly also from the calcareous and crystalline rocks to the south-west of ,cartama4 ,satisfactory as it seems to be in some respects1 this explanation is open to serious objections1 of which ,i will mention only two4 ,the first is that1 though the pause between the two parts of the shock does increase with the distance1 it does not increase rapidly enough2 at ,seville1 it should be two or three minutes1 instead of some seconds in length4 ,a more fatal objection1 however1 is that1 if the explanation were correct1 every earthquake-shock should consist of two parts1 and this is only the case with a small minority4 ,on the other hand1 if the velocities of the waves composing each part were the same1 the slight increase in the length of the interval is readily accounted for1 as we have seen1 by the gradual extinction of its weak terminal vibrations4 ,but in any case1 the long interval that elapsed between the beginnings of the two parts at a place so near the epicentre as ,ventas de ,zafarraya1 shows that each part was due to a distinct impulse2 and1 judging from the directions of the respective movements1 it would seem that the focus of the first impulse was situated at a greater depth than the focus of the second4 ,whether the epicentres corresponding to the two foci were coincident or more or less separate is not clear from the nature of the shock2 but it is probable that they were nearly or quite detached1 and that a second epicentre was situated near the eastern focus of the ellipse bounding the meizoseismal area4 ,,sound-,,phenomena4 ,in the ,neapolitan earthquake1 the sound was only heard in a district of about #c1cjj square miles immediately surrounding the epicentres1 while the whole area disturbed by the shock was not less than #ci1jjj square miles4 ,a similar limitation was noticed in the ,andalusian earthquake4 ,according to the ,spanish ,commission1 the sound was heard at only one place 7,cordova7 outside the provinces of ,granada and ,malaga2 and its audibility was a rule confined to the area within which buildings were damaged by the shock4 ,it was compared at different places to the noise of a passing train or a carriage heavily laden running on a paved road1 of distant thunder1 a great storm1 or the discharge of heavy guns4 ,at every place where the sound was heard1 it distinctly preceded the shock1 frequently allowing time for escape from houses that were afterwards ruined4 ,its duration within the meizoseismal area was on an average about five or six seconds1 rarely perhaps did it exceed ten seconds4 ,at some places in the same area1 it overlapped the beginning of the shock1 but generally it was separated from the latter by a very short interval1 estimated at a second4 ,from this precedence of the sound1 the ,italian ,commission conclude that the sound-waves travelled more rapidly than those which formed the shock1 an inference that depends on the assumption that both waves started simultaneously from within precisely the same focal limits4 ,a different explanation1 not based on these assumptions1 will be considered more fully in ,chapter ,,viii1 dealing with the recent earthquakes of ,hereford and ,inverness4 ,,velocity ,,of ,,the ,,earth-,,waves4 ,if1 in a highly-civilised country1 the time-records of an earthquake vary within wide limits1 it is not surprising that those given for the ,andalusian earthquake should be wholly untrustworthy4 ,even the clocks in public buildings and railway stations differed by as much as #be minutes in their indications4 ,an interesting observation is1 however1 described in the ,french report and is worth repeating1 though it does not lead to any accurate result4 ,at the time of the principal shock1 two telegraph-clerks were in communication1 one at ,malaga and the other at ,velez-,malaga4 ,the latter1 surprised by the shock1 suddenly stopped his message2 and1 about six seconds later1 the arrival of the earth-waves at ,malaga explained the interruption to his colleague4 ,as1 according to the ,french report1 ,velez-,malaga is #i kms4 7or about #e-a_/b miles7 nearer than ,malaga to the mean epicentral point1 it follows that the velocity of the earth-waves must have been about #a4e kms41 or nearly a mile1 per second4#cd ,the only observations of any real value in determining the velocity are those given by the stopped clock at the observatory of ,san ,fernando 7,cadiz7 and by the magnetographs at ,lisbon1 ,parc ,saint-,maur1 ,greenwich1 and ,wilhelmshaven4 ,taking the times at ,cadiz1 ,lisbon1 ,greenwich1 and ,wilhelmshaven at #i4ah1 #i4ai1 #i4be1 and #i4bi ,p4,m4 respectively 7,paris mean time7 and the mean epicentral point as coinciding with ,alhama1 the ,french ,commission estimates roughly the mean surface-velocity between ,cadiz and ,lisbon at #c4f kms4 per second1 between ,cadiz and ,greenwich at #d4e kms4 per second1 between ,cadiz and ,wilhelmshaven at #c4a kms4 per second1 and between ,greenwich and ,wilhelmshaven at #a4f kms4 per second4 ,dr4 ,agamennone1 however1 notices that the distances from ,alhama are not correctly measured1 and substitutes for the above figures #d4hc1 #c4dc1 #b4hb1 and #a4ge kms4 per second respectively4 ,these results apparently show a decrease in the velocity with the outward spread of the earth-waves1 but1 as ,dr4 ,agamennone again points out1 a comparatively small error in the time at ,cadiz would neutralise the apparent decrease4 ,it is not to be supposed that the astronomical clock at this observatory was wrong by more than a second or two1 but the behaviour of clocks during an earthquake is so irregular--some stopping at once1 others staggering on for some seconds before arrest--that the ,cadiz time may differ from the true time by several seconds4 ,besides this possible error1 there is also considerable uncertainty in the records from the magnetic observatories1 owing to the slow rate at which the photographic paper travels4 ,at ,parc ,saint-,maur this rate is only #aj mm4 per hour1 and at the other observatories about #ae mm4 per hour4 ,allowing1 therefore1 for an error of half-a-minute in the time-record at ,cadiz1 of one minute in those of ,lisbon1 ,greenwich1 and ,wilhelmshaven1 and of two minutes in that at ,parc ,saint-,maur1 and taking the mean epicentral point as determined by the ,italian observers1 ,dr4 ,agamennone1 applying the method of least squares1 finds the probable value of the velocity of propagation to be #c4ae kms4 7or nearly #b miles7 per second1 with a possible error of 4#ai kms4 per second4 ,this result agrees closely with the value found for the long slow undulations of more recent earthquakes4 ,,miscellaneous ,,phenomena4 ,connection between ,geological ,structure and the ,intensity of the ,shock4--,while a great part of the injury to buildings must be attributed to their faulty construction1 the connection between the nature of the underlying rock and the amount of damage was very clearly marked4 ,other conditions being the same1 houses built on alluvial ground suffered most of all2 and the destruction was also great in those standing on soft sedimentary rocks such as clays and friable limestones4 ,on the other hand1 when compact limestones or ancient schists formed the foundation-rock1 the amount of damage was conspicuously less than in other cases4 ,the members of both the ,french and the ,italian ,commissions agree in ascribing the peculiar form and relative positions of the isoseismal lines to geological conditions4 ,to the east of the epicentre1 the schists and crystalline limestones form a deep1 uniform1 and compact mass2 while1 to the west1 the old crystalline rocks are covered by jurassic1 cretaceous1 and eocene formations1 constituting a less homogeneous and less elastic mass1 in which the intensity of the shock would fade off much more rapidly1 with the result that the epicentre occupies the western focus of the elliptical boundary of the meizoseismal area 7,fig4 #ai74ce ,that mountain-ranges have an important influence on the form of isoseismal lines is evident from both maps 7,figs4 #ai and #bj71 but especially from that published by the ,french ,commission 7,fig4 #bj74 ,the resistance offered by the ,sierra ,nevada to the propagation of the earth-waves is shown in the former map by the approximation of the first and second isoseismals at the east end1 and in the latter by the great bay in the third isoseismal line4 ,whichever interpretation of the evidence is the more accurate1 the action of the mountainous mass is clearly to lessen rapidly the intensity of the shock--an effect which is probably due to the abrupt changes in the direction and nature of the strata encountered normally by the earth-waves4 ,on the opposite side of the epicentre1 the waves meet the ,sierra de ,ronda obliquely4 ,in traversing this range1 the shock lost a great part of its strength1 while it continued to be felt severely along its eastern foot1 thus giving rise to the south-westerly extension of the third isoseismal in ,fig4 #bj1 and1 though to a less extent1 that of the second in ,fig4 #ai4 ,fissures1 ,landslips1 etc4--,the earthquake resulted in many superficial changes1 such as fissures1 landslips1 and derangement of the underground water-system--all changes of the same order as the destruction of buildings--but1 so far as known1 in no fault-scarps or other external evidence of deep-seated movements4 ,some of the fissures were of great length4 ,one of the most remarkable occurred at ,guevejar1 a village built on the south-west slope of the ,sierra de ,cogollos4 ,it was in the form of a horse-shoe1 and was about two miles long1 from ten to fifty feet wide1 and of great depth4 ,in its neighbourhood1 innumerable small cracks appeared1 some perpendicular and others parallel to the great fissure4 ,the ground within1 a bed of clay resting on limestone1 also slid down towards the river4 ,houses near the centre of the fissured tract were shifted as much as thirty yards within the first month1 and others near its extremity about ten feet2 while the accumulation of the material at the south end of the fissure resulted in the formation of a small lake1 of about #bej to #cej square yards in area and about #cj feet deep4 ,all streams within the fissured zone disappeared1 and the spring1 which provided the drinking-water of the village1 ceased to flow4 ,the underground water-system was generally affected throughout the central area4 ,in some places1 mineral springs disappeared2 in others1 new springs broke out or old ones flowed more abundantly4 ,at ,alhama1 the increased flow was accompanied by a permanent rise in temperature from #dg to #ej ,c41 and by a marked change in character4 ,,after-,,shocks4 ,frequent after-shocks are a characteristic of the earthquakes of ,southern ,spain4 ,after the ,cordova earthquake of #aagj1 they continued for at least three years4 ,the ,murcian earthquake of #ahbh was followed by #cjj minor shocks during the next twenty-four hours1 and for more than a year slight tremors were often felt4 ,for some time after the great earthquake of #ahhd1 the movements of the ground were extremely numerous in the immediate neighbourhood of the epicentre1 farther away they were rarer and of less intensity1 and outside the area of damaged buildings they were nearly absent4 ,thus1 during the night of ,december #be-bf1 #aaj after-shocks were counted at ,jatar1 from #ad to #ag at ,alcaucin1 ,ventas de ,huelma1 ,motril1 ,cacin1 ,durcal1 ,malaga1 etc42 about #aa at ,la ,mala and ,albuuelas2 #i at ,velez-,malaga and ,lenteje2 and from #e to #g at ,frigiliana1 ,riogordo1 and ,cartama4 ,the strongest of these shocks occurred at #b4bj ,a4,m41 and1 though none was violent1 several helped to complete the ruin of many houses that had been damaged by the principal shock4 ,from this time1 after-shocks occurred almost daily until the end of ,may1 after which they became much less frequent4 ,according to the list given in the ,italian report1 which closes at the end of ,january #ahhf1 #bcg shocks were felt1 #bc up to the end of ,december1 #cj in ,january #ahhe1 #be in ,february1 #bg in ,march1 #df in ,april1 and #dc in ,may4 ,in ,june #ahhe1 only three are recorded1 and the average number during each of the following seven months lies between five and six4 ,this list1 however1 does not include the very weak shocks1#cf for nearly all those contained in it were felt as far as ,malaga or its neighbourhood4 ,the shocks varied considerably in intensity as well as in frequency1 five of them being much more violent than the rest4 ,one that occurred on ,december #cjth was felt strongly in all the damaged area1 two others on ,january #crd and #eth caused fresh injury to buildings1 a fourth1 on ,february #bgth1 disturbed an area bounded roughly by the second isoseismal of the principal earthquake 7,fig4 #ai71 while the fifth and strongest1 that of ,april #aath1 was felt over a large part of the zone beyond4 ,at places within and near the meizoseismal area1 earth-sounds were sometimes heard without any sensible shock2 occasionally1 also1 tremors were felt with no attendant sound2 but1 as a rule1 the shocks were accompanied by sound1 and in every such case1 as in the principal earthquake1 the sound preceded the shock1 or at most was partly contemporaneous with it4 ,several of the after-shocks resembled the principal earthquake in their division into two parts separated by an interval of rest or weaker movement from half a second to a second in length1 though the whole duration of the shock itself in no case exceeded five or six seconds4 ,occasionally1 the likeness was still closer1 in the succession of sound1 subsultory motion and concluding horizontal undulations4 ,,geology ,,of ,,the ,,meizoseismal ,,area ,,and ,,origin ,,of ,,the ,,earthquakes4 ,the meizoseismal area and surrounding zones lie in the midst of the mountainous region that separates the basin of the ,guadalquiver from that of the ,mediterranean1 the essential structure of which1 according to the geologists of the ,french ,commission1 is outlined in ,fig4 #bd4 ,in this sketch-map1 the lightly-shaded bands correspond to an upper series of crystalline schists1 and the cross-shaded bands to the lower series of mica-schists and dolomites that form the anticlinal folds of the ,sierra de ,ronda1 the ,sierra de ,mijas1 and the ,sierra ,tejeda4 ,in addition to the faulting and intense folding in the direction of their strikes1 these rocks are also intersected by three nearly parallel transverse faults of post-,triassic age1 which1 aided by subsequent denudation1 have cut up the whole range into a number of distinct sierras4 ,they are represented by the broken lines in ,fig4 #bd4 ,illustration3 ,,fig4 #bd4--,structure of meizoseismal area of ,andalusian earthquake4 7,fouqu1 etc47 ,one of these faults1 that which passes near ,motril1 traverses the meizoseismal area1 whose boundary1 as laid down by the ,french ,commission1 is indicated by the dotted line on the sketch-map4#cg ,in the neighbourhood of ,zafarraya1 the fault intersects the broken anticlinal fold of the ,sierra ,tejeda1 and the epicentre is thus situated in one of the most disturbed tracts of the whole region4 ,the evidence1 both seismic and geological1 is insufficient to support any precise view as to the origin of the earthquake1 but there can be little doubt that it was closely connected with movements along one or more of the system of faults that intersect not far from ,zafarraya4 ,,references4 #a4 ,,agamennone1 ,g4--,alcune considerazioni sui different metodi fino ad oggi adoperati nel calcolare la velocit di propagazione del terremoto andaluso del #be dicembre #ahhd4 ,roma1 ,r4 ,accad4 ,lincei1 ,rend41 vol4 iii41 #ahid1 pp4 #cjc-caj4 #b4 ---- ,velocit superficiale di propagazione delle onde sismiche in occasione della grande scossa di terremoto dell' ,andalusia del #be dicembre #ahhd4 ,ibid41 vol4 iii41 #ahid1 pp4 #cag-cbe4 #c4 ,,castro1 ,m4,f4 de4--,terremotos de ,andaluca3 ,informe de la comision nombrada para su estudio dando cuenta del estado de los trabajos en #g de marzo de #ahhe4 7,madrid1 #ahhe2 #ajg pp47 #d4 ,,fouqu1 ,f41 etc4--,mission d',andalousie3 tudes relatives au tremblement de terre du #be dcembre #ahhd1 et la constitution gologique du sol branl par les secousses4 ,paris1 ,acad4 ,sci4 ,mm41 vol4 xxx41 pp4 #a-ggb4 #e4 ,,macpherson1 ,j4--,tremblements de terre en ,espagne4 ,paris1 ,acad4 ,sci41 ,compt4 ,rend41 vol4 c41 #ahhe1 pp4 #cig-cii4 #f4 ,,nogu,s1 ,a4,f4--,phnomnes gologiques produits par les tremblements de terre de l',andalousie1 du #be dcembre #ahhd au #af janvier #ahhe4 ,ibid41 pp4 #bec-bef4 #g4 ,,rossi1 ,m4,s4 de4--,gli odierni terremoti di ,spagna ed il loro eco in ,italia4 ,bull4 ,vulc4 ,ital41 anno xii41 #ahhe1 pp4 #ag-ca4 #h4 ,,taramelli1 ,t41 and ,g4 ,,mercalli4--,i terremoti ,andalusi cominciati il #be dicembre #ahhd4 ,roma1 ,r4 ,accad4 ,lincei1 ,mem41 vol3 iii41 #ahhe1 pp4 #aaf-bbb4 #i4 ,paris1 ,acad4 ,sci41 ,compt4 ,rend41 vol4 c41 #ahhe1 pp4 #bd-bg1 #acf-ach1 #aif-aig1 #bef-beg1 #eih-fja1 #aaac-aabj1 #adcf 7the last three by ,f4 ,fouqu74 ,,footnotes3 #ca ,these times correspond to about #i4aj and #i4be ,p4,m41 ,greenwich mean time4 ,the earthquake stopped a clock at the ,royal ,observatory of ,san ,fernando 7,cadiz71 at #h;h4 #dcm4 #ed4es4 mean local time1 corresponding to #i;h4 #hm4 #dds41 ,g4,m4,t4 #cb ,the earthquake is also said to have been registered at the observatory of ,moncalieri1 near ,turin1 but ,i have not been able to ascertain the time of occurrence4 ,a movement felt at about #aj4bj ,p4,m4 at ,ramsbury1 in ,wiltshire1 was attributed to the earthquake1 though the time is about an hour too late4 ,on ,december #bfth1 an astronomical clock was stopped at ,brussels and its pillar displaced2 and1 on the evening of the same day1 the large telescope at the observatory was also found to have been shifted4 ,these effects1 it is suggested1 were caused by the ,andalusian earthquake1 but the connection between them seems to me very doubtful4 #cc ,the ,french observers have also applied a method depending on the time of occurrence of the shock4 ,joining places where the recorded times were the same1 they notice that the perpendicular bisectors of these lines intersect within an area which agrees practically with that determined by the azimuths4 ,the inaccuracy of the time-records must1 however1 lessen the significance of this result4 #cd ,dr4 ,agamennone points out that1 according to the ,italian report1 the difference in distance is #bb kms4 7or #ac-c_/d miles71 leading to a velocity of about #c4f kms41 or #b4c miles per second4 #ce ,it should be remembered that it is not improbable that there were two detached epicentres1 coinciding roughly with the two foci of this curve4 #cf ,only eight are recorded during the night of ,december #be-bf4 ,on several occasions during ,april and ,may #ahhe1 groups of slight shocks were felt2 but as their individual times are not given1 they are regarded as equivalent to one shock each in the above totals4 #cg ,the boundary1 as drawn in this figure1 differs slightly from that given in ,fig4 #bj4 ,,chapter ,v4 ,,the ,,charleston ,,earthquake ,,of ,,august #ca,,st1 #ahhf4 ,the ,charleston earthquake stands alone among the great earthquakes described in this volume1 and indeed among nearly all great earthquakes1 in visiting a region where seismic disturbances were almost unknown4 ,calabria and ,ischia1 the ,riviera and ,andalusia1 ,assam and the provinces of ,mino and ,owari in ,japan1 are all regions where earthquake-shocks are more or less frequent and occasionally of destructive violence4 ,but1 from the foundation of ,charleston in #afhj until #ahhf1 that is1 for more than two centuries1 it is probably not too much to say that few counties in ,great ,britain were so free from earthquakes as the ,state of ,south ,carolina4#ch ,the practical isolation of the earthquake of #ahhf left its trace on the character of the investigation4 ,not only were the observers untrained1 but the investigators themselves were unprepared4 ,for instance1 the scale of intensity used in drawing the isoseismal lines was not adopted until after the first letters of inquiry were issued4 ,on the other hand1 nothing could exceed the energy and ability with which the epicentral tracts were examined by ,mr4 ,earle ,sloan and the collection of time-records made by ,mr4 ,everett ,hayden4 ,to them1 and to ,major ,c4,e4 ,dutton1 whose valuable monograph supersedes all other accounts1 we are indebted for the two chief additions to our knowledge resulting from the study of the ,charleston earthquake4 ,these are the determination of the double epicentre1 and the measurement of the velocity with which the earth-waves travelled4 ,,damage ,,caused ,,by ,,the ,,earthquake4 ,the land-area disturbed by the earthquake and the isoseismal lines are shown in ,fig4 #be1 the small black oval area 7which ,includes ,charleston7 being that within which the greatest damage to buildings occurred4 ,the chief part of the epicentre1 however1 lies from #ab to #ae miles to the west and north-west of ,charleston1 in a forest-clad district1 containing only two villages and various scattered houses4 ,the city of ,charleston1 whose population between #ahhj and #ahia increased from fifty to fifty-five thousand1 is built on a peninsula between the ,cooper ,river on the east and the ,ashley ,river on the south-west4 ,originally1 this was an irregular tract of comparatively high and dry land1 intersected by numerous creeks1 which1 as the city grew1 were filled up to the general level of the higher ground4 ,it is on this made land as a rule that the more serious damage to buildings occurred4 ,at #i4ea ,p4,m4 7standard time of the #geth meridian71 the great earthquake occurred1 and1 one minute later1 there was left hardly a building in the city that was not injured more or less seriously4 ,the destruction1 as ,major ,dutton remarks1 was not of that sweeping and unmitigated order which has befallen other cities1 and in which every structure built of material other than wood has been levelled completely to the earth in a chaos of broken rubble1 beams1 tiles1 and planking1 or left in a condition practically no better4 ,the number of houses entirely demolished was not great1 but several hundred lost a large part of their walls1 and many were condemned as unsafe and afterwards pulled down4 ,a board of inspectors1 appointed to investigate the condition of the houses1 reported that not one hundred out of fourteen thousand chimneys examined by them escaped damage1 and that #ie per cent4 of those injured were broken off at the roof4 ,the total cost of the necessary repairs1 it was estimated1 would amount to about one million pounds4 ,according to the official records1 #bg persons were killed in ,charleston during the earthquake1 but1 by cold1 exposure1 etc41 this number was brought up to not less than #hc4 ,the number of persons wounded was never ascertained4 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,in drawing the isoseismal lines 7represented by the continuous curves in ,fig4 #be71 ,major ,dutton made use of the well-known ,rossi-,forel scale of seismic intensity1 a translation of which is given below4#ci ,the curves range from the highest degree1 #aj1 corresponding to disastrous effects on buildings1 down to the lowest but one1 #b1 which would be applied to a shock felt only by a small number of persons at rest4 ,it is evident1 ,i think1 that these lines cannot be regarded as drawn with great accuracy4 ,the number of records 7nearly #djjj1 from about #a1fjj places71 great as it is1 is hardly sufficient for the purpose2 and many were collected from newspapers4 ,the circulars of inquiry also contained no distinct questions corresponding to the different degrees of the scale employed1 and therefore it is not always certain that the intensity recorded was the maximum observed4 ,but1 if the curves might have varied in detail with a larger and more accurate series of observations1 they must represent in their main features the distribution of seismic intensity throughout the disturbed area4 ,one point of importance is the partial earthquake-shadow in the region of the ,appalachian ,mountains shown by the southward incurving of the isoseismals #d1 #e1 and #f1 and especially by the first two of these lines4 ,another is the close grouping of the isoseismals in the ,state of ,mississippi1 illustrating a rapid fading of intensity as the earth-waves crossed the unconsolidated materials of the ,mississippi delta4 ,illustration3 ,,fig4 #be4--;,isoseismal lines of ,charleston earthquake4 7,dutton1 etc47 ,owing to the short distance between the epicentre and the sea-coast1 it is impossible to make more than a rough estimate of the extent of the disturbed area4 ,even when the boundary lies on land1 it traverses some districts which are thinly populated and others where the inhabitants are unobservant1 and unlikely to notice the slow oscillations which were alone perceptible at great distances4 ,the shock was1 however1 felt at ,boston 7#hjj miles from the epicentre71 ,la ,crosse on the upper ,mississippi 7#iej miles to the north-west71 at several places in ,cuba 7between #gjj and #gaj miles71 and in ,bermuda 7#iej miles74 ,to the south1 the limits are unknown1 there being no report from ,yucatan1 the nearest point of which is distant about #icj miles4 ,if we assume the disturbed area to have a mean radius of #iej miles1 then it must have covered no less than #b1hjj1jjj square miles4 ,and1 that this estimate is not excessive1 will be evident from the fact that the land-area disturbed 7including parts of the great lakes and inlets in the sea-coast7 amounted to about #ibj1jjj square miles4 ,,preparation ,,for ,,the ,,earthquake4 ,the preparation for the earthquake seems to have begun about three months before4 ,during ,june1 and even earlier1 slight but undoubted tremors are said to have been felt in ,charleston1 but no record of them was kept until about #h ,a4,m4 on ,august #bgth1 when a decided earthquake occurred at ,summerville1 a village twenty-two miles to the north-west4 ,the shock and sound were simultaneous1 the shock a single jolt or heavy jar1 the sound loud and sudden2 they were such as might have been caused by the firing of a heavy cannon or the explosion of a boiler or blast of gunpowder4 ,at #d4de ,a4,m4 on ,august #bhth1 the shock and sound were repeated1 only more strongly1 the former being distinctly felt as far as ,charleston4 ,during that day and the next1 there were several other shocks at ,summerville1 and then rest and quiet succeeded until the evening of ,august #cast4 ,,nature ,,of ,,the ,,shock4 ,at #i4ea ,p4,m4 7to take one of the best descriptions71 the attention of an observer in ,charleston was vaguely attracted by a sound that seemed to come from the office below1 and was supposed for a moment to be caused by the rapid rolling of a heavy body1 as an iron safe or a heavily-laden truck1 over the floor4 ,accompanying the sound there was a perceptible tremor of the building1 not more marked1 however1 than would be caused by the passage of a car or dray along the street4 ,for perhaps two or three seconds the occurrence excited no surprise or comment4 ,then by swift degrees1 or all at once--it is difficult to say which--the sound deepened in volume1 the tremor became more decided1 the ear caught the rattle of window-sashes1 gas-fixtures1 and other movable objects2 the men in the office 444 glanced hurriedly at each other and sprang to their feet4444 ,and then all was bewilderment and confusion4 ,the long roll deepened and spread into an awful roar1 that seemed to pervade at once the troubled earth and the still air above and around4 ,the tremor was now a rude1 rapid quiver1 that agitated the whole lofty1 strong-walled building as though it were being shaken--shaken by the hand of an immeasurable power1 with intent to tear its joints asunder and scatter its stones and bricks abroad4444 ,there was no intermission in the vibration4444 ,from the first to the last it was a continuous jar1 adding force with every moment1 and1 as it approached and reached the climax of its manifestation1 it seemed for a few terrible seconds that no work of human hands could possibly survive the shocks4 ,the floors were heaving under-foot1 the surrounding walls and partitions visibly swayed to and fro1 the crash of falling masses of stone and brick and mortar was heard overhead and without4444 ,for a second or two it seemed that the worst had passed1 and that the violent motion was subsiding4 ,it increased again and became as severe as before4 ,none expected to escape4 ,a sudden rush was simultaneously made to endeavor to attain the open-air and fly to a place of safety2 but1 before the door was reached all stopped short1 as by a common impulse1 feeling that hope was vain--that it was only a question of death within the building or without1 of being buried beneath the sinking roof or crushed by the falling walls4 ,the uproar slowly died away in seeming distance4 ,the earth was still1 and oh6 the blessed relief of that stillness4 ,if somewhat sensational in form1 this report gives an extremely vivid and generally accurate account of the great shock4 ,other observers in ,charleston concur in dividing the movement into five phases4 ,the preliminary tremors and murmuring sound lasted about twelve seconds1 and1 although they increased in strength1 they were succeeded somewhat suddenly by the violent oscillations of the second phase1 followed by a third phase of much less intensity and a fourth of stronger oscillations1 these three phases lasting altogether about fifty seconds4 ,the fifth phase1 in which the tremors died out rather rapidly1 continued about eight seconds2 so that the total duration of the earthquake was not less than seventy seconds4 ,the variation of the intensity with the time is represented roughly by the curve in ,fig4 #bf4 ,illustration3 ,,fig4 #bf4--;,curve of intensity at ,charleston4 7,dutton47 ,at ,charleston1 there were thus two decided maxima of intensity1 nearly equal in strength1 though the first seems to have been slightly more powerful than the second4 ,as in the ,andalusian earthquake1 the intervening tremors were imperceptible at a distance from the epicentre1 and the earthquake appeared in the form of two distinct shocks1 separated by an interval the average duration of which was estimated at slightly less than half a minute4 ,at most places1 the first shock is described as the stronger1 but the difference in intensity of the two parts could not have been great1 for both were noticed at several places more than #fjj miles from the epicentre4 ,visible ,earth-,waves4--,many persons in the meizoseismal area assert that they saw waves moving along the surface of the ground4 ,at ,charleston1 according to an observer who was facing a street-lamp at the time1 the progress of the waves as they passed the house1 going towards the south-east1 was plainly observed1 although they travelled with incomparable swiftness4 ,the shadow of each moving ridge cast from the gas-light was distinctly seen4 ,the waves were not in long rollers1 but had rather the appearance of 'ground-swells' in deep water1 the height of which from crest to trough he estimated at not less than two feet4 ,in the words of another observer1 ,the vibrations increased rapidly and the ground began to undulate like the sea4 ,the street was well lighted1 having three gas-lamps within a distance of #bjj feet1 and ,i could see the earth waves as they passed as distinctly as ,i have a thousand times seen the waves roll along ,sullivan's ,island beach4 ,the first wave came from the south-west1 and as ,i attempted to make my way 444 ,i was borne irresistibly across from the south side to the north side of the street4 ,the waves seemed then to come from both the south-west and north-west1 and crossed the street diagonally1 intersecting each other1 and lifting me up and letting me down as if ,i were standing on a chop sea4 ,i could see perfectly1 and made careful observations1 and ,i estimate that the waves were at least two feet in height4 ,,the ,,double ,,epicentre4 ,for seismological purposes1 it is unfortunate that the epicentral district should be one containing so few buildings and other objects that could preserve the effects of the shock4 ,it is for the most part a barren1 forest-clad region1 in places swampy1 with occasional scattered houses4 ,but it is crossed by three lines of railway diverging from ,charleston1 and the damage which they suffered supplements to some extent the defects arising from the scarcity of buildings4 ,these railway lines are the ,south ,carolina1 the ,north-,eastern1 and the ,charleston and ,savannah1 denoted by the letters ,a1 ,b1 and ,c1 respectively1 in ,figs4 #bh and #bi4dj ,it will be convenient to follow ,major ,dutton1 and trace the variation of intensity exhibited along each line4 ,for six miles along the ,south ,carolina ,railway 7,a7 the damage to the line1 though indicative of a strong shock1 was of little consequence4 ,in the first half of this distance no repairs were required1 but at #c-b_/c miles the rails were bent and the joints between them opened2 at #e miles1 the fish-plates were torn from their fastenings and the joints between the rails opened seven inches2 and at nearly #f miles the joints were again opened1 and the road-bed depressed six inches4 ,after this point1 deflections of the line and elevations and depressions of the road-bed were no longer rare4 ,near the #i-mile point1 the intensity of the shock seemed to increase most rapidly2 lateral displacements of the line became more frequent as well as greater in amount4 ,the distortions of the lines were probably greatest between #aj and #aa miles2 here they were often displaced laterally1 sometimes depressed or elevated1 and occasionally twisted into ,s-shaped curves1 while many hundred yards of the track were shoved bodily towards the south-east4 ,the buckling always took place when this lateral shoving encountered a rigid obstacle1 usually a long rigid trestle4 ,at the north-western end of the trestle the accumulation of rails resulted in a sharp kink4 ,corresponding extensions of the track by the opening of the joints and shearing of the fish-plate bolts occurred some distance to the north-westward4 ,at #aa-a_/b miles1 the lines were again stretched and the joints opened by about seven inches2 but1 from this point for more than four miles1 the sharp kinks revealing a sliding of the track were entirely absent1 though there were still long slight flexures in the lines and changes of level in the road-bed4 ,the railway in this section traverses a district which is partly a swamp and partly a rice-field2 and thus it may be1 as ,major ,dutton suggests1 that the ground was less fitted to preserve the effects of the shock4#da ,at about #ah miles1 the line reaches higher and firmer ground2 and1 from here to ,summerville 7#ba-b_/c miles71 there were many sinuous flexures4 ,for six miles farther1 violent distortions of the rails ceased to occur1 the rate of decrease in intensity being most marked near the #bc-mile point4 ,the last flexure occurred at ,jedburgh 7#bg-a_/b miles7 at the south end of a long1 heavy trestle 7,fig4 #bg74 ,illustration3 ,,fig4 #bg4--;,flexure of rails at ,jedburgh4 7,dutton47 ,there is thus a certain symmetry in the damage to this line with respect to a point about #ae or #af miles from the ,charleston terminus4 ,the changes of intensity are most rapid at distances of about #i and #bc miles from the terminus4 ,also1 on the south-east side of the #af-mile point1 the longitudinal displacements of the line are always to the south-east2 on the other side1 always to the north-west4 ,major ,dutton therefore infers that the epicentre must be on a line drawn nearly through the #af-mile point at right angles to the railway4 ,somewhat similar changes were noted along the ,north-,eastern ,railway 7,b71 the ,charleston terminus of which is about three-quarters of a mile to the south-east of that of the ,south ,carolina ,railway4 ,slight flexures in the line occurred at distances of #a-a_/b and #d miles from the terminus1 and at about #f miles the road-bed was depressed1 in one part by as much as #bb inches4 ,at about #f-a_/c miles1 the joints between the rails were opened #ad inches1 and there were slight sinuous flexures in the line near the #g-mile and #h-mile points4 ,the indications of great intensity then rapidly increased1 the rate of change being greatest near the #i-mile point4 ,here1 there was a long lateral flexure with a shift of #d inches eastward4 ,half-a-mile farther1 the fish-plates were broken and the rails parted #h-a_/b inches4 ,a little beyond the #aj-mile point1 an embankment #ae feet high was pushed #d-a_/b feet eastward along a chord of #aej feet4 ,at the #ab-mile point and beyond1 fish-plates were broken1 lines were bent and the joints opened2 the road-bed was cut by a series of cracks1 one of which was #ba inches wide1 while the beginning of a long trestle was shifted #h-a_/c feet to the west4 ,from #ab-a_/b to #ad-a_/b miles1 several buildings were damaged or destroyed by a movement which was clearly more vertical than horizontal4 ,near the #af-mile point1 the ground was fissured and thrown into ridges1 the rails being similarly bent in a vertical plane4 ,soon after this1 the line reaches a broad1 sandy tract1 and1 though the thickness of the sand is probably not much more than #dj feet in any place1 the disturbances diminish almost at once1 and1 for a distance of more than two miles1 there was little damage done to the line4 ,at ,mount ,holly ,station 7#ah miles71 the intensity was so slight that the houses suffered no injury more serious than the loss of chimneys4 ,half-a-mile farther1 the ground becomes less sandy1 and the effects of the shock more distinct4 ,the lines were bent in places for about a quarter of a mile1 after which they again pass into the sandy area with a decrease of damage1 the last flexure being near the #ba-mile point4 ,the rate of change of intensity in this part of the line appears to have been greatest at a distance of about #ai-a_/b miles from the terminus1 but the exact distance is obviously somewhat uncertain4 ,there is again a rough symmetry in the damage to the line1 the central point being about #ad miles from the ,charleston terminus4 ,a line drawn through this point at right angles to the ,north-,eastern ,railway 7or rather to that part of it between the #i-mile and #ai-a_/b-mile points7 should pass through the epicentre4 ,it meets the corresponding line for the ,south ,carolina ,railway in a point which is indicated in ,figs4 #bg and #bh by a small circle 7,w74 ,houses and other buildings are rare in the surrounding district2 but1 as the intensity of the shock diminished outwards in all directions1 this point must mark approximately the position of the epicentre4 ,as it is close to the ,woodstock ,station on the ,south ,carolina ,railway1 it is called by ,major ,dutton the ,woodstock epicentre4 ,the ,charleston and ,savannah ,railway 7,c7 uses the same lines as the ,north-,eastern for the first seven miles from ,charleston1 and then turns off in a south-westerly direction4 ,for #d-a_/b miles from the junction the signs of disturbance were few and unimportant4 ,the railway then crosses the ,ashley ,river1 the banks of which slid towards one another and jammed the drawbridge2 but for four miles farther there was no serious damage done to the lines4 ,at about #af-a_/b miles the effects of the shock became rapidly more apparent4 ,for nearly #a-a_/b mile the entire railroad was deflected into an irregular curve1 the displacement being greatest at the bridge1 where it crosses the ,stono ,river4 ,here1 it was as much as #cg inches to the south4 ,after ,rantowles ,station 7#ah miles71 there were many displacements1 both lateral and vertical4 ,at #ah-a_/b miles1 a long southward deflection began1 the amount of which reached #be inches at the #ai-mile point1 #ej inches half-a-mile farther on1 and was still greater at #bj-b_/c miles4 ,for two miles more1 sinuous flexures were continuous1 but1 at the #bb-b_/c-mile point1 they rapidly disappeared1 the railroad passing on to higher and firmer ground4 ,between #be and #bg miles1 there were occasional slight flexures in the line or depressions of the railroad2 but1 after the #bg-a_/d-mile point1 they seldom occur1 and1 when they do1 are of little consequence4 ,some of the effects described in the last paragraph may1 as ,major ,dutton suggests1 be due to the varying nature of the surface-rocks4 ,it is important to notice1 however1 that disturbances of the lines were exceedingly rare in the section that lies nearest to the ,woodstock epicentre1 and that they increase in violence for some distance from that region1 the maximum intensity being reached a mile or two to the west of ,rantowles ,station4 ,this points clearly to the existence of a second focus4 ,unfortunately1 there are very few houses or other objects in the neighbourhood1 and the position of the corresponding epicentre cannot be determined accurately4 ,major ,dutton places it in the position indicated by a small circle 7,r71 and calls it the ,rantowles epicentre from its vicinity to the station of that name4 ,if the meizoseismal area had been a thickly populated one1 the evidence of ruined and damaged houses would have provided materials for the construction of isoseismal lines surrounding the two epicentres4 ,it is difficult1 as it is1 to gauge the equality of the effects on objects so different as railway-lines and buildings2 and the isoseismals shown in ,figs4 #bh and #bi can therefore lay no claim to accuracy4 ,fig4 #bh shows the epicentral isoseismals as they are drawn by ,mr4 ,earle ,sloan4 ,they do not correspond to the degrees of any definite scale of seismic intensity2 but they may be taken as representing the impressions of a very careful observer1 who traversed the district immediately after the occurrence of the earthquake1 and who1 when drawing these lines1 was biassed by no preconceived theory4 ,major ,dutton1 relying chiefly on ,mr4 ,sloan's written notes1 interprets the evidence differently1 and obtains the series of curves shown in ,fig4 #bi4 ,in this case1 also1 the isoseismals correspond to no expressed standard of intensity4 ,they are intended merely to represent the forms of the curves1 and1 by their less or greater distance apart1 the more or less rapid rate at which the intensity varied4 ,the chief difference between the two maps concerns the form of the ,woodstock isoseismals4 ,major ,dutton draws them approximately circular1 leaving the map blank towards the north1 where hardly any evidence was forthcoming4 ,mr4 ,sloan attributes the scantiness of effects here to a diminution of intensity1 and makes the lines curve in towards the epicentre4 ,they certainly must do so in crossing the ,north-,eastern ,railway2 and the somewhat southerly trend of ,mr4 ,sloan's curves to the east of this railway seems to me to furnish the better representation of the distinctly greater intensity in that region4 ,illustration3 ,,fig4 #bh4--;,epicentral isoseismal lines of ,charleston earthquake according to ,mr4 ,sloan4 7,dutton47 ,illustration3 ,,fig4 #bi4--;,epicentral isoseismal lines of ,charleston earthquake according to ,major ,dutton4 7,dutton47 ,more important1 however1 than this divergence of opinion is the agreement in one respect between the two sets of curves4 ,both show a marked expansion around the points known as the ,woodstock and ,rantowles epicentres1 especially about the former1 and a contraction in the intermediate region4 ,the evidence of these isoseismals therefore confirms that of the damaged railway lines1 and establishes ,major ,dutton's inference that there were two distinct foci1 the epicentres of which were about thirteen miles apart4 ,,origin ,,of ,,the ,,double ,,shock4 ,in the last chapter1 it was shown that the double shock of the ,andalusian earthquake could be due only to two distinct impulses taking place either within the same focus or1 more probably1 in two detached foci4 ,similar reasoning applies to the ,charleston earthquake4 ,the double maximum or double shock was observed in no less than fourteen ,states4 ,moreover1 the interval between the two maxima at ,charleston appears from ,fig4 #bf to have been about #cd seconds in length4 ,thus1 the duplication of the shock cannot have been a merely local phenomenon1 nor can it have resulted from the separation into two parts of the earth-waves proceeding from a single disturbance4 ,each maximum must therefore be connected with a distinct impulse4 ,combining this inference with ,major ,dutton's discovery of the double focus1 no doubt can remain as to the origin of the repeated shock4 ,it is clear1 also1 that the impulse at the ,woodstock focus was the stronger of the two2 for the isoseismals spread out more widely round the corresponding epicentre1 and there was no rapid decline of intensity from that point1 such as might be associated with a weaker disturbance within a shallow focus4 ,illustration3 ,,fig4 #cj4--,planes of oscillation of stopped pendulum clocks at ,charleston4 ,again1 since the earlier part of the shock is almost uniformly described as the stronger1 it follows that the ,woodstock focus was the first in action4 ,a curious fact recorded by ,major ,dutton supports this inference4 ,in ,charleston1 four clocks were stopped by the shock1 the errors of which at the time were certainly less than eight or nine seconds4 ,the planes in which their pendulums oscillated are shown by the lines lettered ,a1 ,b1 ,c1 and ,d in ,fig4 #cj1 the broken lines ,w and ,r representing respectively the directions from ,charleston of the ,woodstock and ,rantowles epicentres4 ,clock ,a stopped at #i;h4 #eam4 #js41 ,b at #i;h4 #eam4 #aes41 ,c at #i;h4 #eam4 #afs41 and ,d 7which had been reset to the second earlier in the day7 at #i;h4 #eam4 #dhs4 ,now1 if the plane of oscillation coincided nearly with the direction of the shock1 the only effect would be a temporary change in the period of oscillation2 but if it was at right angles to the direction of the shock1 the clock might be stopped by the point of the pendulum catching behind the graduated arc in front of which it oscillated4 ,the planes of the first three clocks1 it will be seen1 were approximately at right angles to the direction of the ,woodstock epicentre1 and ,b and ,c were indeed stopped in the manner just described2 while the plane of shock ,d was nearly perpendicular to the direction of the ,rantowles epicentre4 ,as the pendulums of ,b and ,c might make a few staggering oscillations before their final arrest1 ,major ,dutton assigns #i;h4 #eam4 #abs4 as the epoch of the first maximum at ,charleston2 and1 as the interval between the two maxima was about #cd seconds1 this would give about #i;h4 #eam4 #dfs4 for the epoch of the second maximum--a time which agrees very closely with that given by clock ,d4 ,thus1 clocks ,a1 ,b1 and ,c must have been stopped by the ,woodstock vibrations1 and clock ,d about half-a-minute later by those coming from the ,rantowles focus4 ,,depth ,,of ,,the ,,seismic ,,foci4 ,two methods of estimating the depth of the seismic focus have been described in the preceding pages--namely1 ,mallet's1 depending on the angle of emergence1 and ,falb's1 based on the interval between the initial epochs of the sound and shock4 ,to these1 ,major ,dutton adds a third method1 in which he relies on the rate at which the intensity of the shock varies with the distance from the epicentre4 ,dutton's ,method of determining the ,depth of the ,focus4--,if the seismic focus is either a point or a sphere1 and the initial impulse equal in all directions1 and if the intensity of the shock diminishes inversely as the square of the distance from the focus1 then the continuous curve in ,fig4 #ca will represent the variation of intensity along a line passing through the epicentre ,e4 ,the form of the curve on these assumptions does not depend in any way on the initial intensity of the impulse2 it is governed solely by the depth of the focus4 ,the deeper the focus1 the flatter becomes the curve1 as we have seen in discussing the ,ischian earthquakes 7p4 #fh74 ,in all directions from the epicentre1 the intensity at first diminishes slowly2 but the rate of change of intensity with the distance soon becomes more rapid1 until it is a maximum at the points ,c1 ,c2 after which it again diminishes and dies out very slowly when the distance becomes great4 ,it will be evident from ,fig4 #ah that the deeper the focus the greater also is the distance ,,ec of the points where the intensity of the shock changes most rapidly4 ,it may be easily shown1 indeed1 that this distance always bears to the depth of the focus the constant ratio of #a to sqrt7#c71 or about #a to #a4gc4db ,now1 if a series of isoseismals could be drawn corresponding to intensities which differ by constant amounts1 we should have a series of circles like those surrounding the ,woodstock epicentre in ,fig4 #bi1 the distance between successive lines at first decreasing gradually until it is a minimum at the dotted circle and afterwards gradually increasing4 ,this dotted circle is obviously that which passes through all points where the intensity of the shock changes most rapidly4 ,major ,dutton calls it the index-circle and1 when its radius is known1 the depth of the focus is at once obtained by multiplying the radius by #a4gc4 ,in #aheh1 ,mallet proposed a method which bears some resemblance to the above1#dc but depending only on the intensity of the longitudinal waves4 ,major ,dutton claims for his method that the effects of the longitudinal and transverse waves are not separated1 that it takes account of the total energy irrespective of direction or kind of vibration4 ,illustration3 ,,fig4 #ca4--;,diagram to illustrate ,dutton's method of determining depth of seismic focus4 ,objections to ,dutton's ,method4--,i have described this method somewhat fully1 though it seems to me open to more serious objections than ,mallet's first method which it is intended to replace4 ,we have1 in the first place1 no reason for supposing that the focus is either a point or a sphere1 or that the initial impulse is uniform in all directions4 ,if the earthquake were caused by fault-slipping1 both assumptions would be untrue1 and it is for those who employ the method to prove their validity4 ,but of greater consequence is the fact that1 if the method were correct1 all earthquakes originating at the same depth must have index-circles of equal radii4 ,if the depth of the focus were1 say1 ten miles1 then the index-circle must have a radius of about six miles1 whether the initial disturbance be of extreme violence or so weak that it is not felt at the surface at all1 much less so far as six miles from the epicentre4 ,the law of the inverse square is of course only true for a perfectly elastic and continuous medium1 and the real curve of intensity is not that of the continuous line in ,fig4 #ca1 but something of the form represented by the dotted line4 ,in this case1 the rate of change of intensity is greatest at some point ,c'1 nearer than ,c to the epicentre1 and the application of ,major ,dutton's rule would give a point ,f'1 nearer the surface than ,f1 for the focus4 ,thus1 assuming that the method can be applied in practice--and the test involved is one so delicate that it would be difficult to apply except with refined measurements--then all that we can assert is that the calculated depth is certainly less than the true depth4 ,dutton's ,estimate of the ,depth of the ,seismic ,foci4--,in applying the method1 the chief difficulty is to obtain a series of isoseismal lines corresponding to equidistant degrees of intensity4 ,as already pointed out1 those given in ,fig4 #bi are merely diagrammatic2 but the index-circle of the ,woodstock focus1 represented by the dotted line1 is made to pass through the places where the rate of change of intensity was found to be greatest4 ,the radius of this circle being very nearly seven miles1 it follows that the resulting depth of the ,woodstock focal point would be about twelve miles4 ,major ,dutton regards this estimate as probably correct within two miles4 ,in the neighbourhood of the ,rantowles epicentre1 the isoseismals in both ,figs4 #bh and #bi are elongated in form4 ,the index-circuit1 as it would be called in such a case1 cannot be drawn completely1 but its radius parallel to the shorter axis of the curves is about #d-a_/b miles1 and the resulting depth of the ,rantowles focal point would be nearly eight miles4 ,,velocity ,,of ,,the ,,earth-,,waves4 ,the recognition of the double epicentre is1 from a geological point of view1 the most important fact established by the investigation of the ,charleston earthquake4 ,but of equal interest1 from a physical point of view1 is the estimate of the velocity of the earth-waves1 which is probably more accurate than that determined for any previous shock4 ,owing to the existence of the standard time system in the ,united ,states1 the exact time is transmitted once a day to every town and village within reach of a telegraph line2 and the effect of small errors in the observations is considerably lessened by the great distance traversed by the earth-waves1 sixty good reports coming from places more than #ejj miles from the epicentre1 and ten from places more than #hjj miles distant4 ,the total number of time-records collected is #caf1 but of these #acj had to be rejected1 either because they were obviously too early or too late1 or because they were only given to the nearest five-minutes' interval4 ,there remain #ahf observations which are divided by ,major ,dutton into four classes according to their probable value4 ,in an earthquake of such great duration 7about #gj seconds at ,charleston71 it is necessary in the first place to select some special phase of the movement as that to which the records mainly refer1 and then to determine as accurately as possible the time of occurrence of this phase at the origin4 ,there can be little doubt as to which phase should be chosen4 ,the shock began with a series of tremors1 which passed somewhat abruptly into the oscillations that formed the first and stronger maximum4 ,these were clearly felt all over the disturbed area1 and1 as the beginning of the first maximum at places near the epicentre and the beginning of the shock at distant stations were probably due to the same vibrations1 this particular phase may be fairly selected as that to which the time-measurements refer4 ,the time of this phase at the origin can only be ascertained from the time at which it reached ,charleston1 and our knowledge of this depends chiefly on the evidence of stopped clocks4 ,how unreliable this may be is well known4 ,clocks may indeed be stopped at almost any phase of the movement2 and1 whenever stopped clocks can be compared with really good personal observations1 they almost invariably show a later time4 ,at ,charleston three good clocks were stopped by the vibrations from the ,woodstock focus1 two of them being in close agreement 7p4 #aba72 and1 allowing for a few oscillations before their final arrest1 ,major ,dutton places the time of arrival of the selected phase at ,charleston at #i;h4 #eam4 #abs4 ,p4,m4 ,the evidence of these clocks is also supported by that of other observations1 which show that #i4ea was certainly the nearest minute to the time of arrival1 and favour a somewhat later instant much more strongly than one a little earlier4 ,now1 the distance of ,charleston from the ,woodstock epicentre is sixteen miles1 and from the corresponding focus 7with the calculated value of its depth7 twenty miles4 ,a first estimate of the velocity gives a value of a little more than three miles a second1 and the time at the ,woodstock focus may therefore be taken as #i;h4 #eam4 #fs4 with a probable error of a few seconds4#dd ,proceeding to the observations at a distance1 we find them1 even after all rejections1 to be very different in value4 ,they were therefore divided into groups consisting of observations which are as nearly as possible homogeneous4 ,the first group contains five records from places between #deb and #fde miles from the ,woodstock epicentre4 ,they give the time to within #ae seconds1 obtained from an accurately kept clock1 or from a clock or watch that was compared with such within a few hours of the earthquake4 ,the resulting velocity is #c4bcf plus or minus 4#aje miles 7or #ebje plus or minus #afh meters7 per second4#de ,in the second group there are eleven observations 7between distances of #dch and #ggj miles7 which satisfy the same conditions as those in the first group1 except that the time is only given to the nearest minute or half-minute4 ,the velocity obtained from them is #c4bbf plus or minus 4#adg miles 7or #eaib plus or minus #bcf metres7 per second4 ,the third group included all but the above records and those obtained from stopped clocks4 ,they are #abe in number 7between distances of #hj and #ibd miles71 but it is uncertain whether they correspond to the selected phase of the movement1 and the errors of the clocks and watches used were unknown4 ,they give a mean velocity of #c4jac plus or minus 4#jbg miles 7or #dhdh plus or minus #dc metres7 per second4 ,in the fourth group1 we have the evidence of #de stopped clocks 7at places between #bj and #hee miles71 which apparently give a velocity of #b4fch plus or minus 4#aje miles 7or #dbde plus or minus 4#afh metres7 per second4 ,at six places1 however1 the times indicated by stopped clocks can be compared with good personal observations2 and these show that the time of traverse from the origin obtained from the former is on an average #a4bh times the time of traverse obtained from the latter4 ,if a similar correction be made for all the stopped clocks1 the corrected velocity of the earth-waves would be from #c4ag to #c4bc miles 7or #eajj to #ebjj metres7 per second4 ,in obtaining the mean value of the velocity from all the observations1 those of the fourth group are omitted1 and the weights of the first three groups are taken inversely as the squares of the probable errors--that is1 as #b3 #a3 #d4 ,the resulting mean velocity is #c4bba plus or minus 4#jej miles 7or #eahd plus or minus #hj metres7 per second2 and1 though it does not follow that all other estimates are erroneous 7for the velocity may vary with the strength of the earthquake and with other conditions71 it is probable that this result is more nearly accurate than any other previously obtained4 ,,miscellaneous ,,phenomena4 ,fissures and ,sand-,craters4--,in point of size1 there was nothing remarkable about the fissures in the ground produced by the ,charleston earthquake4 ,the largest were only a few hundred yards long1 and1 except near the river-banks1 they rarely exceeded an inch in width4 ,they seem1 however1 to have been unusually abundant2 for1 within an area of nearly #fjj square miles surrounding the two epicentres1 they were almost universal1 and over a much wider area they still occurred in great numbers1 though with somewhat less continuity4 ,from many of these fissures water was ejected1 carrying with it large quantities of sand and silt1 and so abundantly that every stream-bed1 even though generally dry in summer1 was flooded4 ,by the passage of the water1 some part of the fissures was often enlarged into a round hole of considerable size1 ending in a craterlet at the surface4 ,certain belts within the fissured area contained large numbers of these craterlets1 of all sizes up to twenty feet or more in diameter4 ,one near ,ten-,mile ,hill was twenty-one feet across4 ,in this district1 they were apparently larger and more numerous than elsewhere2 many acres of ground being covered with sand1 which1 close to the orifices1 was two feet or more in depth4 ,here and there1 the water was ejected with considerable violence1 as was manifest from the heights to which it spurted4 ,the testimony of witnesses on this point is of course doubtful1 for the earthquake occurred after nightfall1 but in a few places the branches and leaves of trees overhanging the orifices were smirched with sand and mud up to a height of fifteen or twenty feet4 ,effects on ,human ,beings4--,it is interesting to notice the behaviour of different races under the influence of a violent earthquake1 and perhaps no greater contrast could be observed than between the calmness exhibited by the ,japanese in the presence of disaster and the wild fear merging into helpless panic that characterised the residents1 and especially the negroes1 of ,charleston4 ,as we dashed down the stairway1 says a writer already quoted 7p4 #ajh71 and out into the street1 from every quarter arose the shrieks1 the cries of pain and fear1 the prayers and wailings of terrified women and children1 commingled with the hoarse shouts of excited men4444 ,on every side were hurrying forms of men and women1 bareheaded1 partially dressed1 some almost nude1 and all nearly crazed with fear and excitement4444 ,a few steps away1 under the gas-lamp1 a woman lies prone and motionless on the pavement1 with upturned face and outstretched limbs1 and the crowd which has now gathered in the street passes her by1 none pausing to see whether she is alive or dead 444 no one knows which way to turn1 or where to offer aid2 many voices are speaking at once1 but few heed what is said4 ,between the selfish rush for safety here described and the calm interest of the most distant observers1 ,major ,dutton records nearly every possible variety of mental effects1 the actions resulting from which may be roughly classified as follows3 ,a4 ,no persons leave their rooms4 ,b4 ,some persons leave their houses4 ,c4 ,most persons run into the streets1 which are full of excited people4 ,d4 ,people rush wildly for open spaces and remain all night out-of-doors4 ,in the map of the isoseismal lines 7,fig4 #be71 the dotted curves bound the areas in which the effects corresponding to the three highest degrees of the above scale were observed4 ,the curve for the first degree 7,a7 coincides of course with the isoseismal line of intensity #b4 ,it will be seen that there is a certain rough correspondence between these curves and the isoseismal lines4 ,the curve ,d and the isoseismal #h are close together2 in other words1 people thought it wiser to camp out-of-doors for the night if the shock was strong enough to damage buildings slightly4 ,the curve ,c and the isoseismal #f are similarly connected2 that is1 if the movement made pictures swing1 etc41 people rushed into the streets4 ,on the whole1 the curve ,b and the isoseismal #c roughly coincide1 or1 if the shock was not strong enough to make doors and windows rattle1 some persons left their houses and public meetings were dispersed4 ,feeling of ,nausea4--,a feeling of nausea was experienced by many persons at the time of the earthquake1 somewhat rarely it appears in the neighbourhood of the epicentre and even outside the isoseismal #g1 but more frequently beyond these limits1 and perceptible as far as the broken line in ,fig4 #be4 ,the most distant places at which it was noticed are ,blue ,mountain ,creek 7,new ,york7 and ,dubuque 7,iowa71 which are respectively #hbc and #hhf miles from ,charleston4 ,,after-,,shocks4 ,as ,summerville lies six miles to the north-west of the ,woodstock epicentre and ,charleston #af miles to the south-east1 it is probable that many of the after-shocks were unfelt and a still greater number unrecorded4 ,in ,charleston1 seven shocks1 all much slighter than the principal shock1 were felt during the night of ,august #ca--,september #a1 and thirty before the end of ,september4 ,of these1 the shock of ,september #crd1 at #aa ,p4,m41 was the strongest1 but those which occurred on ,september #ast1 #bnd1 #bast1 and #bgth were also described as severe1 and the remainder as moderate or slight4 ,for weeks after the great shock1 curious sensations were distinctly perceptible during the still hours of the night as though the crust of the earth were resting on a gelatinous mass in constant motion4 ,the last shock felt in ,charleston seems to have been one recorded on ,march #ahth1 #ahhg4 ,at ,summerville1 many shocks occurred that were scarcely perceptible in ,charleston1 and those noticed in both places were usually stronger1 and the motion more nearly vertical1 at ,summerville4 ,the peculiar characteristic of all of them was the deep1 solemn1 powerful boom1 like the report of a heavy cannon1 usually accompanied by a quick1 short jar4 ,sometimes it was prolonged into a heavy roar or rumble1 as if many reports were delivered in a volley4 ,the number of them was never recorded4 ,on ,september #crd1 ,mr4 ,w4,j4 ,mc,gee1 of the ,united ,states ,geological ,survey1 arrived at ,summerville4 ,during the evening of that day1 detonations were heard at intervals1 averaging perhaps half-an-hour1 accompanied occasionally by very slight spasmodic tremors of an instant's duration4 ,they were much like peals of thunder at a distance of half-a-mile or more1 though rather more muffled4 ,it was my impression1 ,mr4 ,mc,gee remarks1 that the sound was sometimes about as grave as the ear can perceive1 resembling somewhat the tremulous roar sometimes accompanying combustion in locomotives4 ,these sounds continued1 but with diminishing frequency1 throughout the remainder of the year and as late as ,july #ast1 #ahhg4 ,,origin ,,of ,,the ,,earthquake4 ,major ,dutton's valuable monograph is a record of the earthquake-phenomena4 ,he offers no theory as to the cause of the shock1 and is therefore in no way responsible for the account given in the remaining part of this chapter4 ,that there were two seismic foci he has shown1 ,i think1 conclusively2 and my object is now to trace out briefly the probable nature of the movements that produced the double shock4 ,referring to ,figs4 #bh and #bi1 it will be seen that1 according to both ,mr4 ,sloan and ,major ,dutton1 the isoseismals surrounding the ,rantowles epicentre are distorted along a line which runs from a few degrees east of north to a few degrees west of south4 ,their oval form is in all probability connected with a focus elongated in about the same direction4 ,near the ,woodstock epicentre1 the isoseismals are drawn differently in the two maps1 and in neither case do they offer any sure guide as to the form of the seismic focus4 ,if1 however1 we follow ,mr4 ,sloan's interpretation of the evidence1 and suppose the earthquake to have been fault-formed1 then it is probable that in this region the fault bends round slightly towards the east4 ,the only other evidence on this point is that afforded by the regions of defective intensity1 real or apparent1 along the three railway-lines diverging from ,charleston4 ,one of these occurred near ,mount ,holly ,station on the ,north-,eastern ,railway 7,b1 ,figs4 #bh and #bi71 another for four miles starting from the #aa-a_/b-mile point on the ,south ,carolina ,railway 7,a71 and a third along the ,charleston and ,savannah ,railway 7,c7 over a distance of four miles from the ,ashley ,river4 ,in the first two cases1 ,major ,dutton suggests that the less amount of damage was due to the nature of the soil traversed by the railway2 but it is on the softer ground that the effects of an earthquake-shock are generally the more disastrous4 ,on the whole1 it seems to me probable that the three tracts referred to are really regions of less intensity1 and it is worthy of notice that they lie along a nearly straight line4 ,to show the bearing of these remarks1 let ,,cd 7,fig4 #cb7 represent the section of a fault and ,,ef that of the surface of the earth1 and suppose the rock-mass ,a to slip slightly and suddenly downwards4 ,then the particles of ,a at the surface of the fault will1 by impulsive friction1 be drawn sharply upwards1 and those of ,b correspondingly downwards2 so that the earth-waves in the two rock-masses will start in opposite phases of vibration4 ,along the line of fault1 every particle of rock1 being urged upwards and downwards almost equally1 will remain practically at rest4 ,thus1 regions of defective intensity may arise from partial interference by the spreading of either earth-wave in the adjoining rock-mass4 ,illustration3 ,,fig4 #cb4--;,diagram to explain origin of regions of defective intensity4 ,if this be the correct explanation1 the path of the originating fault may be taken as that indicated by the broken line in ,fig4 #bh1 a line which is nearly parallel to the chief branches of the isoseismal curves4#df ,as both epicentres lie on the west side of this line1 the fault must hade or slope in this direction4 ,the distortion of the ,woodstock isoseismals towards the north-west confirms the latter inference1 for the intensity of the shock is greater on the side towards which the fault hades4 ,from the comparative absence of earthquakes in ,south ,carolina1 we may infer that the fault is one subject to displacements at wide intervals of time4 ,the gradually increasing stress along its surface was relieved at one or two points in or near the ,woodstock focus on ,august #bgth and #bhth1 and perhaps during the preceding months4 ,but the first great slip took place suddenly in that focus1 and spread gradually southwards--for there was no interruption in the movement--until about half-a-minute later it reached the ,rantowles focus1 where the second great slip occurred4 ,eight or ten minutes afterwards there was another slip--in what part of the fault is uncertain--and this was followed at irregular intervals by many small movements gradually diminishing in frequency and in focal area4 ,within a year from the first disturbance1 the fault-system attained once more its usual condition of rest4 ,,references4 #a4 ,,dutton1 ,c4,e4--,the ,charleston ,earthquake of ,august #cast1 #ahhf4 ,amer4 ,geol4 ,survey1 ,ninth ,annual ,report1 pp4 #bji-ebh4 #b4 ,nature1 vol4 xxxv41 #ahhg1 pp4 #ca-cc2 vol4 lxiii41 #aija1 pp4 #afe-aff4 ,,footnotes3 #ch ,the authorities for this statement are ,mallet's ,catalogue of ,recorded ,earthquakes 7,brit4 ,assoc4 ,rep41 #aheb1 pp4 #a-agf2 #ahec1 pp4 #aag-bab2 #ahed1 pp4 #a-cbf71 which closes with the year #ahdb1 and ,fuchs' ,statistik der ,erdbeben von #ahfe-ahhe4 ,according to ,mallet1 there was an earthquake in ,s4 ,carolina in ,november #aggf1 and the ,new ,madrid earthquake of ,december #afth1 #ahaa1 was felt at ,charleston4 ,fuchs records two earthquakes at ,charleston on ,may #abth1 #ahgj1 and ,december #abth1 #ahgf2 and two in ,s4 ,carolina on ,december #abth and #acth1 #ahgi4 #ci #a4 ,recorded by a single seismograph1 or by some seismographs of the same pattern1 but not by several seismographs of different kinds1 the shock felt by an experienced observer4 #b4 ,recorded by seismographs of different kinds2 felt by a small number of persons at rest4 #c4 ,felt by several persons at rest2 strong enough for the duration or direction to be appreciable4 #d4 ,felt by several persons in motion2 disturbance of movable objects1 doors1 windows2 creaking of floors4 #e4 ,felt generally by every one2 disturbance of furniture and beds2 ringing of some bells4 #f4 ,general awaking of those asleep2 general ringing of bells2 oscillation of chandeliers1 stopping of clocks2 visible disturbance of trees and shrubs2 some startled persons leave their dwellings4 #g4 ,overthrow of movable objects1 fall of plaster1 ringing of church bells1 general panic1 without damage to buildings4 #h4 ,fall of chimneys1 cracks in the walls of buildings4 #i4 ,partial or total destruction of some buildings4 #aj4 ,great disasters1 ruins1 disturbance of strata1 fissures in the earth's crust1 rock-falls from mountains4 #dj ,in order to simplify these figures1 the rivers1 most of the inlets1 and other details are omitted4 ,small figures are added along the railway lines to denote the distance in miles from the stations in ,charleston4 #da ,if this were so1 the decrease in intensity would be only apparent2 but it may have been real1 and a possible explanation on this supposition is given later on 7p4 #ace74 #db ,if c be the depth of the focus1 a the intensity at unit distance from the focus1 and y the intensity on the surface at distance x from the epicentre1 then ya_/7c#bx#b7 ,the inclination of the curve at any point is given by dy_/dx-#b99;a99x_/7c#bx#b7b1 and this is a maximum when d#by_/dx#b or 7#c99x#b-;c#b7_/7;c#bx#b7c is zero1 which is satisfied when cx99sqrt7#c7 #dc ,british ,association ,report1 #aheh1 pp4 #aja-ajc4 #dd ,the above time would have to be increased by one second if the depth of the focus were very small1 and diminished by one second if it were as great as #bc miles2 the difference in either case being less than the probable error4 #de ,the method employed is as follows3 ,let t#j be the computed time 7#i;h4 #eam4 #fs47 at the focus1 x seconds the error in this estimate1 t the reported time at a given place1 ,d its distance from the focus in miles1 and y the number of seconds required to travel one mile2 then1 assuming that y does not vary with the distance1 we have x,dytt#j4 ,an equation of this form is obtained from each observation1 and the method of least squares is then applied to determine the most probable values of x and y4 #df ,this seems to me the more probable course4 ,it is possible1 however1 that the fault-line may pass from ,mount ,holly ,station to the east of the ,woodstock epicentre as shown in ,fig4 #bh1 and then to the west of the ,rantowles epicentre1 the fault changing its direction of hade in the intermediate district4 ,,chapter ,,vi4 ,,the ,,riviera ,,earthquake ,,of ,,february #bc,,rd1 #ahhg4 ,few earthquakes have aroused a more widespread interest than those which struck the thronged cities of the ,riviera on ,february #bcrd1 #ahhg4 ,the first and greatest of the shocks occurred at about #f4bj ,a4,m41 the second nine minutes later1 and the third1 intermediate in strength1 at about #h4ea ,a4,m4#dg ,all three shocks were of destructive violence1 the damage wrought by them extending along the coast and for a short distance inland from ,nice to beyond ,savona4 ,most of the injury to property and nearly all the loss of life were1 however1 concentrated on the eastern side of the frontier2 and it therefore fell to the lot of the ,italian ,government to provide for the scientific investigation of the earthquakes1 as well as to meet the wants of those deprived of home and support4 ,professors ,taramelli and ,mercalli1 who two years before had studied the earthquakes in ,andalusia1 were again nominated1 the former to examine the geology of the central regions1 and the latter to report on the seismic phenomena4 ,their joint memoir forms one of the most complete accounts that we possess of any earthquake1 and is the chief authority for the description given in this chapter4 ,another valuable monograph is that prepared by ,professor ,a4 ,issel1 of ,genoa1 who received an independent appointment from the same ,ministry4 ,a third official commission was also sent to estimate the amount of damage caused by the earthquakes in the ,italian towns and villages4 ,in ,france1 the destruction of property was much less serious1 and attention was confined chiefly to the records of the shock provided by magnetographs and other instruments in distant observatories4 ,in ,switzerland1 the effects remarked were merely those due to the evanescent vibrations of a remote earthquake2 but many interesting records were collected by the permanent seismological commission established in that country4 ,,damage ,,caused ,,by ,,the ,,earthquakes4 ,owing to variations in the nature1 foundation1 and site of buildings1 there is always great diversity in the destructive effects of an earthquake4 ,in one and the same town1 most of the houses may be razed to the ground1 while in their midst may be found some that are shattered but still standing1 and others perhaps that are practically unharmed4 ,the stronger after-shocks often complete the ruin of the partially damaged houses2 though in such cases the real loss is as a rule comparatively small4 ,the close succession of the two strong after-shocks of ,february #bcrd made it impossible as a rule to separate their effects from those due to the first shock2 but it has been roughly estimated that about one-quarter of the total damage was caused by the two after-shocks together4 ,to them also must be referred in part the comparatively small number of wounded1 many persons buried beneath the ruins having no doubt perished from subsequent falls before they could be extricated4 ,taking all three shocks together1 the total loss to property1 according to ,professor ,mercalli1 must be valued at about #bb million francs in ,italy alone4 ,for the province of the ,alpes ,maritimes in ,france1 full details are wanting1 but the loss there cannot fall far short of three million francs4 ,the total amount of damage must therefore be placed at about a million pounds4 ,from the figures given by the official commissions1 it appears that the earthquakes were most disastrous at ,diano ,marina and ,diano ,castello2 while other places1 such as ,oneglia1 ,bussana1 ,baiardo1 ,pompeiana1 and ,vallecrosia1 suffered only a little less severely4 ,at ,mentone about #aee houses1 and at ,nice about #fa houses1 were rendered uninhabitable1 and many others were badly injured4 ,in ,italy1 #fcc persons were killed1 #dcb seriously wounded1 and #ajd slightly wounded2 in ,france1 #g persons were killed and #cj seriously wounded1 the number of persons slightly wounded being unknown4 ,the majority of the deaths occurred in two or three places4 ,thus1 at ,diano ,marina1 #aij persons were killed and #ajb wounded2 at ,baiardo1 #bbj were killed and #fj wounded2 at ,bussana1 there were #ec killed and #bg wounded4 ,the death-rates were1 however1 comparatively small1 amounting for the above places to not more than #h-a_/b1 #ad1 and #f-a_/b per cent41 respectively2 figures which only slightly exceed those obtained for places in the meizoseismal area of the ,andalusian earthquake4 ,though the damage can hardly be regarded as excessive1 it was nevertheless largely due to the peculiar architecture prevalent in the ,riviera4 ,arches in the walls are common even in the upper storeys1 and1 in ,oneglia and ,diano ,marina1 if not also in other places1 the floors are nearly always brick arches abutting against the walls and without other lateral support4 ,professor ,mercalli believes that1 in private houses1 more than #ij per cent4 of the dead bodies were crushed beneath these fallen arches4 ,the height of the buildings is also great in proportion to the foundation and to the thickness of the walls2 and the main walls are interrupted by numerous apertures1 from the corners of which nearly all the fissures sprang4 ,in some of the coast towns1 the houses are built of rounded stones gathered from the beach1 or of rubble with stones of all shapes and sizes1 bound by cement of the poorest quality4 ,lastly1 as much of the damage due to previous earthquakes had been badly repaired1 it is evident that the destructiveness of the ,riviera earthquakes must to a great extent be referred to preventable causes4 ,the occurrence of the principal shock shortly after six on the morning of ,ash ,wednesday must also have increased the death-rate2 for many persons1 after a night of amusement1 had lain down for a short time and were sleeping heavily2 while others had already risen and were collected in the churches2 the circumstances in either case rendering escape more difficult4 ,taking account1 however1 of this accidental increase in the number of victims1 ,professor ,mercalli considers that the earthquake of #ahhg was the most disastrous of all those which have visited either the ,riviera or northern ,italy in the last three centuries2 though1 during the nineteenth century1 there were three ,italian earthquakes of far greater destructive power1 but all confined to the southern part of the peninsula--namely1 the ,neapolitan earthquakes of #ahje and #aheg1 and the ,ischian earthquake of #ahhc4 ,,preparation ,,for ,,the ,,earthquakes4 ,it is difficult1 as usual1 to specify the exact moment when the first earthquake of the #ahhg series took place2 but it is evident that the preparation for the great shock was very brief4 ,at ,oneglia1 it is alleged that faint shocks and sounds were observed many times1 chiefly at night1 during the month preceding ,february #bcrd2 though they were not at the time supposed to be of seismic origin4 ,a slight shock is also reported from ,diano at about midnight on ,february #ba-bb4 ,the first undoubted shock occurred on ,february #bbnd1 at about #h4cj ,p4,m41 or ten hours before the principal earthquake4 ,though very slight1 it was felt throughout the ,riviera and in part of ,piedmont4 ,another shock1 also weak1 took place at about #aa ,p4,m42 and a third1 sensible only in the eastern part of the ,ligurian ,apennines1 on ,february #bcrd1 at about #a ,a4,m42 at which time the tide-gauge at ,genoa recorded some abnormal oscillations4 ,an hour later1 a more important1 though by no means a strong1 shock occurred2 this was perceptible all over the ,riviera1 in ,piedmont1 and in ,corsica2 in other words1 it disturbed a region agreeing closely with the central area of the disastrous shock4 ,at about #e ,a4,m41 a fifth shock1 somewhat weaker than the preceding1 was felt over the same area1 concurrently1 or nearly so1 with another abnormal oscillation of the tide-gauge at ,genoa2 while a sixth shock was noticed at several places a few minutes before the great shock4 ,during the night of ,february #bb-bc1 nervous persons in many towns and villages were agitated without apparent reason4 ,birds and animals1 more sensitive than human beings to faint tremors1 were more distinctly affected1 especially for some minutes before the earthquake4 ,horses refused food1 were restless or tried to escape from their stables1 dogs howled1 birds flew about and uttered cries of alarm4 ,as these symptoms were noticed at more than one hundred and thirty places within the ,italian part of the central area1 there can be little doubt that they were caused by microseismic movements for the most part insensible to man4 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,the only complete map of the isoseismal lines is that drawn by ,professor ,mercalli4#dh ,in this map1 reproduced in ,fig4 #cc1 the continuous curves represent the principal isoseismal lines2 the dotted curves define the disturbed areas of two of the stronger after-shocks4 ,the meizoseismal area1 bounded by the curve marked #a in ,fig4 #cc1 is also shown on a larger scale in ,fig4 #cd4 ,at the places denoted by small circles in the latter figure1 the principal shock was disastrous1 some of the houses in each being either totally or partially ruined4 ,at those marked by a small cross1 the shock was almost ruinous2 in other words1 numerous houses were damaged1 but in no case was the injury of a serious character4 ,the meizoseismal area is thus a narrow band1 skirting the ,riviera coast from ,mentone to ,albissola1 a distance of #ajf miles1 and extending inland for not more than from nine to twelve miles4 ,the greatest intensity1 corresponding to the ruin of many houses with considerable loss of life1 was reached at only a few places between ,bussano and ,diano ,marina1 all lying within a littoral band about twenty miles in length and three to three and a half miles in width4 ,if1 however1 the epicentre had lain on land1 the area would have been much greater1 ,professor ,mercalli estimates about four times greater1 than its actual amount4 ,illustration3 ,,fig4 #cc4--;,isoseismal lines of the ,riviera earthquake4 7,mercalli47 ,the curve marked #b 7,fig4 #cc7 bounds the almost ruinous zone2 its expansion towards the north and contraction towards the west1 north-west1 and east1 being its most noteworthy features4 ,the next zone1 that of slight damage1 is contained between the isoseismals #b and #c1 the latter curve probably grazing the north end of ,corsica4 ,beyond this lies the strong zone1 in which the shock was generally felt without causing any damage to buildings4 ,its boundary 7marked #d7 passes near ,marseilles1 ,como1 and ,parma1 and includes nearly the whole of ,corsica2 towards the north-west1 in the valley of ,aosta1 it curves in towards the isoseismal #c4 ,in the outermost zone of all the shock was slight1 and towards the margin was only just perceptible4 ,the boundary1 which of course defines that of the disturbed area1 reaches as far north as ,basle and ,dijon1 to ,perpignan on the west1 ,trento1 ,venice1 and ,pordenone on the east1 and to the south as far as ,tivoli 7near ,rome7 and the northern end of ,sardinia4 ,in eastern ,switzerland1 it shows a marked curve inwards2 possibly1 as ,professor ,mercalli suggests1 from the vibrations having to cross the northern ,apennines in a direction nearly at right angles to their axis4 ,except for this bay1 however1 the curve differs little from a circle1 the centre of which lies in the sea1 a little to the south of ,oneglia1 close to the position assigned by other evidence to the epicentre4 ,the radius of this circle being about #bfd miles1 it follows that the disturbed area must have contained about #bai1jjj square miles--by no means a large amount for so strong an earthquake4 ,,position ,,of ,,the ,,epicentres4 ,it is evident1 from the form of the meizoseismal area shown in ,fig4 #cc1 that a mere fringe of it lies upon land1 and that the epicentre must be situated some distance out at sea4 ,other facts may be mentioned which point to the same conclusion4 ,there were1 for instance1 no purely vertical movements observed1 even in the districts where the damage done by the shock was greatest4 ,nor were any large landslips to be seen in those areas2 there were no lasting changes in the underground water-system2 and in general1 as ,professor ,mercalli remarks1 all the superficial distortions of the ground which are so characteristic of the epicentral area of a great earthquake were conspicuous by their absence4 ,there is evidence1 again1 of some disturbance of the sea-bed in the death and flight of fishes from great depths and in the seismic sea-waves recorded by the tide-gauges at ,genoa and ,nice4 ,these phenomena will be described in a later section1 but reference should be made here to an interesting observation at ,oneglia on the occurrence of some of the stronger after-shocks4 ,persons on the coast1 it is said1 saw the sea curling and moving1 and immediately afterwards the shock was felt4 ,in determining the position of the epicentre1 ,professor ,mercalli had recourse as usual to observations on the direction of the shock1 especially those derived from the oscillation of lamps or other suspended objects1 the projection or fall of bodies free to move1 fractures1 etc41 in damaged houses1 and the stopping of pendulum clocks4 ,such observations were made at #abj places--#gb in the western ,riviera and the ,alpes ,maritimes1 and #dh at ,piedmont1 ,lombardy1 and ,tuscany4 ,at many of these places the movement was extremely complicated4 ,in nearly all parts of the area most strongly shaken1 for instance1 the direction of the shock changed more than once2 and it was therefore necessary to select whenever possible the principal direction of the shock at each place4 ,in some towns1 such as ,oneglia1 ,mentone1 ,antibes1 ,cuneo1 etc41 the shock had two dominant directions1 and these appeared to be sensibly at right angles to one another2 an inclination which1 as ,professor ,mercalli suggests1 may be due in part to the approximation of the real directions to those of the principal walls of the houses in which the observations were made4 ,most of the lines of direction1 when plotted on the map1 converge towards an area lying between the meridians of ,oneglia and ,san ,remo1 and between nine and fifteen miles from the coast4 ,for places near the epicentre1 the most trustworthy1 in ,mercalli's opinion1 are those made at ,oneglia1 ,mentone1 ,taggia1 ,bordighera1 ,castel ,vittorio1 ,nice1 and ,genoa2 and the points in which these lines ,intersect one another are ,indicated by small crosses on the map of the meizoseismal area 7,fig4 #cd74 ,all of them lie at sea at distances between six and fifteen miles to the south of ,oneglia4 ,the most probable position of the principal epicentre is that marked by the small circle ,a1 which is situated about fifteen miles south of ,oneglia4 ,illustration3 ,,fig4 #cd4--,meizoseismal area of the ,riviera earthquake4 7,taramelli and ,mercalli47 ,there are1 however1 several lines of direction which can have no connection with this epicentre4 ,besides the east and west lines at ,nice1 ,mentone1 and ,antibes1 there are others at the same places which run north and south or nearly so4 ,professor ,mercalli believes that they were due to vibrations coming from a second focus lying to the south of ,nice1 and there are also several lines of direction at more distant places which converge towards the neighbourhood of the corresponding epicentre4 ,this conclusion receives unexpected support from some of the best time-records4 ,at the railway-stations of ,loano and ,pietra ,ligure1 the times of occurrence were given as #f;h4 #bjm4 #es4 and #f;h4 #bjm4 respectively--estimates which are probably accurate to within a few seconds2 for1 at the moment of the shock1 the officer who brought the exact time along the railway-line from ,genoa was at ,loana1 and had just passed through ,pietra ,ligure4 ,on the other hand1 the estimates for ,mentone and ,nice--namely1 #f;h4 #ahm4 #ces4 and #f;h4 #aim4 #dcs41 if not equally exact1 cannot err by many seconds1 certainly not by so much as one minute4 ,since the distances of ,loana and ,pietra ,ligure from the principal epicentre are #ca and #cb miles1 and those of ,mentone and ,nice #bh and #cg miles1 it is therefore clear that the vibrations which arrived first at ,nice and ,mentone must have come from a local focus1 where the impulse preceded that at the principal focus by several seconds4 ,,depth ,,of ,,the ,,principal ,,focus4 ,inaccurate as are all the methods of determining the depth of focus1 it seems probable1 as ,professor ,issel argues1 that the principal ,riviera focus was situated at a considerable distance from the surface4 ,in no part of the meizoseismal area was the shock a really violent one2 yet its intensity must have faded very slowly outwards1 for it was strong enough to stop clocks at places in ,switzerland and elsewhere not less than #bej miles from the origin4 ,professor ,mercalli regards ,mallet's method with greater favour than most seismologists4 ,he points to the gradual increase in the angle of emergence from the outer zones disturbed by the ,riviera earthquake towards the meizoseismal area1 where several good observations were made from fissures in walls parallel to the dominant direction of the shock4 ,the angles of emergence which he considers as most trustworthy are those of #ce at ,taggia1 #dj at ,oneglia1 and about #cj at ,bordighera4 ,the corresponding depths for the focus are #aj4d1 #aj4d1 and #aa4f miles1 giving an average of about #aj-c_/d miles4 ,there are no similar observations forthcoming for the depth of the secondary focus near ,nice and ,mentone2 but ,professor ,mercalli observes that it must have been shallower than the other1 for the vertical component of the vibrations from this focus was much less sensible than that of the motion coming from the principal focus4 ,,nature ,,of ,,the ,,shock4 ,the ,double ,shock4--,in the valuable collection of records made by ,professors ,taramelli and ,mercalli there appears at first sight to be the utmost diversity in the evidence with regard to the nature of the shock4 ,thus1 in the province of ,p4 ,maurizio alone1 the shock was described as subsultory first and then undulatory or vorticose at #be places1 undulatory and then subsultory at #bb1 undulatory and then subsultory and again undulatory or vorticose at #ac1 and subsultory first1 then undulatory1 and finally subsultory and vorticose at two places4 ,it is clear that the shock was of considerable duration1 not less than half-a-minute as a rule1 and that there were several phases in the movement2 and it would seem that one or more of these phases may have passed unnoticed owing to the alarm occasioned by the shock1 and to the fact that most of the observers were asleep when the earthquake began4 ,defects of memory must also have an influence not to be neglected1 for1 even with the simple shocks felt in the ,british ,isles1 persons in the same or neighbouring places differ greatly in their testimony4 ,but1 if we confine ourselves to the accounts of careful persons alone1 the discrepancies to a large extent disappear4 ,indeed1 all over the ruinous area 7,fig4 #cc7 the shock maintained a nearly uniform character4 ,at ,oneglia1 for instance1 there were two well-marked phases1 the first of which began with a brief subsultory movement1 followed by more horizontal undulations of longer period2 a pause1 lasting but for an instant1 was succeeded by vibrations which1 though not vertical1 were highly inclined to the horizon2 they continued throughout the second phase1 but1 towards the end1 new undulations were superposed1 and these1 coming from different directions1 resulted in an apparently vorticose movement4 ,professor ,mercalli represents the motion diagrammatically by the curve a in ,fig4 #ce4 ,at ,diano ,marina1 as will be seen from the curve b1 the shock again consisted of two phases1 each beginning with a few subsultory vibrations and ending with horizontal undulations of much longer period4 ,in the first phase1 the undulations were marked by a dominant direction1 but1 towards the close of the second phase1 there was no determinate direction1 and the impression was again that of a vorticose shock4 ,at ,savona1 the movement1 which is represented by the curve c1 must have lasted from twenty-five to thirty seconds4 ,it also consisted of two phases1 with subsultory vibrations and undulations in the same order2 and it was noticed that the second part of the shock was much stronger than the first4 ,according to some observers1 the concluding movements were vorticose4 ,illustration3 ,,fig4 #ce4--,nature of shock of ,riviera earthquake4 7,taramelli and ,mercalli47 ,in the zone surrounding the ruinous area1 the vertical component of the motion was observed to diminish with the intensity2 but1 in other respects as well as in duration1 the shock retained the same general form4 ,at ,genoa1 ,turin1 ,acqui1 ,alessandria1 ,antibes1 and other places1 two distinct phases were perceived1 occasionally separated by a brief pause1 the first being invariably the weaker4 ,at some places1 the observers speak of two shocks at about #f4bj ,a4,m41 separated by an interval of a few seconds2 and this division was noticeable as far as ,sal on the shore of ,lake ,garda and ,vicenza in ,venetia4 ,only in ,switzerland and other districts near the boundary of the disturbed area did the weaker part of the shock become insensible1 the other consisting of horizontal oscillations1 remarkable for their slowness and regularity1 and lasting for as much as twenty or thirty seconds4 ,we may thus conclude1 with ,professor ,mercalli1 that the earthquake resulted from the almost immediate succession of two distinct shocks1 in each of which the nearly vertical vibrations were more marked at the beginning1 while the slower undulations predominated towards the close1 those of the second phase generally becoming vorticose through the superposition of movements coming from different directions4 ,the second part of the shock in all of the more carefully written accounts is described as the stronger1 especially as regards the subsultory vibrations in the meizoseismal area2 except in the immediate neighbourhood of ,nice1 where the second phase was generally regarded as the weaker1 or at any rate as not stronger than the first4 ,origin of the ,double ,shock4--,these observations show1 not only that the principal earthquake consisted of two distinct shocks1 but also that the shocks originated in different foci4 ,for1 if the vibrations of both had started from one focus1 the second shock would have been everywhere the stronger2 instead of which there was a small area near ,nice where the intensity of the first was the greater4 ,this points clearly to the existence of another focus situated not far from ,nice2 and it is evident that the greater intensity of the first part in that district was due solely to the proximity of this focus1 for1 still farther to the west1 at ,antibes1 the second part was again the stronger4 ,there is thus a striking agreement in the inferences drawn from observations on the direction1 time of occurrence1 and nature of the shock4 ,in the face of such concurring testimony1 little doubt can remain as to the existence of two foci1 one to the south of ,oneglia and the other to the south of ,nice1 the initial impulse at the latter being decidedly the weaker1 and preceding that at the eastern focus by an interval of some seconds1 long enough at any rate for the resulting vibrations to reach the ,oneglia focus and to spread beyond it before the vibrations from that focus started on their outward journey4 ,seismographic ,records4--,in #ahhg1 the ,riviera and the districts adjoining it were unprovided with accurately constructed seismographs4 ,the observatories at ,alessandria1 ,milan1 ,monza1 ,parma1 ,florence1 and other places in ,italy contained seismoscopes and other pendulums1 and these all registered the fact that an earthquake had occurred1 and in many cases traced a series of elliptical or elongated curves4 ,a record of the shock was also given by a ,cecchi seismograph at ,perpignan in ,france1 but the distance from the epicentre was too great to allow details to be shown4 ,the most valuable record was that obtained from a ,cecchi seismograph at the observatory of ,moncalieri1 near ,turin1 about ninety miles north of the principal epicentre4 ,in this seismograph1 the pendulums are provided with pointers1 the tips of which touch vertical sheets of paper attached to the sides of an upright rectangular box4 ,when an earthquake occurs1 this box is made to descend slowly with a uniform velocity1 while the moving pointers trace curves upon the smoked paper4 ,the north-and-south component of the horizontal motion is inscribed on the sheet of paper facing west1 and the east-and-west component on the paper facing south4 ,illustration3 ,,fig4 #cf4--,seismographic record of the ,riviera earthquake at ,moncalieri4 7,denza47 ,during the principal ,riviera earthquake1 the former pendulum furnished an indistinct record1 while the other traced the diagram reproduced in ,fig4 #cf4 ,the movement1 as here represented1 began at about #f;h4 #bam4 #ejs4 ,a4,m4 7mean time of ,rome7 with a series of small tremors1 which lasted for about twelve seconds4 ,then followed some large oscillations1 always in a nearly east-and-west direction1 which at #f;h4 #bbm4 #bas4 gave place to a second series of tremors similar to those at the beginning of the shock1 but of greater amplitude4 ,these continued for at least twelve seconds1 at the end of which time the motion of the smoked paper ceased4 ,the total duration of the movement at ,moncalieri cannot therefore have been less than forty-three seconds4 ,interesting as this record is1 it is doubtful how far it represents accurately the movement of the ground4 ,the ,moncalieri instrument was erected before the modern type of seismograph was designed1 in which some part remains steady1 or very nearly steady1 during the complicated movements of the ground that take place in an earthquake4 ,it will be noticed that the curve in ,fig4 #cf shows no sign of the division of the shock into two distinct parts1 and this may perhaps be due to the swinging of the pendulum itself2 in which case1 the curve described by the pointer would be the resultant of the oscillations of the ground and the proper motion of the pendulum4 ,,sound-,,phenomena4 ,the sounds that preceded and accompanied the ,riviera earthquake have attracted but little study1 although they seem to have been widely observed4 ,no attempt was made to define the limits of the area over which they were audible2 but ,professor ,mercalli states that in the two outer zones 7,fig4 #cc7 the sound generally passed unobserved4 ,it was1 however1 heard near ,piacenza in ,lombardy and ,reggio in ,emilia1 places which are about #aae and #adj miles from the principal epicentre4 ,in the area in which the shock was most violent1 the sound resembled that of trains and vehicles in motion2 while1 outside this area it generally appeared to be like the hissing of a violent wind4 ,in only a few places was it compared to detonations1 the crashes of artillery or distant thunder4 ,some observers describe the sound as appearing at first as if a strong wind were rising1 and then as the roaring of a heavy railway-train passing4 ,nearly all the observers1 who were awake at the beginning of the earthquake1 agree in asserting that the sound distinctly preceded any movement of the ground4 ,from this1 as in the case of the ,andalusian earthquake1 ,professor ,mercalli infers that the sound-vibrations travelled with the greater velocity2 but1 as will be shown in ,chapter ,,viii41 the general precedence of the sound admits of another and more probable explanation4 ,,the ,,unfelt ,,earthquake4 ,if the ,andalusian earthquake first drew general attention to the distant spread of unfelt earth-waves1 the ,riviera earthquake showed that this was no isolated phenomenon4 ,we know now that the propagation of such waves is only limited by the surface of the earth1 but in #ahhg some doubt was felt at first as to the nature of the disturbance1 whether it was magnetic or mechanical in its origin4 ,in #ahhd1 the only observatories at which magnetographs were disturbed were those of ,lisbon1 ,parc ,saint-,maur 7near ,paris71 ,greenwich1 and ,wilhelmshaven4 ,in #ahhg1 the magnetographs registered the ,riviera earthquake at these and several other observatories1 the distribution of which is shown in ,fig4 #cg4 ,in this sketch-map1 the position of the principal epicentre is represented by the small cross1 while the nearly circular line shows the boundary of the disturbed area4 ,illustration3 ,,fig4 #cg4--;,distribution of observatories at which magnetographs were disturbed by the ,riviera earthquake4 ,three of the observatories1 those of ,nice1 ,lyons1 and ,perpignan1 lie inside this area4 ,at ,nice 7which is thirty-seven miles from the principal epicentre71 ,m4 ,perrotin states that the magnetograph curves show nothing of any interest1 except a notable magnetic perturbation on the vertical force curve1 the time of which1 however1 is not stated4#di ,at ,lyons 7#baa miles71 the declination1 horizontal force and vertical force1 magnets were all disturbed at #f;h4 #bem4 #dgs4 ,a4,m41 and ,perpignan 7#bfd miles71 all three magnets1 but especially those for the declination and horizontal force1 were set abruptly oscillating at #f;h4 #bem4 #bjs4 ,elsewhere in ,france1 the disturbances were noticed at the observatories of ,parc ,saint-,maur and ,montsouris1 near ,paris 7about #ddg miles71 and at ,nantes 7#ech miles74 ,at ,parc ,saint-,maur1 all three curves show a very clear trace of the earthquake at #f;h4 #bem4 #ces41 the oscillations lasting several minutes1 and at ,montsouris they also began at the same time4 ,at ,nantes1 the perturbations were so slight that they escaped notice on a first examination4 ,in ,austria1 disturbances were observed at ,pola 7#bie miles7 and ,vienna 7#ejf miles71 beginning at #f;h4 #bhm4 #ces4 and #f;h4 #cjm4 #ces41 respectively4 ,they reached ,brussels 7#ebb miles7 at #f;h4 #bim4 #bgs41 and ,utrecht 7#fjj miles7 at #f;h4 #bhm4 #chs4#ej ,at ,wilhelmshaven 7#fij miles71 only the vertical force magnet was affected1 the oscillations beginning at #f;h4 #cjm4 #ces41 and lasting for fourteen minutes4 ,at #f;h4 #bgm4 #ees41 the declination and horizontal force magnets of ,greenwich observatory 7#fdb miles7 were set vibrating1 but no similar disturbances were revealed by the vertical force curve or by the two earth-current registers4 ,at ,kew 7#feb miles71 the horizontal force magnetograph was moved by the earthquake at about #f;h4 #bim4 #ees4 ,the curves at ,stonyhurst and ,falmouth show no sign of any disturbance1 nor do those at ,pawlovsk in ,russia1 or ,seville4 ,at ,lisbon 7#iea miles71 however1 the three curves indicate disturbances at #f;h4 #cbm4 #ces41 but so feeble are they that they would have escaped discovery if the occurrence of the earthquake had been unknown4 ,the effects registered on the magnetograms are quite different from those which correspond to ordinary magnetic perturbations2 but they are not unlike those produced by the action of the momentary currents which are used for making the hour-marks1 except that the earthquake-oscillations lasted several minutes 7see ,fig4 #ba74 ,in each case1 then1 the magnetic bars must have received a succession of several or many impulses4 ,now1 the effect of these impulses on each magnet must depend on the relations which exist between the period of oscillation of the magnet1 the rate of damping of such oscillations1 and the interval between the successive impulses4 ,also1 the apparent commencement of the phenomena may be delayed if two impulses of contrary sense should follow one another before the bar is perceptibly displaced4 ,it is therefore to be expected1 as ,m4 ,mascart points out1 that the disturbances of the three instruments need not be of the same order of magnitude1 that with different forms of apparatus the effects may be very variable1 and that the deflection of one instrument may precede that of another at one and the same place4 ,in all the magnetographs1 the record is made on photographic paper1 which travels so slowly that the time of a movement can only be ascertained to the nearest minute4 ,as the disturbances on the ,french curves were apparently almost simultaneous1 and as no two of the others differed in time of occurrence by more than five minutes1 there is thus some colour for ,m4 ,mascart's contention that the magnetic apparatus registered1 not the movements of the ground1 but the passage of electric currents produced in the ground at a certain epoch of the earthquake4#ea ,on the other hand1 it is important to notice that1 in the central part of the disturbed area1 at ,nice1 two1 if not all three1 of the magnetographs were unaffected at the time of the earthquake4 ,at first sight1 this fact seems equally opposed to a mechanical explanation of the disturbance4 ,but1 when the vibrations are very rapid1 as they are in the neighbourhood of the epicentre1 the magnetic bars1 owing to their mode of suspension1 have not sufficient time to be sensibly deflected in the brief interval between successive phases of the impulse4 ,the magnetograms of the ,montsouris observatory show1 for instance1 hardly any perceptible trace of disturbance during the passage of railway trains along two adjacent lines4 ,the farther1 however1 the earth-waves travel from the origin1 the longer becomes the period of their vibrations4 ,in ,switzerland1 they were remarkable for their slowness1 even to the unaided senses4 ,thus1 at places more or less remote from the ,riviera1 the magnets would receive impulses at intervals approximating to their own periods of vibration1 and they would then oscillate freely for some time4 ,again1 notwithstanding some variations1 it will be remarked that on the whole the retardation of the initial epoch of the disturbances increases with the distance from the epicentre4 ,it thus seems clear1 ,i think1 that the cause of the disturbances must be sought in the shock itself2 although their initial epochs at different places are too roughly defined for ascertaining the velocity with which the earth-waves travelled4 ,,effects ,,of ,,the ,,earthquake ,,at ,,sea4 ,the ,riviera earthquake1 owing to its submarine origin1 was marked by certain phenomena that were absent from the other earthquakes described in this volume4 ,nature of the ,earthquake at ,sea4--,at the time of the earthquake1 several vessels were close to the epicentral area4 ,one1 about three miles off ,diano ,marina1 was shaken twice at about #f4bj ,a4,m41 and so violently that it seemed as if the masts would be broken off4 ,another1 about ten miles south of ,p4 ,maurizio1 also experienced two shocks1 a few minutes apart1 as if each time it had struck the bottom4 ,these observations are chiefly interesting in showing that the double shock was felt at sea as well as on land4 ,as transverse vibrations are not propagated through water1 it follows that the second part of the shock cannot1 as some maintain1 have been composed of transverse vibrations4 ,destruction of ,fishes4--,during the days immediately following the earthquake1 a large number of deep-sea fishes were found dead or half-dead either in shallow water or stranded on the beach1 especially in the neighbourhood of ,nice4 ,among them were numerous specimens1 mostly dead and floating1 of ,alepocephalus rostratus1 a typical deep-sea form1 several of ,pomatomus telescopium1 ,scopelus elongatus1 and ,s4 humboldti1 and many of ,dentex macrophthalmus and ,spinax niger4 ,the death and flight of these fishes must have been due to a sudden shock1 almost like that caused by the explosion of dynamite1 and communicated simultaneously to the whole surface of their bodies4 ,seismic ,sea-,waves4--,immediately after the earthquake1 the sea retired a short distance1 variously estimated at from ten to thirty metres1 laying bare some rocks that were usually immersed4 ,at ,p4 ,maurizio1 the surface was lowered by a little more than a metre2 and after a few minutes it rose to nearly a metre above its original level1 returning to it after a series of continually-decreasing oscillations4 ,at ,san ,remo1 a fall of about the same amount took place1 the sea returning after five minutes1 and a ship anchored in the harbour broke from her moorings4 ,again1 at ,antibes1 the sea was suddenly lowered by about a metre1 so that ships afloat in the harbour were aground for some instants1 and then returned with some impetuosity to its original level4 ,illustration3 ,,fig4 #ch4--,record of tide-gauge at ,nice4 7,issel47 ,the evidence of eye-witnesses is confirmed by that of the tide-gauges at ,nice and ,genoa1 the curves of which are reproduced in ,figs4 #ch and #ci4 ,at ,nice1 the first arrest of the curve in its usual course occurred at #f4cj ,a4,m42#eb the sea-level sank somewhat abruptly1 and after a few marked oscillations gradually returned to its normal position at #g4ej ,a4,m4 ,at ,genoa1 the shock caused the writing-pen of the tide-gauge to dent the paper on which the record is made1 and soon afterwards the curve shows a series of irregular oscillations1 about eight taking place every hour1 and gradually decreasing until they ceased to be perceptible about two hours after the principal earthquake4 ,illustration3 ,,fig4 #ci4--,record of tide-gauge at ,genoa4 7,issel47 ,,miscellaneous ,,phenomena4 ,connection between ,geological ,structure and the ,intensity of the ,shock4--,as with the ,andalusian earthquake1 faulty construction and defective materials were responsible for much of the damage caused by the ,riviera earthquake4 ,but1 if we may judge from the sharp local variations in its amount1 the nature of the surface-rocks must have exerted a still more potent influence4 ,at ,cervo1 for example1 the injury to property amounted to less than #c per head of the population2 at ,diano ,marina1 only two or three miles to the west1 it rose to #bb per head4 ,the death-rate at ,cervo was about one-tenth1 and at ,diano ,marina about #h-a_/b per cent4 ,again1 at ,mentone1 the damage must have been considerable1 for about #aee houses were rendered uninhabitable2 while ,monte ,carlo1 only a few miles farther west1 escaped almost unharmed4 ,now1 ,mentone and ,diano ,marina are for the most part built on clay or alluvial deposits1 and ,monte ,carlo on a foundation of limestone4 ,even within the limits of a single town1 variations no less striking were perceptible4 ,in ,mentone1 the greatest damage occurred to houses of two storeys built on alluvial soil in the low-lying parts near the sea and in the valleys4 ,the effect of the foundation in this part was well shown in the case of two equally well-built houses not more than #cjj yards apart4 ,one in the valley1 with doubtful foundations1 was very much shattered2 the other1 built on rock1 was uninjured4 ,the large hotels1 especially those on high ground1 suffered least1 few of them having their main walls seriously damaged4 ,these buildings rise to heights of from four to six storeys1 and of necessity have a firm and solid foundation4 ,professors ,taramelli and ,mercalli have made a careful study of the subject of this section4 ,the general conclusions at which they arrive are that the intensity of the shock was greatest at places built on pliocene conglomerates1 beds of clay superposed on compact old rocks1 patches of alluvium1 miocene formations of some thickness formed of repeated alternations of strata of incoherent marls and limestones or compact sandstones1 beds of chalk1 or somewhat rotten dolomite4 ,the shock was also more destructive on the summits of isolated hills and ridges and on the steep slopes of mountains4 ,the influence of the form of the ground was1 however1 subordinate to that exerted by the nature of the subsoil4 ,thus1 at ,mentone1 as we have seen1 and also at ,nice and ,genoa1 houses built on rock in elevated positions suffered much less than those situated on the plains below that are composed of sand and recent alluvium4 ,observations of the ,earthquake in ,railway-,tunnels4--,observations made in mines at various times and places have proved that an earthquake is felt less strongly in deep workings1 if felt at all1 than on the surface of the ground4 ,in the railway-tunnels of the ,riviera1 as ,professor ,issel has shown1 the same result was established during the earthquake of #ahhg4 ,on the line which runs northward from ,genoa to ,piedmont1 a tunnel more than five miles in length pierces the hilly ground between ,ponterosso and ,ronco1 the greatest thickness of rock above being about a thousand feet4 ,at the time of the earthquake1 the tunnel was not everywhere opened out to its full width1 and men were at work in different sections4 ,outside1 the shock was strong enough to damage buildings4 ,inside1 at about #bjj yards from the south end1 only a feeble shock was felt2 at #a1cej and #a1fbe yards1 some bricks were seen to fall from the facing1 but the shock was not otherwise perceived1 and only a few yards farther nothing unusual was noticed by the men at work4 ,again1 in an unfinished tunnel1 about three-quarters of a mile long1 between the harbour of ,genoa and the eastern railway-station1 the vibrations were very slightly felt4 ,even in the tunnels traversed by the coast railway from ,genoa to ,nice--that is1 in those situated within the meizoseismal area--the shock was either very weak or not felt at all1 and not one of the tunnels suffered the slightest injury4 ,to men at work inside a long tunnel1 the conditions for observing earthquakes are somewhat imperfect1 but these facts1 nevertheless1 bring out very clearly the inferior intensity of the shock at some depth below the surface4 ,,after-,,shocks4 ,while the unfelt earth-waves of the great earthquake were still wending their way over the zone that surrounds the disturbed area1 the central regions were again shaken1 at #f4bi ,a4,m41 by a shock strong enough to produce fresh ruins in the stricken towns along the coast4 ,nearly two and a half hours of quiet followed1 broken only by a few subterranean rumblings in the central part of the meizoseismal area4 ,then1 at #h4ea ,a4,m41 occurred another shock1 short and sharp1 and inferior in strength only to the principal earthquake4 ,both of these after-shocks were felt in ,western ,switzerland2 indeed1 they were perceptible nearly as far as the great shock2 the second1 however1 a little farther than the first1 for it alone was noticed at such places as ,vicenza1 ,forl1 and ,florence4 ,the shock at #f4bi was usually described as long and its vibrations as undulatory only2 that at #h4ea as rather subsultory than undulatory and of very brief duration4 ,the latter1 however1 was followed after an interval of a few seconds by another shock so weak that it generally passed unobserved4 ,both shocks were preceded by a rumbling sound4 ,during the next two days1 tremors and earth-sounds were frequent in the ,riviera2 once an hour1 on an average1 the greater part of the meizoseismal area was shaken by vibrations more or less slight4 ,but1 between one shock and another1 at ,diano ,marina and ,alassio1 and even as far as ,nice1 it only required attention from a careful observer to perceive an almost continual throbbing of the ground4 ,only one of these shocks1 that of ,february #bdth1 at #b4aj ,a4,m41 was strong enough to cause slight damage to buildings4 ,it disturbed an area1 not exceeded by any of the later shocks1 the boundary of which1 shown by the dotted line ,a in ,fig4 #cc1 extends to the north and east as far as ,piacenza and ,spezia1 while to the west it includes ,cannes4 ,the centre of the curve so drawn lies on land1 but1 as the shock was not felt in ,corsica1 there is no evidence as to the southerly extension of the disturbed area2 and it is probable1 as ,professor ,mercalli suggests1 that the shock originated in the eastern or ,oneglia focus of the great earthquake4 ,after ,february #beth1 slight shocks were felt during the next fortnight1 at the rate of three or four a day1 until ,march #aath1 when the last after-shock resulting in slight damage occurred at about #c4ab ,p4,m4 ,the boundary of its disturbed area1 represented in ,fig4 #cc by the dotted line ,b1 passes a little to the east of ,savona1 and then through ,alessandria1 ,moncalieri1 and ,marseilles4 ,the shock1 however1 was not observed in ,corsica1 so that the exact position of the epicentre is unknown2 but ,professor ,mercalli believes it to coincide with the western or ,nice epicentre of the principal earthquake4 ,at the moment of the shock1 the sea was observed from ,alassio to curl and to rise slightly1 while the tide-gauge at ,nice1 which had traced a continuous curve earlier in the day1 showed a characteristic notch about #c4g ,p4,m4 ,of the remaining after-shocks1 only two attained any notable degree of strength4 ,one1 on ,may #bjth at about #h4ae ,a4,m41 disturbed an area nearly concentric with that of the great earthquake1 and with a boundary coinciding nearly with the isoseismal #b in ,fig4 #cc4 ,again1 on ,july #agth at #aa4cj ,p4,m41 occurred a shock felt over an area nearly as large as that disturbed on ,february #bdth at #b4aj ,a4,m41 and situated in the same part of the country4 ,altogether1 during the year following the ,riviera earthquake1 ,professor ,mercalli records #aij after-shocks1 most of them slight or only just felt4 ,with the exception of the first two 7on ,february #bcrd71 none was observed outside the isoseismal #d of the principal earthquake 7,fig4 #cc72 and1 of the rest1 only the four whose dates are given above disturbed an area of more than one-eighth of that of the great shock4 ,some of them1 like the shock of ,march #aath1 were stronger in the western part of the meizoseismal area2 but the majority affected most the eastern portion and seem to be closely associated with the ,oneglia focus4 ,from ,february #bfth to ,april #bjth1 ,professor ,rumi made observations on the after-shocks by means of the ,foucault pendulum erected at ,genoa for demonstrating the rotation of the earth4 ,in nearly every case1 the oscillations took place along a north-east and south-west line1 or in the same direction as the first great shock--a resemblance which supports the inference that many of the after-shocks originated within the ,oneglia focus4 ,,origin ,,of ,,the ,,earthquakes4 ,recent ,movements in the ,riviera4--,the earliest movements that resulted in the great range of the ,maritime ,alps and the ,ligurian ,apennines date from pre-,carboniferous times1 when the central crystalline massifs in part emerged4 ,at the end of the ,liassic epoch1 the secondary formations of the district were uplifted1 and it was at this time that the range assumed its characteristic curved form4 ,later still1 at the close of the ,eocene period1 an elevation of more than #ijjj feet took place1 for upper ,eocene beds are found at this height in the ,maritime ,alps4 ,since that time1 other important movements have occurred4 ,pliocene deposits have been found in the ,riviera at an altitude of #a1hjj feet4 ,recent soundings in the ,gulf of ,genoa have also shown that all the valleys of the ,riviera between ,nice and ,genoa are continued far below the level of the sea to depths of not less than #cjjj feet4 ,thus1 at the end of the ,pliocene or beginning of the ,quaternary period1 there was an elevation of nearly #ejjj feet1 accompanied or followed by the erosion of the valleys which1 later on1 during the ,quaternary period1 were submerged about #cjjj feet4 ,even in still more recent times1 probably in the ,palolithic age1 minor movements continued4 ,traces of recent elevation1 varying in amount from a few feet to sixty feet or more1 occur at the ,balzi ,rossi in the ,alpes ,maritimes1 near ,bergeggi1 and in ,genoa2 while evidences of submergence are to be found near ,monaco1 at ,beaulieu and at ,diano ,marina4 ,it is important to notice that the great movements dating from the end of the ,eocene period are almost confined to the ,maritime ,alps and the western portion of the ,riviera4 ,in the parts of ,piedmont lying to the north of ,cuneo and in the eastern ,riviera1 they produced hardly any sensible effect4 ,seismic ,history of the ,riviera4--,the movements just referred to are those which1 in course of time1 have become sensible to the eye4 ,they represent the sum of a long-continued series of displacements that may once have been on a large scale1 but are now comparatively small4 ,the earthquakes that occur in the ,riviera show1 however1 that the final stage has not yet been reached4 ,their epicentres indicate the regions in which slips are still taking place1 and the magnitude of these slips is roughly measured by the intensity of the resulting shocks4 ,the map in ,fig4 #dj is one of a series drawn by ,professor ,mercalli to represent the distribution of seismic activity in ,piedmont and the ,riviera4 ,it corresponds to the period from #ahja to #ahie4 ,the whole area is divided into a number of seismic districts1 each of which is distinguished by a particular degree of activity4 ,in estimating this quantity1 ,professor ,mercalli takes intensity as well as frequency into account4 ,thus1 the lowest degree1 represented by the lightest tint of shading1 corresponds to one or two strong earthquakes with a few moderate or slight shocks2 the eighth and highest to four or five ruinous or disastrous earthquakes followed by trains of after-shocks4 ,the map shows very clearly that1 during the last century1 the seismic activity was greatest in the ,maritime ,alps and the western ,riviera--that is1 in the very districts in which the recent mountain-making movements have been most conspicuous4#ec ,illustration3 ,,fig4 #dj4--;,distribution of seismic activity in the ,riviera4 7,mercalli47 ,in all these districts1 ,professor ,mercalli distinguishes several well-marked seismic centres1 to each of which he traces the origin of two or more earthquakes4 ,in the districts with which we are at present concerned1 those of the ,alpes ,maritimes and the western ,riviera1 the most important centres are situated near ,oneglia 7in the sea71 near ,taggia1 in the valleys of the ,vesubia and ,tinea 7near ,nice71 and in the sea to the south of ,nice4 ,to the first of these centres belongs the disastrous earthquake of ,february #bcrd1 #ahhg1 as well as its after-shocks on ,february #bdth1 ,may #bjth1 ,july #agth1 and ,september #cjth of the same year1 also the ruinous earthquakes of #afab and #ahed1 and several others of a lesser degree of intensity4 ,all of these were longitudinal earthquakes1 the axes of their meizoseismal areas being parallel to the neighbouring mountain-ranges4 ,a few miles to the west of ,oneglia lies the ,taggia centre1 with which were connected the disastrous earthquake of #ahca1 the violent earthquake of #ahgd1 and other strong or very strong shocks4 ,these were for the most part transversal earthquakes1 their axes being perpendicular to those of the ,oneglia centre4 ,some of the strongest earthquakes in this region originated in a centre lying to the north of ,nice in the valleys of the ,vesubia and ,tinea4 ,among them may be mentioned the ruinous earthquakes of #adid1 #aeef1 #aefd1 and #afdd1 and probably also the disastrous earthquake of #abbg4 ,a fourth centre1 and one of considerable interest1 is that which lies at sea1 a short distance to the south of ,nice1 and nearly along the continuation of the valleys above-mentioned4 ,this is the secondary centre of the earthquake of #ahhg1 and probably also of that of ,december #bith1 #aeed4 ,it is occasionally in action apart from the ,oneglia centre1 as on ,november #bgth1 #agga1 ,june #aith1 #ahjf1 and ,december #bast1 #ahfa2 but such shocks1 though rather strong1 never reach a high degree of intensity4 ,origin of the ,earthquakes of #ahhg4--,the most important feature in the principal earthquake of #ahhg is its origination in two distinct foci1 which are sometimes in action almost simultaneously1 but more often separately4 ,the earthquakes belonging to the two foci differ greatly in intensity and number1 and the stronger part of the shock in #ahhg originated in the focus associated with the more disastrous and more frequent earthquakes4 ,the existence of two foci would of course give rise to a meizoseismal area elongated in the direction of the line joining them4 ,it is clear1 however1 that the ,oneglia focus was also extended in the same direction2 for1 in the after-shock of ,february #bdth1 the isoseismals drawn by ,professor ,mercalli are parallel to this line2 and this was also the case in the shock of ,march #aath4 ,as both foci were under the sea1 it is difficult to locate them with precision2 but it seems very probable that they occupy portions of a submarine fault that runs parallel or nearly so to the ,apennine axis between the meridians of ,oneglia and ,nice4 ,a brief period of preparation is a characteristic of the ,riviera earthquakes4 ,in #ahhg1 two at least of the preliminary shocks on ,february #bcrd 7those of about #b and #e ,a4,m47 originated in the ,oneglia focus4 ,at #f4bj ,a4,m4 the first and weaker movement took place in the western focus2 and1 a few seconds after the resulting vibrations reached the eastern focus1 the second and greater slip took place there4 ,the occurrence of seismic sea-waves is probably evidence of the formation of a small1 though sensible1 fault-scarp in the same region4 ,to relieve the additional stresses thus brought into action along the fault-surface1 numerous small slips took place in different parts1 some as far to the west as the ,nice focus1 but the greater number probably within or close to the focus in the neighbourhood of ,oneglia4 ,,references4 #a4 ,,bertelli1 ,t4--,osservazioni fatte in occasione di una escursione sulle ,riviera ,ligure di ponente dopo i terremoti ivi seguiti nell' anno #ahhg4 ,boll4 ,mens4 dell' ,oss4 di ,moncalieri1 vol4 viii41 #ahhh1 ,nos4 #f1 #g1 #h4 #b4 ,,charlon1 ,e4--,note sur le tremblement de terre du #bc fvrier #ahhg4 ,bull4 del ,vulc4 ,ital41 anno xiv41 #ahhg1 pp4 #ah-bc4 #c4 ,,denza1 ,f4--,alcune notizie sul terremoto del #bc febbraio #ahhg 7,turin74 #d4 ,,issel1 ,a4--,il terremoto del #ahhg in ,liguria4 ,boll4 del ,r4 ,com4 ,geol4 d',italia1 anno #ahhg1 supplemento1 pp4 #a-bjg4 #e4 ,,mercalli1 ,g4--,i terremoti della ,liguria e del ,piemonte4 7,naples1 #ahig1 #adf pp47 #f4 ,,oddone1 ,e4--,i dati sismici della ,liguria in rapporto alla frequenza ed alla periodicit4 ,boll4 della ,soc4 ,sismol4 ,ital41 vol4 ii41 #ahif1 pp4 #adj-aea4 #g4 ,,offret1 ,a4--,sur le tremblement de terre du #bc fvrier #ahhg4 ,discussion des heures observs dans la zone picentrale4 ,paris1 ,acad4 ,sci41 ,compt4 ,rend41 vol4 civ41 #ahhg1 pp4 #aaej-aaec4 #h4 ----4 ,tremblements de terre du #bc fvrier #ahhg4 ,heures de l'arrive des secousses en dehors de l'picentre4 ,ibid41 pp4 #abch-abdb4 #i4 ,,rossi1 ,m4,s4 ,,de4--,relazione sui terremoti del febbraio #ahhg4 ,bull4 del ,vulc4 ,ital41 anno xiv41 #ahhg1 pp4 #e-ag4 #aj4 ----4 ,bibliografia3 ,sul terremoto ligure del #bc febbraio #ahhg4 ,ibid41 pp4 #fj-fb1 #ajg-aab1 #aae-abh4 #aa4 ,,taramelli1 ,t41 and ,g4 ,,mercalli4--,il terremoto ligure del #bc febbraio #ahhg4 ,annali dell' ,uff4 ,centr4 di ,meteor4 e di ,geodin41 vol4 viii41 parte iv41 #ahhh4 7,roma1 #bih pp47 #ab4 ,,uzielli1 ,g4--,le commozioni telluriche e il terremoto del #bc febbraio #ahhg 7,turin74 #ac4 ,nature1 vol4 xxxv41 #ahhg1 pp4 #dch1 #dfb1 #ecd-ece2 vol4 xxxvi41 #ahhg1 pp4 #d1 #aea-aeb4 #ad4 ,paris1 ,acad4 ,sci4 ,compt4 ,rend41 vol4 civ41 #ahhg1 pp4 #eef-eeg1 #fjf-fab1 #fcd-fce1 #fei-ffg1 #gdd-gde1 #geg-geh1 #gei-gfj1 #gfd-gff1 #hbb-hbc1 #hcj-hce1 #hhd-hij1 #iej-iea1 #ajhh-ajhi1 #abdc-abde1 #acej-aceb1 #adaf-adai2 vol4 cv41 #ahhg1 pp4 #bjb-bjc2 vol4 cviii41 #ahhi1 p4 #aahi2 vol4 cix42 #ahhi1 pp4 #afd-aff1 #bgb-bgd1 #ffj4 ,,footnotes3 #dg ,the above times and all others in this chapter are given in ,rome mean time1 which is #ejm4 earlier than ,greenwich mean time4 #dh ,professor ,uzielli has also published a map of the isoseismal lines for the ,italian part of the disturbed area4 #di ,it seems doubtful whether this movement was connected with the earthquake4 ,m4 ,offret does not include ,nice in his list of observatories at which magnetographs were disturbed4 #ej ,this is the time given by ,m4 ,offret4 ,according to ,m4 ,mascart1 it should be #f;h4 #bem4 #djs4 #ea ,in order to test the truth of this explanation1 ,m4 ,moureaux suspended a bar of copper at the ,parc ,saint-,maur observatory by two threads in the same way as the horizontal force-magnet4 ,the direction of this bar was also registered photographically1 and it remained unmoved during the ,verny earthquake of ,july #abth1 #ahhi1 and the ,dardanelles earthquake of ,october #beth1 #ahhi1 while one or more of the magnets were disturbed4 ,the experiment1 however1 was ineffective2 for1 in order that the magnet may rest in a horizontal position1 its centre of gravity must be at unequal distances from the two points of support4 #eb ,the hour-marks in ,fig4 #ch refer to ,paris mean time1 and those in ,fig4 #ci to ,genoa mean time4 #ec ,in the seventeenth century1 the maximum seismic activity was manifested in the neighbourhood of ,nice1 and in the eighteenth century in ,piedmont4 ,,chapter ,,vii4 ,,the ,,japanese ,,earthquake ,,of ,,october #bh,,th1 #ahia4 ,although several years have elapsed since the occurrence of the greatest of ,japanese earthquakes1 the final report that will embody the labours of all its investigators is yet to be written4 ,several important contributions to it1 however1 have already been made4 ,professor ,koto1 in an admirable memoir1 has traced the course of the great fault-scarp and discussed the origin of the earthquake2 ,professor ,omori1 with equal care and thoroughness1 has investigated the unrivalled series of after-shocks2 ,mr4 ,conder studied the damaged buildings from an architect's point of view2 ,professor ,tanakadate and ,dr4 ,nagaoka devoted themselves to a re-determination of the magnetic elements of the central district1#ed while1 by the compilation of his great catalogue of ,japanese earthquakes during the years #ahhe-ib1 ,professor ,milne has provided the materials for a further analysis of the minor shocks that preceded and followed the principal earthquake4 ,the part of ,japan over which the earthquake was sensibly felt is shown in ,fig4 #da4 ,the small black area in the centre is that in which the shock was most severe and the principal damage to life and property occurred4 ,the other bands1 more or less darkly shaded according to the greater or less intensity of the shock1 will be referred to afterwards4 ,fig4 #de represents the meizoseismal area on a larger scale2 and1 as the greater part of it lies within the two provinces of ,mino and ,owari1 the earthquake is generally known among the ,japanese themselves as the ,mino-,owari earthquake of #ahia4 ,illustration3 ,,fig4 #da4--,sketch-,map of ,disturbed ,area and ,isoseismal ,lines4 7,masato47 ,,the ,,meizoseismal ,,area4 ,more than half of the meizoseismal area occupies a low flat plain of not less than #djj square miles in extent4 ,on all sides but the south1 the plain1 which is a continuation of the depression forming the ,sea of ,is1 is surrounded by mountain ranges1 those to the west1 north1 and north-east being built up mainly of ,palozoic rocks1 and those on the east side of granite4 ,a network of rivers and canals converts what might otherwise have been unproductive ground into one of the most fertile districts in ,japan4 ,a great garden1 as it has been aptly termed1 the whole plain is covered with rice-fields1 and supports a population of about #ghg to the square mile--a density which is exceeded in only six counties of ,england4 ,as a rule1 the soil is a loose1 incoherent1 fine sand1 with but little clayey matter2 and it is1 no doubt1 to its sandy nature that the disastrous effects of the earthquake were largely due4 ,in the northern half of the district1 the meizoseismal area is much narrower1 and here it crosses a great mountain-range running from south-west to north-east and separating the river-systems of the ,japan sea from those of the ,pacific4 ,to the north1 the meizoseismal area terminates in another plain1 in the centre of which lies the city of ,fukui1 where the destructiveness of the earthquake was only inferior to that experienced in the provinces of ,mino and ,owari4 ,there is also a detached portion of the area lying to the east of ,lake ,biwa1 but it is uncertain whether the exceptional intensity there was due to the nature of the ground or to the occurrence of a secondary or sympathetic earthquake in its immediate neighbourhood4 ,illustration3 ,,fig4 #db4--;,general ,plan of ,geological ,structure of ,meizoseismal ,area4 7,koto47 ,the general plan of the geological structure of the central district is represented in ,fig4 #db4 ,the thick line1 partly continuous and partly broken1 shows the course of the great fault1 to the growth of which the earthquake chiefly owed its origin2 while the thin continuous lines represent the changing direction of strike of the ,palozoic rocks which surround the ,mino-,owari plain1 and the arrowheads the direction of the dip4 ,it will be seen that the direction of the strike forms an ,s-shaped curve1 and it is clear that the present torsion-structure of the district could not have been produced without the formation of many fractures at right angles and parallel to the lines of strike4 ,professor ,koto points out that the regular and parallel valleys of the rivers ,tokuno-yama1 ,neo1 ,mugi1 and ,itatori1 indicated by broken lines in ,fig4 #db1 have probably been excavated along a series of transverse fractures running from north-west to south-east2 while fractures which are parallel to the line of strike may be responsible for the zigzag course of the valleys4 ,,damage ,,caused ,,by ,,the ,,earthquake4 ,the great earthquake occurred at #f4cg ,a4,m41 practically without warning1 and in a few seconds thousands of houses were levelled with the ground4 ,within the whole meizoseismal area there was hardly a building left undamaged4 ,the road from ,nagoya to ,gifu1 more than twenty miles in length1 and formerly bordered by an almost continuous succession of villages1 was converted into a narrow lane between two long drawn-out banks of dbris4 ,in some streets1 says ,professor ,milne1 it appeared as if the houses had been pushed down from the end1 and they had fallen like a row of cards4 ,or1 again1 a mass of heaped-up rubbish might be passed1 where sticks and earth and tiles were so thoroughly mixed that traces of streets or indications of building had been entirely lost4 ,at ,gifu1 ,ogaki1 ,kasamatsu1 and other towns1 fires broke out after the earthquake4 ,in ,kasamatsu the destruction was absolutely complete2 nothing was left but a heap of plaster1 mud1 tiles1 and charred timbers4 ,at ,ogaki1 not more than thirty out of #hjjj houses remained standing1 and these were all much damaged4 ,within the whole district1 according to the official returns1 #aig1ecj buildings were entirely destroyed1 #gh1bif half destroyed1 and #e1icd shattered and burnt2 while #g1bgi persons were killed1 and #ag1cic were wounded4 ,next to buildings1 the embankments which border the rivers and canals suffered the most serious damage1 no less than #cag miles of such works having to be repaired4 ,railway-lines were twisted or bent in many places1 the total length demolished being more than ten miles4 ,in cuttings1 twenty feet or more in depth1 both rails and sleepers were unmoved2 it was on the plains that the effects of the earthquake were most marked4 ,the ground appeared as if piled up into bolster-like ridges between the sleepers1 and in many places the sleepers had moved end-ways4 ,when the line crossed a small depression in the general level of the plain1 the whole of the track was bowed1 as if the ground were permanently compressed at such places4 ,effects of compression1 says ,professor ,milne1 were most marked on some of the embankments1 which gradually raise the line to the level of the bridges4 ,on some of these1 the track was bent in and out until it resembled a serpent wriggling up a slope4444 ,close to the bridges the embankments had generally disappeared1 and the rails and sleepers were hanging in the air in huge catenaries4 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,the land area disturbed by the earthquake and the different isoseismal lines are shown in ,fig4 #da4 ,the most severely shaken district1 that in which the destruction of buildings and engineering works was nearly complete1 contains an area of #d1bhf square miles1 or about two-thirds that of ,yorkshire4 ,this is indicated on the map by the black portion4 ,outside this lies the very severely shaken district1 #ag1cbe square miles in area1 extending from ,kobe on the west to ,shizuoka on the east1 in which ordinary buildings were destroyed1 walls fractured1 embankments and roads damaged1 and bridges broken down4 ,the third or severely shaken district contains #bj1ahc square miles2 and in this some walls were cracked1 pendulum clocks stopped1 and furniture1 crockery1 etc41 overthrown4 ,tokio and ,yokohama lie just within this area4 ,in the fourth region the shock was weak1 the motion being distinctly felt1 but not causing people to run out-of-doors2 and in the fifth it was slight1 or just sufficient to be felt4 ,these two regions together include an area of #ea1igf square miles4 ,thus1 the land area disturbed amounts altogether to #ic1ggj square miles--i4e41 to a little more than the area of ,great ,britain4 ,according to ,professor ,omori1 the mean radius of propagation was about #cbc miles1 and the total disturbed area must therefore have been about #ccj1jjj square miles1 or nearly four times the area of ,great ,britain4 ,considering the extraordinary intensity of the shock in the central district1 this can hardly be regarded as an over-estimate4 ,the isoseismal lines shown in ,fig4 #da are not to be regarded as drawn with great accuracy2 for there is no marked separation between the tests corresponding to the different degrees of the scale of intensity4 ,the seismographs at ,gifu and ,nagoya were thrown down within the first few seconds1 and failed to record the principal motion4 ,but a great number of well-formed stone lanterns and tombstones were overturned1 and1 from the dimensions of these1 ,professor ,omori calculated the maximum horizontal acceleration necessary for overturning them at fifty-nine places within the meizoseismal area4#ee ,at five of these it exceeded #djjj millimetres per second per second1 an acceleration equal to about five-twelfths of that due to gravity4 ,making use of these observations1 ,professor ,omori has drawn two isoseismal lines within the central district1 which are shown in ,fig4 #dd4 ,at every point of the curve marked #b1 the maximum acceleration was #bjjj millimetres per second per second1 and of that marked #a1 #hjj millimetres per second per second4 ,the dotted line within the curve marked #b represents the boundary of the meizoseismal area1 which1 it will be observed1 differs slightly from that given by ,professor ,koto 7see ,fig4 #de74 ,the difference1 however1 is apparently due to the standard of intensity adopted1 ,professor ,koto's boundary agreeing rather closely with the curve marked #b in ,fig4 #dd4 ,,nature ,,of ,,the ,,shock4 ,little has yet been made known with regard to the nature of the shock1 and the published records of the accompanying sound are so rare that it seems as a rule to have passed unheard4 ,the seismographs at ,gifu and ,nagoya registered the first half-dozen vibrations1 and were then buried beneath the fallen buildings4 ,in the following table1 the data from these two stations are therefore incomplete3-- ,,principal ,,measurements ,,obtained ,,from ,,seismographic ,,records4 ---------------------------------------- -------------------------- ,tokio ,gifu4 ,nagoya4 ,osaka4 7,imp4 ,univ474 ---------------------------------------- -------------------------- ,maximum horizontal motion #ah mm4 #bf mm4 #cj mm4 #ce mm4 ,period of ditto #b4j secs4 #a4c sec4 #a4j sec4 #b4j secs4 ,maximum vertical motion #aa4c mm4 #f4b mm4 #h mm4 #i4e mm4 ,period of ditto #j4i sec4 #a4e sec4 #a4j sec4 #b4d secs4 ---------------------------------------- -------------------------- ,if the period of the principal vibrations were known1 the observations of ,professor ,omori on the overturning of bodies would enable us to determine the range of motion at different places4 ,for instance1 the maximum acceleration at ,nagoya was found by these observations to be #b1fjj millimetres per second per second1 and if we take the period of the greatest horizontal motion to be the same as that of the initial vibrations--namely1 #a4c second1 the total range 7or double amplitude7 would be #bbc millimetres1 or #h4h inches4 ,with the same period1 and the maximum acceleration observed 7at ,iwakura and ,konaki7 of more than #d1cjj millimetres per second per second1 the total range would be greater than #ad4e inches4#ef ,in the meizoseismal area1 many persons saw waves crossing the surface of the ground4 ,at ,akasaka1 according to one witness1 the waves came down the streets in lines1 their height being perhaps one foot1 and their length between ten and thirty feet4 ,to the north of the same area1 we are told that the shoreline rose and fell1 and with this rising and falling the waters receded and advanced4 ,even at ,tokio1 which is about #age miles from the epicentre1 the tilting of the ground was very noticeable4 ,after watching his seismographs for about two minutes1 ,professor ,milne next observed the water in an adjoining tank1 #hj feet long and #bh feet wide1 with nearly vertical sides4 ,at the time it was holding about #ag feet of water1 which was running across its breadth1 rising first on one side and then on the other to a height of about two feet4 ,still clearer is the evidence of the seismographs in the same city4 ,instead of a number of irregular waves1 all the records show a series of clean-cut curves4 ,the heavy masses in the horizontal pendulums were tilted instead of remaining as steady points4 ,they were not simply swinging1 for the period of the undulations differed from that of the seismograph when set swinging1 and also varied in successive undulations4 ,it was ascertained afterwards1 by measurement with a level1 that to produce these deflections1 the seismograph must have been tilted through an angle of about one-third of a degree4 ,direction of the ,shock4--,shortly after the earthquake1 ,professor ,omori travelled over the meizoseismal area and made a large number of observations on the directions in which bodies were overturned1 taking care to include only those in which the direction of falling would not be influenced by the form of the base1 such as the cylindrical stone lanterns so frequently found in ,japanese gardens4 ,at some places these bodies fell in various directions1 at others with considerable uniformity in one direction4 ,for instance1 at ,nagoya1 out of #bjj stone lanterns with cylindrical stems1 #aai fell between west and south1 and #cf between east and north2 the numbers falling within successive angles of #ae being represented in ,fig4 #dc4 ,the mean direction of fall is ,w4 #cj ,s41 coinciding with that in which the majority of the lanterns were overturned4 ,similar observations were made at forty-two other places within and near the meizoseismal area1 and the resulting mean direction for each such place in the ,mino-,owari district is shown by short lines in ,fig4 #dd1 the arrow indicating the direction towards which the majority of bodies at a given place were overturned4 ,it will be seen from this map that the direction of the earthquake motion was generally at right angles1 or nearly so1 to that of the neighbouring part of the meizoseismal zone1 and that on both sides of it1 the majority of overturned bodies at each place fell towards this zone4 ,illustration3 ,,fig4 #dc4--,plan of ,directions of ,fall of ,overturned ,bodies at ,nagoya4 ,illustration3 ,,fig4 #dd4--,map of ,mean ,directions of ,shock and ,isoseismal ,lines in ,central ,district4 7,omori47 ,,velocity ,,of ,,the ,,earth-,,waves4 ,the times of the great earthquake and of sixteen minor shocks on ,october #bhth and #bith and ,november #fth were determined at the ,central ,meteorological ,observatory at ,tokio1 and at either two or three of the observatories of ,gifu1 ,nagoya1 and ,osaka1 each of which is provided with a seismograph and chronometer4 ,the after-shocks referred to originated near a point about #f miles west of ,gifu1 and the difference between the distances of ,tokio and ,osaka from this point is #hi-a_/b miles1 of ,tokio and ,nagoya #adg miles1 and of ,tokio and ,gifu #afe miles4 ,the mean time-intervals between these three pairs of places were #fg1 #aaa1 and #abh seconds respectively2 and these give for the mean velocity for each interval #b4a kilometres 7or #a4c mile7 per second4 ,thus there appears in these cases to be no sensible variation in the velocity with the distance from the origin4 ,as might be expected1 an earthquake of such severity was recorded by magnetometers at several distant observatories4 ,disturbances on the registers of ,zikawei 7,china71 ,mauritius1 ,utrecht1 and ,greenwich have been attributed to the ,japanese earthquake1 but the times at which they commenced are too indefinite to allow of any determination of the surface-velocity of the earth-waves to great distances from the origin4 ,,the ,,great ,,fault-,,scarp4 ,as in all disastrous earthquakes1 the surface of the ground was scarred and rent by the shock4 ,from the hillsides great landslips descended1 filling the valleys with dbris2 and slopes which were formerly green with forest1 after the earthquake looked as if they had been painted yellowish-white4 ,innumerable fissures cut up the plains1 the general appearance of the ground1 according to ,professor ,milne1 being as if gigantic ploughs1 each cutting a trench from #c to #ab feet deep1 had been dragged up and down the river-banks4 ,but by far the most remarkable feature of the earthquake was a great rent or fault1 which1 unlike the fissures just referred to1 pursued its course regardless of valley1 plain1 or mountain4 ,although at first sight quite insignificant in many places1 and some time hardly visible to the untrained eye1 ,professor ,koto has succeeded in tracing this fault along the surface for a distance of forty miles1 and he gives good reasons for believing that its total length must be not less than seventy miles4 ,illustration3 ,,fig4 #de4--,map of ,meizoseismal ,area4 7,koto47 ,illustration3 ,,fig4 #df4--,ploughshare ,appearance of the ,fault near ,fujitani4 7,koto47 ,illustration3 ,,fig4 #dg4--,the ,fault-scarp at ,midori4 7,koto47 ,the general character of the fault-scarp changes with the surface features4 ,on flat ground1 where the throw is small1 it cuts up the soft earth into enormous clods1 or makes a rounded ridge from one to two feet high1 so that it resembles1 more than anything else1 the pathway of a gigantic mole 7,fig4 #df74 ,when the throw is considerable--and in one place it reaches from #ah to #bj feet--the fault-scarp forms a terrace1 which from a distance has the appearance of a railway embankment 7,fig4 #dg74 ,or1 again1 where the rent traverses a mountain ridge or a spur of hills1 it caused extensive landslips1 one side of it descending considerably in level1 carrying the forest with it1 but with the trees complicatedly interlocked or prostrate on the ground4 ,illustration3 ,,fig4 #dh4--;,displacement of ,field ,divisions by the ,fault near ,nishi-,katabira4 7,koto47 ,at its southern end1 the fault was seen for the first time crossing a field near the village of ,katabira4 ,the field was broken into clods of earth1 and swollen up to a height of #e-a_/b yards1 while a great landslip had descended into it from an adjoining hill4 ,a little farther to the north-west1 the ground was sharply cut by the fault1 the north-east side having slightly subsided and at the same time been shifted horizontally through a distance of #c-a_/d to #d feet to the north-west ,adjoining fields were formerly separated by straight mounds or ridges running north and south and east and west1 and these mounds were cut through by the fault and displaced1 as shown in ,fig4 #dh4 ,from this point the fault runs in a general north-westerly direction1 the north-east side being always slightly lowered with respect to the other and shifted to the north-west4 ,near ,seki it takes a more westerly direction1 and continues so to a short distance east of ,takatomi1 where the north side is lowered by five feet1 and moved about #a-a_/d feet to the west4 ,at the north end of ,takatomi1 a village in which every house was levelled with the ground1 the fault is double1 and the continuous lowering towards the north has converted a once level field into sloping ground4 ,at this point1 the small river ,toba1 flowing south1 is partially blocked by the fault-scarp1 and an area of about three-quarters of a square mile1 on which two villages stand1 was converted into a deep swamp 7,fig4 #di71 so that1 as the earthquake occurred at the time of the rice-harvest1 the farmers were obliged to cut the grain from boats4 ,after passing ,takatomi1 the fault again turns to the west-north-west1 but1 the throw being small1 it resembles here the track of an enormous mole4 ,at ,umhara it crosses a garden between two persimmon trees1 appearing on the hard face of the ground as a mere line2 but the trees1 which were before in an east-and-west line1 now stand in one running north and south1 without being in the least affected by the movement 7,fig4 #ej74 ,from here to ,kimbara1 where the fault enters the ,neo valley1 the north side is always depressed and shifted westwards by about #f-a_/b feet4 ,illustration3 ,,fig4 #di4--,map of ,swamp formed by stoppage of ,river ,toba by ,fault-scarp4 7,koto47 ,illustration3 ,,fig4 #ej4--,shifting of ,trees by fault at ,umhara4 7,koto47 ,it was in the ,neo valley that the supreme efforts of the earthquake were manifested4 ,landslips were so numerous that the greater part of the mountain slopes had descended into the valley1 the whole appearance of which had changed4 ,unfamiliar obstacles1 remarks ,professor ,koto1 made themselves apparent1 and small hills covered with forest had come into sight which had not been seen before4 ,but the ground was not only lowered and shifted by the fault2 it was permanently compressed1 plots originally #dh feet in length afterwards measuring only #cj feet4 ,in fact1 it appears1 in the words of ,professor ,milne1 as if the whole ,neo valley had become narrower4 ,a few miles after entering the ,neo valley1 the throw of the fault reaches its maximum at ,midori4 ,but instead of the relative depression of the east side1 which prevails throughout the rest of the line1 that side is here about #bj feet higher than the other4 ,it is1 however1 shifted as usual towards the north1 by about #ac feet2 and this displacement is rendered especially evident by the abrupt break in the line of a new road to ,gifu 7,fig4 #dg74 ,that the east side has really risen is clear1 for1 a little higher up1 the river has changed from a shallow rapid stream #cj yards wide into a small lake of more than twice the width1 and so deep that a boatman's pole could not reach the bottom4 ,at ,itasho1 about a mile north of ,midori1 both sides are nearly on the same level1 the fault appearing like a mole's track2 and seven miles farther1 at ,nagoshima1 the east side is relatively depressed by more than a yard1 and at the same time shifted about #f-a_/b feet to the north4 ,illustration3 ,,fig4 #ea4--;,daily frequency of after-shocks at ,gifu and ,nagoya4 ,at ,nogo1 the main ,neo valley turns off at right angles to the east1 and the fault continues its course up a side valley1 the east side1 with respect to the other1 being continually depressed and shifted towards the north4 ,it was traced by ,professor ,koto through ,fujitani 7,fig4 #df71 where there were many unmistakable evidences of the violence of the shock1 as far as the eastern shoulder of ,haku-san2 and here1 after following the fault for #dj miles1 the lateness of the season compelled him to return4 ,there can be no doubt1 however1 that it runs as far as ,minomata2 and it is probable1 from the linear extension of the meizoseismal area1 that it does not entirely die out before reaching the city of ,fukui1 #gj miles from its starting-point at ,katabira4 ,,minor ,,shocks4 ,for some hours after the earthquake1 shocks were so frequent in the meizoseismal area that the ground in places hardly ever ceased from trembling4 ,without instrumental aid1 detailed record was of course impossible2 but fortunately the buried seismographs at ,gifu and ,nagoya were uninjured1 and in about seven hours both were once more in working order4 ,to the energy by which this result was accomplished1 we owe our most valuable registers of the after-shocks of a great earthquake4 ,illustration3 ,,fig4 #eb4--,monthly frequency of after-shocks at ,gifu4 7,omori47 ,until the end of #ahic--that is1 in little more than two years--the total number of shocks recorded at ,gifu was #c1cfe1 and at ,nagoya #a1bih4 ,none of these approached the principal earthquake in severity4 ,nevertheless1 of the ,gifu series1 #aj were described as violent and #ig strong2 while of the remainder1 #a1hjh were weak1 #a1jda feeble1 and #dji were sounds alone without any accompanying shock4 ,the slight intensity of most of the shocks is also evident from the inequality in the numbers recorded at ,gifu and ,nagoya1 from which it appears that nearly two-thirds were imperceptible more than about #be miles from the chief origin of the shocks4 ,only #gj of the after-shocks during the first two years were registered at ,osaka1 and not more than #cj at ,tokio4 ,distribution of ,after-shocks in ,time4--,the decline in frequency of the after-shocks was at first extremely rapid1 the numbers recorded at ,gifu during the six days after the earthquake being #cjc1 #adg1 #aaf1 #ii1 #ib1 and #ha1 and at ,nagoya #ahe1 #ic1 #gi1 #ef1 #cj1 and #ca2 in fact1 half of the shocks up to the end of #ahic occurred by ,november #bcrd at ,gifu1 and by ,november #fth at ,nagoya4 ,the daily numbers at these two places are represented in ,fig4 #ea1 in which the crosses correspond to the numbers at ,gifu1 and the dots to those at ,nagoya2 and the curves drawn through or near the marks represent the average daily number of shocks from ,october #bith to ,november #bjth4 ,it will be seen that these curves are hyperbolic in form1 the change from very rapid to very gradual decline in frequency taking place from five to ten days after the great earthquake4 ,fig4 #eb illustrates the distribution in time of the after-shocks at ,gifu to the end of #ahic1 the ordinates in these cases representing the number of shocks during successive months4#eg ,a similar rapid and then gradual decline in frequency characterises the strong and weak shocks recorded at ,gifu4 ,of the ten violent shocks1 only one occurred after the beginning of ,january #ahib2 and of the #ig strong shocks1 only three after ,april #ahib4 ,but at the commencement of the series1 feeble shocks 7i4e41 shocks that could just be felt7 and earth-sounds without any accompanying movement were comparatively rare1 and did not become really prominent until two months had elapsed4 ,of the #cjh after-shocks recorded in #ahic1 none could be described as strong1 only #aj were weak1 while #bfc were feeble shocks and #ce merely earth-sounds4 ,the last two diagrams show at a glance that the decline in frequency of after-shocks is very far from being uniform4 ,some of the fluctuations are due to the occurrence of exceptionally strong shocks1 each of which is followed by its own minor train of after-shocks4#eh ,others seem to be periodic1 and possibly owe their origin to external causes unconnected with the earthquake4#ei ,method of representing the ,distribution of ,after-shocks in ,space4--,the maps in ,figs4 #ed-eg show the distribution of the after-shocks in space during four successive intervals of two months each4 ,they are founded on ,professor ,milne's great catalogue of ,japanese earthquakes1 which give1 among other data1 the time of occurrence and the position of the epicentre for every shock until the end of #ahib4 ,for the latter purpose1 the whole country is divided by north-south and east-west lines into numbered rectangles1 each one-sixth of a degree in length and breadth2 and the position of an epicentre is denoted by the number of the rectangle in which it occurs4 ,the area included within the maps is bounded by the parallels #cd #dj' and #cf #bj' lat4 ,n41 and by the meridians #b #aj' and #c #ej' long4 ,w4 of ,tokio1 so that ten rectangles adjoin each side of the map4 ,the number of epicentres lying within each rectangle having been counted1 curves are then drawn through the centres of all rectangles containing the same number of epicentres1 or through points which divide the line joining the centres of two rectangles in the proper proportion4 ,taking1 for example1 the curve marked #e1 if the numbers in two consecutive rectangles are #c and #g1 the curve bisects the line joining their centres2 if the numbers are #a and #f1 the line joining their centres is divided into five equal parts1 and the curve passes through the first point of division reckoned from the centre of the rectangle in which six epicentres are found4 ,thus the meaning of the curve marked1 say1 #e may be stated as follows3--,if any point in the curve be imagined as the centre of a rectangle whose sides are directed north-south and east-west1 and are respectively one-sixth of a degree of latitude and longitude in length2 then the number of epicentres within this rectangle is at the rate of #e for the time considered4 ,preparation for the ,great ,earthquake4--,at first sight1 there appears to have been but little direct preparation for the great earthquake4 ,except for a rather strong shock on ,october #beth1 at #i4ad ,p4,m41 it occurred without the warning of any preliminary tremors4 ,but a closer examination of the evidence shows1 as we should indeed expect1 that there was a distinct increase in activity for many months beforehand4 ,the region had become seismically sensitive4 ,of the hundred rectangles included in the maps in ,figs4 #ec-eg1 there are thirteen lying along the meizoseismal area of the earthquake of #ahia1 in which nearly all the after-shocks originated4 ,during the five years #ahhe-hi1 #ec out of #abe earthquakes 7or #db per cent47 had their epicentres lying within the thirteen rectangles2 or1 in other words1 the average frequency in one of the rectangles of the meizoseismal area was five times as great as in one of those outside it4 ,in #ahij and #ahia 7until ,october #bgth71 the percentage in the thirteen rectangles rose to #fa1 and the average frequency in one of them to ten times that of one of the exterior rectangles4 ,the curves in ,fig4 #ec illustrate the distribution of epicentres during the latter interval4 ,it will be seen that they follow roughly the course of the meizoseismal area southwards to the ,sea of ,is1 and that to the south-east they continue for several miles the short branch of the meizoseismal area which surrounds the southern end of the fault-scarp4 ,illustration3 ,,fig4 #ec4--;,distribution of preliminary ,shocks in ,space4 7,davison47 ,thus1 the preparation for the great earthquake is shown1 first1 by the increased frequency of earthquakes originating within its meizoseismal area2 and1 secondly1 by the uniformity in the distribution of epicentres throughout the same region1 the marked concentration of effort which characterises the after-shocks being hardly perceptible during the years #ahij-ia4 ,illustration3 ,,fig4 #ed4--;,distribution of ,after-shocks in ,space 7,november-,december #ahia74 7,davison74 ,distribution of ,after-shocks in ,space4--,we have seen that the after-shocks were subject to a fluctuating decline in frequency1 rapid at first1 and more gradual afterwards4 ,it is evident1 from ,figs4 #ed-eg1 that a similar law governs the area within which the after-shocks originated4 ,during the first two months1 epicentres occur over nearly the whole of the meizoseismal area1 but afterwards they are confined to a smaller district1 which slowly1 though not continually1 decreases in size4 ,illustration3 ,,fig4 #ee4--;,distribution of ,after-shocks in ,space 7,january-,february1 #ahib74 7,davison47 ,illustration3 ,,fig4 #ef4--;,distribution of ,after-shocks in ,space 7,march-,april74 7,davison47 ,the most important feature in the distribution of the epicentres is the central region of extraordinary activity2 but there are also districts of minor and more short-lived activity near the three extremities of the meizoseismal band4 ,the seat of chief seismic action shifts slightly from one part to another of the epicentral region1 especially about the end of #ahia1 as will be seen by comparing the innermost curves of ,figs4 #ed and #ee4 ,thus1 with the decline in frequency of the after-shocks and the decrease in their sphere of action1 there took place concurrently a gradual but oscillating withdrawal of that action to a more or less central region of the fault4 ,illustration3 ,,fig4 #eg4--;,distribution of ,after-shocks in ,space 7,may-,june1 #ahib74 7,davison47 ,sound ,phenomena of ,after-shocks4--,while comparatively few observers seem to have noticed any noise with the principal earthquake1 many of the after-shocks were accompanied by sounds4 ,professor ,omori describes them as belonging to two types4 ,they were either rushing feeble noises like that of wind1 or loud rumbling noises like those of thunder1 the discharge of a gun1 or the fall of a heavy body4 ,in the ,neo valley1 sounds of the second type were most frequent and distinct1 but they either occurred without any shock at all1 or the attendant tremor was very feeble2 while1 on the other hand1 severe sharp shocks were generally unaccompanied by distinctly audible sounds4 ,it is remarkable1 also1 that sounds were less frequently heard with the early than with the later after-shocks4 ,in ,november #ahia1 the percentage of audible shocks was #ag1 and from ,december to the following ,april always lay between #aj and #ab4 ,in ,may the percentage suddenly rose to #ci1 and until the end of #ahib was always greater than #cb1 while in ,november #ahib1 it rose as high as #di4 ,this1 of course1 agrees with ,professor ,omori's observation that sounds attended feeble shocks more often than strong ones4 ,the distribution of the audible after-shocks in space is shown in ,fig4 #eh4 ,these curves are drawn in the same way as those in ,figs4 #ec-eg1 but they represent the percentages1 not the actual numbers1 of shocks accompanied by sound4 ,it will be noticed that all three groups of curves lie along the meizoseismal area1 or the continuation of the south-east branch2 while the axis of the principal group of curves lies to the west of the central regions in which most after-shocks originated4 ,illustration3 ,,fig4 #eh4--;,distribution of ,audible ,after-shocks in ,space 7,november #ahia-;,december #ahib74 7,davison47 ,the explanation of these peculiarities is no doubt connected with the comparative inability of the ,japanese people to perceive the deep sounds which in ,europe are always heard with earthquake shocks4 ,the sounds are rarely heard by them more than a few miles from the epicentre4#fj ,we may therefore conclude that slight after-shocks originated nearer the surface than strong ones1 that the mean depth of the foci decreased with the lapse of time1 and that the axes of the systems of curves in ,fig4 #eh mark out approximately the lines of the growing faults4 ,the separation of the two westerly groups of curves appears to show that the main branch of the meizoseismal area is connected with a fault roughly parallel to that traced by ,professor ,koto1 but of which no scarp 7if it existed7 could be readily distinguished among the superficial fissures produced by the great shock4 ,,effect ,,of ,,the ,,earthquake ,,on ,,the ,,seismic ,,activity ,,of ,,the ,,adjoining ,,districts4 ,so great and sudden a displacement as occurred along the fault-scarp could hardly take place without affecting the stability of adjoining regions of the earth's crust1 and we should naturally expect to find a distinct change in their seismic activity shortly after ,october #bhth4 ,in ,fig4 #ei two such regions are shown1 bounded by the straight dotted lines4 ,the district in which the principal earthquake and its after-shocks originated is enclosed within the undulating dotted lines4 ,the continuous lines inside all three districts are the curves corresponding to #aj and #e epicentres for the years #ahhe-ib4 ,not far from the axes of the outer groups of curves there are probably transverse faults1 approximately parallel to the great fault-scarp and the main branch of the meizoseismal band1 and distant from them about #de and #ee miles respectively4 ,illustration3 ,,fig4 #ei4--,map of ,adjoining ,regions in which ,seismic ,activity was affected by the ,great ,earthquake4 7,davison47 ,in the district represented in the north-east corner of ,fig4 #ei1 #bi earthquakes originated between ,january #ast1 #ahhe1 and ,october #bgth1 #ahia1 and #cj between ,october #bhth1 #ahia1 and ,december #cast1 #ahib1 #g of the latter number occurring in ,november #ahia4 ,in the south-west district1 the corresponding figures before and after the earthquake are #bj and #cf1 #h of the latter occurring in ,november #ahia4 ,thus1 in the north-east district1 for every shock in the interval before the earthquake there were six in an equal time afterwards1 and at the rate of #aj during ,november #ahia2 and in the south-west district1 for every shock before the earthquake there were #aj afterwards1 and at the rate of #af during ,november #ahia4 ,now1 it is unlikely that the gradual increase of stress should be so nearly proportioned everywhere to the prevailing conditions of resistance as to give rise to a marked and practically simultaneous change in seismic activity over a large area2 whereas the paroxysmal occurrence of a strong earthquake might alter the surrounding conditions with comparative rapidity1 and so induce a state of seismic excitement in the neighbourhood4 ,it therefore seems very probable that the increased activity in the two districts here described was a direct consequence of the occurrence of the great earthquake4 ,,origin ,,of ,,the ,,earthquake4 ,the preponderance of preliminary earthquakes within the meizoseismal area and the outlining of the fault-system by the frequency curves of #ahij-ia 7,fig4 #ec7 point to the previous existence of the originating fault or faults1 and to the earthquake being due1 not to the formation of a new fracture1 as has been suggested1 but to the growth of an old fault4 ,the last severe earthquake in the ,mino-,owari plain occurred in #ahei1 so that for more than thirty years there had been but little relief to the gradually increasing stresses4 ,now1 the distribution of stress must have been far from uniform throughout the fault-system1 and also the resistance to displacement far from proportional to the stresses at different places4 ,at certain points1 therefore1 the effective stress would be greater than elsewhere1 and it would be at these points that fault-slips would first occur4 ,such slips tend to remove the inequalities in effective stress4 ,thus1 the function of the slight shocks of #ahij and #ahia was1 briefly1 to equalise the effective stress over the whole fault-system1 and so to clear the way for one or more great slips throughout its entire length4 ,as to which side of the fault moved during the great displacement1 or whether both sides moved at once1 we have no direct evidence but as regards the neighbourhood of ,midori1 and there the conditions were exceptional4 ,professor ,koto thinks that it was probably the rock on the north-east side that was generally depressed and always shifted to the north-west ,but the disturbance in reality seems to have been more complicated4 ,that this was the case1 that displacement occurred along more than one fault1 is probable from the branching of the meizoseismal area1 the isolation of the audibility curves of the after-shocks 7,fig4 #eh71 and the sudden increase in seismic activity both to the north-east and south-west of the epicentre4 ,the detached portion of the meizoseismal area near ,lake ,biwa may also point to a separate focus4 ,the whole region1 indeed1 was evidently subjected to intense stresses1 and the depression on the north-east side of the fault-scarp can hardly fail to have been accompanied by other movements1 especially along a fault running near the western margin of the main branch of the meizoseismal area4 ,the later stages of the movements are somewhat clearer4 ,from a study of the after-shocks1 we learn that the disturbed masses began at once to settle back towards the position of equilibrium4 ,at first the slips were numerous and took place over the whole fault-system1 but chiefly at a considerable depth1 where no doubt the initial displacement was greatest4 ,after a few months1 stability was nearly restored along the extremities of the faults2 slips were confined almost entirely to the central regions1 while a much larger proportion of them took place within the superficial portions of the faults4 ,the official records bring down the history to the end of #ahic4 ,since that time more than one strong shock has been felt in the ,mino-,owari plain2 but the stage of recovery from the disturbances of #ahia is probably near its end1 and we seem rather to be entering on a period in which the forces are once more silently gathering that sooner or later will result in another great catastrophe4 ,,references4 #a4 ,,conder1 ,j4--,an ,architect's ,notes on the ,great ,earthquake of ,october #ahia4 ,japan ,seismol4 ,journ41 vol4 ii41 #ahic1 pp4 #a-ia4 #b4 ,,davison1 ,c4--,on the ,distribution in ,space of the ,accessory ,shocks of the ,great ,japanese ,earthquake of #ahia4 ,quart4 ,journ4 ,geol4 ,soc41 vol4 liii41 #ahig1 pp4 #a-ae4 #c4 ---- ,on the ,effect of the ,great ,japanese ,earthquake of #ahia on the ,seismic ,activity of the ,adjoining ,districts4 ,geol4 ,mag41 vol4 iv41 #ahig1 pp4 #bc-bg4 #d4 ---- ,on the ,diurnal ,periodicity of ,earthquakes4 ,phil4 ,mag41 vol4 xiii41 #ahif1 pp4 #dfc-dgf1 especially pp4 #dff-dfh4 #e4 ---- ,on ,earthquake-,sounds4 ,phil4 ,mag41 vol4 xlix41 #aijj1 pp4 #ca-gj--;especially pp4 #di-ec1 #fj-fa4 #f4 ,,koto1 ,b4--,the ,cause of the ,great ,earthquake in ,central ,japan1 #ahia4 ,journ4 ,coll4 ,sci4 ,imp4 ,univ4 ,japan1 vol4 v41 #ahic1 pp4 #bie-cec4 #g4 ,,masato1 ,h4--,report on ,earthquake ,observations in ,japan4 ,cent4 ,meteor4 ,obs4 of ,japan 7,tokio1 #ahib71 pp4 #af-ah1 #da1 and map #cj4 #h4 ,,milne1 ,j4--,a ,note on the ,great ,earthquake of ,october #bhth1 #ahia4 ,japan ,seismol4 ,journ41 vol4 i41 #ahic1 pp4 #abg-aea2 ,brit4 ,assoc4 ,rep41 #ahib1 pp4 #aad-abh4 #i4 ---- ,a ,catalogue of #h1cca ,earthquakes recorded in ,japan between #ahhe and #ahib4 ,japan ,seismol4 ,journ41 vol4 iv41 #ahie1 pp4 #a-cfg--;especially pp4 #acd-bcd1 #cjc-cec4 #aj4 ---- ,on ,certain ,disturbances in the ,records of ,magnetometers and the ,occurrence of ,earthquakes4 ,brit4 ,assoc4 ,rep41 #ahih1 pp4 #bbf-bea--;especially pp4 #bbg1 #bcb1 #bcd1 #bda1 and #bde4 #aa4 ,,milne1 ,j41 and ,w4,k4 ,,burton4--,the ,great ,earthquake in ,japan4 ,journ4 ,coll4 ,sci4 ,imp4 ,univ4 ,japan1 vol4 v41 #ahic1 pp4 #bie-ceb4 #ab4 ,,omori1 ,f4--,on the ,after-shocks of ,earthquakes4 ,journ4 ,coll4 ,sci4 ,imp4 ,univ4 ,japan1 vol4 vii41 #ahid1 pp4 #aaa-bjj2 abstract in ,japan ,seismol4 ,journ41 vol4 iii41 #ahid1 pp4 #ga-hj4 #ac4 ---- ,a ,note on the ,great ,mino-,owari ,earthquake of ,october #bhth1 #ahia4 ,pub4 ,earthquakes ,inves4 ,com4 in ,foreign ,languages1 ,no4 #d1 ,tokio1 #aijj1 pp4 #ac-bd4 #ad4 ---- ,sulla velocit di propagazione e sulla lunghezza delle onde sismiche4 ,ital4 ,soc4 ,sismol4 ,boll41 vol4 i41 #ahie1 pp4 #eb-fj--;especially pp4 #eb-eg4 #ae4 ---- ,sull' intensit e sull' ampiezza del movimento nel gran terremoto giapponese del #bh ottobre #ahia4 ,ital4 ,soc4 ,sismol4 ,boll41 vol4 ii41 #ahif1 pp4 #ahi-bjj4 #af4 ---- ,note on the ,after-shocks of the ,mino-,owari ,earthquake of ,oct4 #bhth1 #ahia4 ,pub4 ,earthquakes ,inves4 ,com4 in ,foreign ,languages1 ,no4 #g1 ,tokio1 #aijb1 pp4 #bg-cb4 #ag4 ---- ,note on the relation between ,earthquake ,frequency and the ,atmospheric ,pressure4 ,tokyo ,phys4-,math4 ,soc4 ,reports1 vol4 ii41 #aijd1 ,no4 #h4 #ah4 ,,tanakadate1 ,a41 and ,h4 ,,nagaoka4 ,the ,disturbance of ,isomagnetics attending the ,mino-,owari ,earthquake of #ahia4 ,journ4 ,coll4 ,sci4 ,imp4 ,univ4 ,japan1 vol4 v41 #ahic1 pp4 #adi-aib4 ,,footnotes3 #ed ,i have not referred to the results of this survey1 for1 though changes in all the magnetic elements 7especially in horizontal intensity7 have taken place between #ahhg and #ahia-ib1 these changes cannot be ascribed with confidence to the earthquake in the absence of a thorough knowledge of the secular variation4 #ee ,from the formula ax99g_/y1 where a is the maximum horizontal acceleration1 g the acceleration due to gravity1 y the height of the centre of gravity1 and x its horizontal distance from the edge about which the body was overturned4 #ef ,these estimates are made1 on the supposition of simple harmonic motion1 from the formula #b99;aalpha99t#b_/7b99pi#b71 where #b;a is the total range or double amplitude1 a the maximum acceleration1 and t the period of the vibration4 #eg ,professor ,omori finds that the mean daily number of earthquakes y during the month x 7reckoned from ,november #ahia7 may be approximately represented by the equation-- y #af4i _/ 7x #j4cig72 or1 taking the semi-daily earthquake numbers during the five days between ,october #bith and ,november #bnd1 #ahia1 by the equation-- y #ddj4g _/ 7x #b4cad71 where y denotes the number of earthquakes observed during the twelve hours denoted by x1 the time being measured from the first half of ,october #bith4 ,it is interesting to notice that1 taking account of the mean annual frequency of earthquakes in ordinary years1 the number of shocks observed at ,gifu during the two years #ahih-ii should1 according to the latter formula1 be #afc2 the actual number recorded was #afj4 #eh ,the last violent shock before the end of #ahic occurred on ,september #gth1 #ahib1 and its effects on the frequency of after-shocks is shown by the daily numbers recorded at ,gifu during the first fortnight in ,september4 ,these are--#b1 #b1 #b1 #c1 #e1 #e1 #bh 7on ,september #gth71 #h1 #h1 #e1 #d1 #c1 #b1 #d1 #c4 #ei ,the periodicity of after-shocks is discussed in the papers numbered #d1 #ab1 #af1 and #ag at the end of this chapter4 ,in these1 the existence of diurnal and other periods is clearly established4 ,professor ,omori also shows that the mean daily barometric pressure is subject to fluctuations with maxima occurring on an average every #e-a_/b days1 and that earthquakes are least frequent on the days of the barometric maxima and minima1 and more frequent in the days immediately preceding and following them4 #fj ,of the ,japanese earthquakes of #ahhe-ib originating beneath the land1 twenty-six per cent4 were accompanied by a recorded sound2 but less than one per cent4 of those originating beneath the sea and not more than ten miles from the coast4 ,,chapter ,,viii4 ,,the ,,hereford ,,earthquake ,,of ,,december #ag,,th1 #ahif1 ,,and ,,the ,,inverness ,,earthquake ,,of ,,september #ah,,th1 #aija4 ,among the earthquakes described in this volume1 the ,hereford and ,inverness earthquakes hold but a minor place4 ,the damage to buildings1 though unusual for this country1 was slight when compared with that caused by the preceding shocks2 there was no loss of life1 not a single person was injured by falling masonry4 ,the interest of the earthquakes lies entirely in the detailed study rendered possible by numerous observations of the shock and sound1#fa and in the bearing of this evidence on the general theory of the origin of earthquakes4 ,,the ,,hereford ,,earthquake ,,of ,,december #ag,,th1 #ahif4 ,the principal earthquake of this series occurred at #e4cb ,a4,m4 on ,december #agth1 and was preceded by at least nine minor shocks 7the first of which was felt at about #aa or #aa4cj ,p4,m4 on ,december #afth71 and followed by two others on the same day1 and by a third and last on ,july #aith1 #ahig4 ,the accounts of these preliminary movements will be found on a later page1 as their bearing will be more fully apparent after the discussion of the principal shock4 ,illustration3 ,,fig4 #fj4--;,isoseismal and ,isacoustic lines of ,hereford earthquake4 7,davison47 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,on the map in ,fig4 #fj1 the continuous curves represent isoseismal lines corresponding to the degrees #h1 #g1 #f1 #e1 and #d of the ,rossi-,forel scale4 ,the isoseismal #h1 which is the most accurately drawn of the series1 is an elongated oval1 #dj miles long1 #bc miles broad1 and containing an area of #gbd square miles4 ,the longer axis is directed ,w4 #dd ,n4 and ,e4 #dd ,s4 ,within this curve1 there are #gc places where buildings are known to have been damaged1 #ee places being in ,herefordshire1 #ag in ,gloucestershire1 and one in ,worcestershire4 ,the most important damage occurred in the city of ,hereford1 which1 in #aija1 contained #d1efe inhabited houses4 ,here1 no fewer than #bah chimneys had to be repaired or rebuilt4 ,the ,cathedral was slightly injured4 ,the finial of a pinnacle of the ,lady ,chapel was thrown down1 a fragment of a stone fell from one of the arches in the south transept1 and the three pinnacles of the western front were fractured4 ,several churches suffered to a similar extent1 while1 at the ,midland ,railway ,station1 all the seven chimney-stacks were shattered4 ,at ,dinedor1 ,fownhope1 ,dormington1 ,withington1 and a few other villages1 the damage was also relatively greater than elsewhere1 these places all lying within a small oval about #h-a_/b miles long1 which surrounds1 not the centre1 but rather the north-west focus1 of the isoseismal #h4 ,the isoseismal #g1 which includes places where the shock was strong enough to overthrow ornaments1 vases1 etc41 is also very nearly an ellipse1 whose axes are #hj and #ef miles in length1 and whose area is #c1ehj square miles4 ,its longer axis1 running from ,w4 #db ,n4 to ,e4 #db ,s41 is practically parallel to that of the inner curve4 ,next in succession comes the isoseismal #f1 surrounding those places where the shock was strong enough to make chandeliers1 pictures1 etc41 swing2 but1 as most of the observers seem to have slept in darkened rooms1 the number of determining points for this curve is less than usual1 and its course is therefore laid down with a somewhat inferior degree of accuracy4 ,the error1 however1 is probably small1 and we may therefore regard the isoseismal #f as another ellipse1 #ada miles long1 #aaf miles broad1 and containing an area of #ac1jjj square miles4 ,its longer axis is again nearly parallel to those of the preceding isoseismals4 ,the next two isoseismals are nearly circular in form4 ,it will be noticed that large portions of them1 and especially of the isoseismal #d1 traverse the sea4 ,in these parts1 the paths of the curves are to some extent conjectural4 ,in drawing them1 the chief guides are their trend before leaving the land and the known intensity along the neighbouring coastlines4 ,the isoseismal #e bounds the area within which the shock was perceptible as a sensible displacement and not merely a quiver4 ,its dimensions are #bcc miles from north-west to south-east1 and #bbi miles from south-west to north-east1 and its area #da1afj square miles4 ,the isoseismal #d1 which includes places where the shock was strong enough to make doors1 windows1 etc41 rattle1 is #cef miles from north-west to south-east1 and #ceg miles from south-west to north-east1 and #ih1jjj square miles in area2 its centre coincides nearly with that of the small oval area in the neighbourhood of ,hereford1 where the damage to buildings was relatively greater than elsewhere4 ,outside the isoseismal #d1 the earthquake was observed at several places4 ,the shock was certainly felt at ,middlesbrough1 #ab-a_/b miles from the curve1 and probably at ,killeshandra 7in ,ireland71 #fe miles distant4 ,thus1 if we consider the boundary of the disturbed area to coincide with the isoseismal #d1 its area would be #ih1jjj square miles1 or #a-b_/c that of ,england and ,wales2 if it were a circle concentric with the isoseismal #d1 and passing through ,middlesbrough1 its area would be #aae1jjj square miles1 or nearly twice that of ,england and ,wales2 while1 if it passed through ,killeshandra1 its area would be #ahe1jjj square miles1 or more than three times the area of ,england and ,wales4#fb ,position of the ,originating ,fault4--,the form1 directions1 and relative positions of the isoseismal lines furnish important evidence with regard to the originating fault4 ,we conclude in the first place that its mean direction is parallel to the longer axes of the three innermost isoseismal lines--that is1 north-west and south-east1 or1 more accurately1 ,w4 #dc ,n4 and ,e4 #dc ,s4#fc ,in this case1 the elongated forms of the isoseismal lines cannot be attributed to variations in the nature of the surface rocks4 ,the district embraced contains about #ac1jjj square miles1 and it is improbable that the axes of the three isoseismals should retain their parallelism over so large an area1 if these variations had any considerable effect4 ,moreover1 in the same district1 an earthquake occurred in #ahfc1 whose meizoseismal area was elongated from north-east to south-west1 or almost exactly perpendicular to the direction in #ahif4 ,secondly1 it will be noticed 7,fig4 #fj7 that the isoseismal lines are not equidistant from one another4 ,on the north-east side1 they are separated by distances of #bj1 #cd1 #ee1 and #ea miles2 and on the south-west side by distances of #ac-a_/d1 #be1 #fj1 and #gg miles4 ,it follows from this that the fault-surface must hade or slope towards the north-east2 for1 near the epicentre1 the intensity is greatest and dies out more slowly on the side towards which the fault hades4 ,if we could ascertain any one place through which the fault passed1 its position would thus be completely determined4 ,unfortunately1 there is no decisive evidence on this point4 ,there are1 however1 several places to the south-west of ,hereford where the intensity of the shock was distinctly less than in the surrounding district1 and it is possible that this was due to their neighbourhood to the fault-line 7see p4 #ace74 ,if so1 the originating fault must have extended from a point about a mile and a half west of ,hereford for a distance of about #af miles to the south-east2 and a fault in this position would certainly satisfy all the details of the seismic evidence4 ,,nature ,,of ,,the ,,shock4 ,throughout the disturbed area1 considerable variations were observed in the nature of the shock4 ,these changes were due to the mere size of the focus1 to its elongated form and1 as will be seen1 to its discontinuity1 and also to the distance of the place of observation from the epicentre4 ,at places near the epicentre1 rapid changes in the direction of the shock were observed owing to the large angle subtended by the focus2 while1 at considerable distances1 this angle being small1 the changes of direction were imperceptible4 ,a further variation with the distance was an increase in the period of the vibrations4 ,close to the epicentre1 the general impression was that of crossing the wake of a steamer in a very short rowing-boat1 or of riding in a carriage without springs4 ,at distances of a hundred miles or more1 the movement is described as being of a pleasant1 gentle1 undulating character1 like that felt during the rocking of a ship at anchor or in a carriage with well-appointed springs4 ,the most remarkable feature of the shock1 however1 was its division into two distinct parts or series of vibrations1 separated by an interval1 lasting two or three seconds1 of absolute rest and quiet4 ,and this was no mere local phenomenon4 ,with the exception of a narrow band that will be referred to presently1 records of the double shock come from nearly all parts of the disturbed area1 even from districts so remote as the ,isle of ,man and the east of ,ireland4 ,the two parts differed in intensity1 in duration1 and in the period of their constituent vibrations4 ,for instance1 at ,oaklands 7near ,chard71 a shivering motion was first felt1 and then1 after about three or four seconds1 a distinct rocking from side to side4 ,at ,exeter1 there was a sudden tremor lasting about two seconds1 followed1 after two or three seconds1 by another and more severe shaking lasting four or five seconds4 ,again1 at ,west ,cross 7near ,swansea71 an undulatory movement for about four seconds was followed soon after by a tremulous shock4 ,at ,liverpool1 the durations of the first part1 interval1 and second part were respectively estimated at about six1 two1 and four seconds4 ,as a first result of the observations1 then1 it appears that in the south-east half of the disturbed area1 the second part of the shock was the stronger1 of greater duration and consisted of longer-period vibrations 7as at a1 ,fig4 #fa72 while1 in the north-west half1 the same features characterised the first part of the shock 7b1 ,fig4 #fa74 ,a closer examination of the records shows1 however1 that the boundary between the two portions of the disturbed area was not a straight line1 but slightly curved1 the concavity facing the south-east4 ,the broken line on the map 7,fig4 #fj71 which is hyperbolic in form1 represents roughly the position of this curved boundary4#fd ,illustration3 ,,fig4 #fa4--,nature of shock of ,hereford earthquake4 ,along this hyperbolic boundary-line1 or rather within a narrow band of which it is the central line1 the shock lost its double character1 and was manifested as a single series of vibrations gradually increasing in intensity and then dying away4 ,close to the edges of this band1 careful observers were able to distinguish two maxima of intensity connected by a continuous series of tremors 7c1 ,fig4 #fa74 ,thus1 within the band1 the two series of vibrations1 which elsewhere were isolated1 must have been superposed on one another2 while1 near the edges of the band1 the concluding tremors of the first series overlapped the initial tremors of the second4 ,origin of the ,double ,series of ,vibrations4--,the ,hereford earthquake thus belongs to the same class as the ,neapolitan1 ,andalusian1 ,charleston1 and ,riviera earthquakes4 ,as in these cases1 the hypothesis of a single focus is inadmissible4 ,the division of the disturbed area into two regions of opposite relative intensity1 duration1 etc41 is sufficient proof that a single series of vibrations was not duplicated by reflection or refraction1 or by separation into longitudinal and transverse waves4 ,it is equally conclusive against a repetition of the impulse within the same focus4 ,we must therefore infer that the focus consisted of two nearly or quite detached portions arranged along a north-west and south-east line1 and that the impulse at the north-west focus was the stronger of the two4 ,the only question that remains to be decided is whether the impulses at the two foci were simultaneous or not4 ,now1 if the impulses occurred at the same instant1 the waves from the two foci would travel with the same velocity1 and would therefore coalesce along a straight band which would bisect at right angles the line joining the two epicentres4 ,but we have already seen that this band is curved1 and it thus follows that the two impulses were not simultaneous4 ,again1 since the concavity of the hyperbolic band faces the south-east1 the waves from the north-west focus must have travelled farther than those from the south-east focus before the two met along the hyperbolic band2 in other words1 the impulse at the north-west focus must have occurred two or three seconds before the impulse at the other4 ,position and ,dimensions of the ,two ,foci4--,there can be little doubt that the impulse at the north-west focus was responsible for the greater damage to buildings at ,hereford1 ,dinedor1 ,fownhope1 etc4 ,the centre of its epicentral area must therefore lie about three miles south-east of ,hereford4 ,it is probable1 also1 that the corresponding centre of the other focus is similarly placed with respect to the south-east portion of the isoseismal #h--that is1 about two or three miles north-east of ,ross4 ,these two points are eight or nine miles apart4 ,now1 since1 as we shall see1 the mean surface-velocity of the earth-waves was about #cjjj feet per second1 and the mean duration of the quiet interval between the two series was #c-a_/b seconds1 the nearest ends of the two foci must have been separated by a distance of not less than two miles4 ,moreover1 since the series of vibrations from the north-west or ,hereford focus lasted a few seconds longer than that from the south-east or ,ross focus1 the former must have been about two miles longer than the latter1 and we may therefore estimate their lengths at about eight and six miles respectively4 ,including the undisturbed intermediate portion1 this would give a total length of focus of about #af miles1 a result we have already inferred from the dimensions of the isoseismal #h4 ,,direction ,,of ,,the ,,shock4 ,although no question was asked with regard to the direction of the shock1 no fewer than #dfi observers made notes on this point4 ,as a general rule1 their determinations are extremely rough1 few referring to more than the eight principal points of the compass4 ,moreover1 in any one place1 the directions assigned to the shock are very varied4 ,for instance1 in the city and suburbs of ,birmingham1 eight observers give the direction along a north and south line1 eight east and west1 eleven north-west and south-east1 and five north-east and south-west1 while there are five other intermediate estimates4 ,but1 when these directions are plotted on a map of the district1 it is seen at once that they are either nearly parallel or perpendicular to the roads in which the observers were living2 that is1 the apparent direction of the shock was at right angles to one of the principal walls of the house4 ,this1 of course1 is a result to be anticipated1 for1 whatever be the direction of the earthquake-motion1 a house tends to oscillate in a plane perpendicular to one or other of its walls4 ,it is extraordinary to how great a distance the direction of the shock is perceptible4 ,records come from ,brighton 7#acg miles from the epicentre71 ,maldon in ,essex 7#add miles71 ,harrogate 7#adg miles71 ,douglas in the ,isle of ,man 7#afg miles71 ,dublin 7#agf miles71 and ,baltinglass in ,co4 ,wicklow 7#ahj miles74 ,nevertheless1 whatever the distance may be1 the sense of direction must be most perceptible in those houses whose principal walls are at right angles to the true direction of the earthquake-motion1 and we should therefore expect to find the observations of direction most frequently made in such houses1 or in others which approximate to this situation4 ,thus1 the average of all the observations within a fairly small area should give a result not very far from the true direction of the shock2 and1 the smaller the area and the farther from the epicentre1 the more reliable should be the result4 ,now1 in ,birmingham the mean direction of the shock is ,e4 #ci ,n41 which differs only by #b from the line joining the city to the epicentre2 in ,london it is ,e4 #ba ,s41 the difference being again #b4 ,in other cases1 the observations from different counties are grouped together1 and the mean direction is taken to correspond to the centre of the county4 ,yet1 even then1 there is often a close agreement between the mean direction of the shock and the direction of the county-centre from the epicentre2 the difference being not more than two or three degrees in the counties of ,buckingham1 ,devon1 ,stafford1 ,warwick1 and ,york4 ,in other cases1 where the deviation exceeds this amount1 either the number of observations is small or the county is near the epicentre and so subtends a large angle4 ,two results of some importance follow from this analysis3 7#a7 that while1 with a few isolated observations1 the method of directions is almost sure to fail1 with a large number of observations closely grouped1 the position of the epicentre may be determined with a fair approach to accuracy2 and 7#b7 that1 at any rate outside a radius of forty miles1 the earth-waves travelled in approximately straight lines outwards from the epicentre4 ,,coseismal ,,lines ,,and ,,velocity ,,of ,,earth-,,waves4 ,coseismal lines were defined by ,mallet as long ago as #ahdi1 but1 owing to the difficulty of ascertaining the correct time1 they have so far been of little service in the investigation of earthquakes4 ,in the case of the ,hereford earthquake1 the distances traversed by the earth-waves are small2 but1 on the other hand1 the time-records are numerous and frequently trustworthy to the nearest minute4 ,rejecting all estimates earlier than #e4cb ,a4,m41 and later than #e4cf1 as well as a number at #e4ce1 there remain fairly good observations from #cha places1 and exceptionally accurate ones from #cc places4 ,the latter were obtained from signalmen and other careful observers who were in possession of ,greenwich time1 or who compared their watches shortly afterwards with well-regulated watches4 ,with evidence so abundant1 a new method of drawing coseismal lines becomes possible4 ,according to this method1 each place of observation is indicated on the map by a mark corresponding to the particular minute recorded4 ,if the records were quite correct1 there would be a central area occupied by the marks corresponding to #e4cb ,a4,m41 surrounded by a series of zones in which the times were respectively #e4cc1 #e4cd1 and #e4ce4 ,the curves separating these zones would be coseismal lines corresponding to the times #e4cb-a_/b1 #e4cc-a_/b1 and #e4cd-a_/b4 ,owing1 however1 to the inevitable inaccuracy of all the time-records1 these different zones intrude on one another1 and the coseismal lines have therefore to be drawn about half-way through the overlapping regions1 special weight being attributed to the apparently more accurate observations4 ,illustration3 ,,fig4 #fb4--;,coseismal lines of the ,hereford earthquake4 7,davison47 ,the coseismal lines obtained in this manner are represented by the continuous curves in ,fig4 #fb4 ,the isoseismals1 which are added for the sake of comparison1 are indicated by the dotted lines4 ,it will be seen that the coseismal lines are elongated in the same direction as the isoseismals1 but to a less extent1 and this no doubt is due to the fact that the epoch selected by the majority of observers was one not far from1 and slightly preceding1 that of the maximum intensity of the shock4 ,now1 the average distance between the two inner coseismals is #cb-c_/d miles1 between the two outer ones 7so far as drawn7 #ce-a_/f miles1 and between the first and third #fg-a_/f miles4 ,the mean surface-velocity between the two inner coseismals is therefore #b1hhb feet per second1 and between the two outer ones #c1jie feet per second4 ,there is thus an apparent increase in the velocity with the distance1 but the accuracy of the coseismal lines is unequal to establishing this as a fact4 ,the mean surface-velocity of #b1iee feet per second between the first and third coseismals is probably1 however1 the most accurate estimate of the surface-velocity yet made in a slight earthquake4 ,,sound-,,phenomena4 ,nature of the ,sound4--,the sound which accompanied the shock was of the same character as that heard during all great earthquakes4 ,it is often described in such terms as a deep booming noise1 a dull heavy rumble1 a grating roaring noise1 or a deep groan or moan2 more rarely as a rustling or a loud hissing rushing sound4 ,as a rule1 it began faintly1 increased gradually in strength1 and then as gradually died away2 and this no doubt is the reason why it sometimes appeared as if an underground train or waggon were approaching quickly1 rushing beneath the observer1 and then receding in the opposite direction4 ,occasionally1 the sound was very loud1 being compared to the noise of many traction-engines heavily laden passing close at hand1 or to a heavy crash or peal of thunder4 ,but its chief characteristic was its extraordinary depth1 as if it were almost too low to be heard4 ,according to one observer1 it was a low rumbling sound1 much lower than the lowest thunder2 and another compared it to the pedal notes of a great organ1 only of a deeper pitch than can be taken in by the human ear1 a noise more felt than heard4 ,it will be seen presently how the sound1 from its very depth1 was inaudible to many persons4 ,a few observers described the sound in terms like those quoted above1 but by far the larger number compared it to some more or less well-known type1 and in many cases the resemblance was so close that the observer at first attributed it to the object of comparison4 ,the descriptions1 which present great varieties in detail1 may be classified as follows3 7#a7 ,one or several traction-engines passing1 either alone or heavily laden1 sometimes driven furiously past2 a steam-roller passing over frozen ground or at a quicker pace than usual2 heavy waggons driven over stone paving1 on a hard or frosty road1 in a covered way or narrow street1 or over hollow ground or a bridge2 express or heavy goods trains rushing through a tunnel or deep cutting1 crossing a wooden bridge or iron viaduct1 or a heavy train running on snow2 the grating of a vessel over rocks1 or the rolling of a lawn by an extremely heavy roller2 7#b7 a loud clap or heavy peal of thunder1 sometimes dull1 muffled or subdued1 but most often distant thunder2 7#c7 a moaning1 roaring1 or rough1 strong wind2 the rising of the wind1 a heavy wind pressing against the house2 the howling of wind in a chimney1 a chimney or oil-factory on fire2 7#d7 the tipping of a load of coal1 stones1 or bricks1 a wall or roof falling1 or the crash of a chimney through the roof2 7#e7 the fall of a heavy weight or tree1 the banging of a door1 only more muffled1 and the blow of a wave on the sea-shore2 7#f7 the explosion of a boiler or cartridge of dynamite1 a distant colliery explosion1 distant heavy rock-blasting and the boom of a distant cannon2 7#g7 sounds of a miscellaneous character1 such as the trampling of many men or animals1 an immense covey of partridges on the wing1 the roar of a waterfall1 the passage of a party of skaters1 and the rending and settling together of huge masses of rock4 ,the total number of comparisons made was #a1bfd4 ,of these1 #de4d per cent4 refer to passing waggons1 etc41 #ae4j per cent4 to thunder1 #ae4e to wind1 #c4i to loads of stones falling1 #b4g to the fall of a heavy body1 #g4b to explosions1 and #aj4c per cent4 to miscellaneous sounds4 ,generally1 the sound adhered throughout to one of the types mentioned above1 and1 if it varied at all1 varied only in intensity4 ,at some places1 however1 the character of the sound was observed to change4 ,for instance1 one person described it as like the rumbling of a train going over a bridge1 with a terrific crash1 such as is heard in a thunderstorm at the instant when the shock was strongest1 the rumbling dying away afterwards for some seconds4 ,inaudibility of the ,sound to some ,observers4--,the total number of observers who give a detailed account of the earthquake is #b1fha1 and1 of these1 #ei per cent4 state that they heard the sound1 #bc per cent4 give no information1 while #ah per cent4 distinctly say that they heard no sound2 that is1 roughly1 out of every five observers1 three heard the sound1 one made no reference to it1 and one failed to hear the sound4 ,in a few cases1 no doubt1 this failure was due to the distance of the observer1 but this is far from being a complete explanation2 for1 in ,herefordshire1 six out of #agi1 and in ,gloucestershire #ag out of #bbg1 observers heard no sound4 ,nor is the peculiarity a local one1 for at ,clifton two out of five observers who were awake did not hear the sound1 at ,birmingham four out of #bc1 and in ,london1 eight out of #ah4 ,even in the same house1 it would happen that one observer would hear a sound as of a heavily-laden traction-engine passing1 while to another it was quite inaudible4 ,again1 a large number of observers who heard the sound expressly state that they were unconscious of any while the shock lasted4 ,the noise at first resembled the approach of a steam-roller or traction-engine up the street1 it became gradually louder1 and then ceased more or less suddenly as the shock began2 while1 to others in the same places1 the sound continued to grow in loudness until the strongest vibrations were felt4 ,even when observers in the same place agreed in hearing the sound1 it presented itself to them under different aspects4 ,thus1 at ,hereford1 a crash or bomb-like explosion was noticed by some1 but not by all1 observers2 at ,ledbury1 the sound according to one began like a rushing wind and culminated in a loud explosive report1 another heard a noise like distant thunder1 which ended when the shock began1 while a third heard no sound at all4 ,at places more distant from the epicentre1 the same diversity1 both in character and intensity1 is manifested4 ,thus1 at ,birmingham1 the accounts refer on the one hand to the distant approach of a train and the rising of the wind1 on the other to the reports of large cannons and to a noise as if tons of dbris had been hurled against the wall of the house2 at ,bangor1 to muffled thunder1 wind through trees1 and a loud rumbling sound4 ,the first explanation of these apparent anomalies which presents itself is inattention on the part of the observers2 but it is one that will not bear examination1 though it may apply in some cases4 ,the sound is too loud1 at any rate near the epicentre1 to escape notice1 and it is generally heard before the shock begins to be felt4 ,moreover1 as described in the last chapter1 three out of every four earthquakes in ,japan are unaccompanied by recorded sound1 and the ,japanese as a race cannot be accused of such constant inattention4 ,the defect1 it can hardly be doubted1 is inherent to the observer1 and not dependent on the conditions in which he is placed4 ,that the higher limit of audibility varies with different persons has long been known2 and there can be no reason for doubting that there is a similar variability in the lower limit4 ,thus1 to some observers1 the sound remains inaudible throughout1 however intently they may be listening4 ,again1 it is found that1 the deeper the sound1 the greater must be the strength of the vibrations required to render them audible4 ,as the vibrations which reach an observer increase in period1 it may therefore happen that1 sooner or later1 the strength of some does not attain or exceed that limiting value1 and1 at that moment1 the sound will cease to be heard4 ,moreover1 for vibrations of a given period1 this limiting value varies for different persons4 ,thus1 to one observer1 the sound may become inaudible1 while another may continue to hear it4 ,lastly1 the vibrations which affect an observer at any moment are of various strength and period4 ,one may hear all perhaps1 while a second may be able to hear some and not others4 ,thus1 to one observer1 the sound may be like a rising wind1 to another like a heavy traction-engine passing2 one may hear the crashes which accompanied the strongest part of the shock1 while a second may be deaf to the same vibrations2 to one the sound may become continually louder and cease abruptly1 to another it may increase to a maximum and then die away4 ,sound-,area4--,while the sound was a very prominent feature of the earthquake in and near the epicentral area1 records at a great distance are naturally difficult to obtain1 and1 on this account1 the number of stations for determining the boundary of the sound-area is too small to allow of it being accurately drawn4 ,as a rule1 however1 it must lie between the isoseismals #e and #d1 but it is less nearly circular than either of these lines4 ,its length1 from north-west to south-east1 is #cbj miles1 its breadth #bhd miles1 and the area contained by it about #gj1jjj square miles1 or roughly two-thirds that of the disturbed area4 ,isacoustic ,lines4--,the dotted lines in ,fig4 #fj represent isacoustic lines--that is1 lines which pass through all places where the percentage of observers who recorded their perception of the sound is the same4 ,for instance1 if we take any point in the line marked #hj and describe a small circle with that point as centre1 then #hj per cent4 of the observers within that circle would hear the earthquake-sound4 ,the isacoustic lines thus show how the audibility of the sound varies throughout the sound area4 ,to draw the curves with a close approach to accuracy1 the unit of area should be small and of constant dimensions2 but1 in the present case1 owing to the comparative paucity of the observations1 a smaller unit than the county would give unreliable results4#fe ,at the centre of each county1 the sound audibility may be regarded as proportional to the percentage of the total number of observers within the county who distinctly heard the sound4 ,to draw the curve marked #ej1 the centre of every county in which the average percentage is less than #ej is joined to the centres of those adjoining counties in which it is above #ej1 and these lines are then divided in the proper ratio so as to give a point where the percentage would be exactly #ej4 ,a number of points at which the percentage is #ej is thus obtained1 and the curve drawn through them is the required isacoustic line4 ,the percentage of audibility varies from #hg in ,herefordshire to #bc in ,essex and the east of ,ireland1 but the only isacoustic lines which can be completely drawn are those that correspond to the percentages between #hj and #ej inclusive4 ,the peculiar form of the isacoustic lines will be evident at a glance4 ,they bear little relation to the isoseismal lines4 ,their greatest extensions are not along the axes of those lines1 but in two directions which are a little east of north-east and south of south-west4 ,they lie indeed along a hyberbolic line which1 towards the south-west1 agrees closely with the curvilinear axis of the hyperbolic band represented by the broken line in ,fig4 #fj4 ,towards the north-east1 the coincidence is not so close1 but this is chiefly owing to the magnitude of the northern counties1 which causes a deflection of the isacoustic lines towards the north4 ,it will be remembered that the hyperbolic band is the area within which the vibrations from the two foci were superposed4 ,now1 the sound accompanied each part of the shock1 and ceased entirely during the interval between them4 ,also1 the stronger series of vibrations was accompanied by the louder sound2 but1 while the difference in strength was considerable between the two parts of the shock1 it was very slight between the two sounds4 ,there is therefore no marked distortion of the isoseismal lines when crossing the hyperbolic band1 while the isacoustic lines are completely diverted from their normal course4 ,thus1 the study of the isacoustic lines strongly confirms the conclusions at which we have arrived above 7p4 #bbc7--namely1 that there were two distinct foci arranged in a north-west and south-east line1 and that the impulse at the former focus occurred a few seconds earlier than that at the latter4#ff ,variations in the ,nature of the ,sound throughout the ,sound-area4--,in one respect1 the sound exhibited a marked uniformity all over the sound-area--namely1 in its great depth2 the word heavy being used in one out of every four accounts of the sound1 whether close to the epicentre or near the boundary of the sound-area4 ,the type of comparison employed varies in different parts of the sound-area4 ,as we recede from the origin1 the sound becomes on the average less like thunder or explosions and more like wind4 ,the references to passing waggons1 etc41 are so numerous that it is possible to draw curves1 in the same way as isacoustic lines1 which represent equal percentages of comparison to this type out of the total number of comparisons4 ,the curves are somewhat incomplete1 but it is noteworthy that those corresponding to the higher percentages cling to the extremities of the hyperbolic band1 probably because the uninterrupted duration of the sound is greater there than elsewhere4 ,the effect of distance from the epicentre1 however1 is most noticeable in connection with changes in the character of the sound4 ,it is only on the immediate neighbourhood of the origin that the explosive reports or crashes were heard in the midst of the rumbling sound4 ,at a moderate distance1 the sound before and after the shock became smoother1 while the sound which accompanied the shock retained to a certain extent its rougher and more rumbling or grating character4 ,close to the boundary of the sound-area1 the irregularities were still further smoothed away1 and the only sound heard was like the low roll of distant thunder4 ,the explanation of these changes depends on the fact that1 as we recede from the epicentre1 the vibrations of every period tend to become inaudible4 ,the limiting vibrations of the whole series will be the first to be lost1 especially those of the longest period4 ,thus1 near the epicentre1 sound-vibrations of many different periods will be heard1 and the sound will be more complex than it is elsewhere4 ,the greater the distance1 the narrower are the limits with regard to period between which the audible vibrations lie1 until1 near the boundary of the sound-area1 the sound becomes an almost monotonous deep growl of nearly uniform intensity4 ,time-relations of the ,sound and ,shock4--,the principal epochs to be compared are the beginning1 the epoch of maximum intensity1 and the end4 ,the beginning of the sound preceded that of the shock in #hb per cent4 of the observations on this epoch1 coincided with it in #ab1 and followed it in #f per cent42 the epoch of maximum intensity preceded that of the shock in #ba per cent4 of the records1 coincided with it in #gc1 and followed it in #f per cent42 while the end of the sound preceded that of the shock in #bb-a_/b per cent41 coincided with it in #bg-a_/b1 and followed it in #ej per cent4 ,thus1 as a general rule1 the beginning of the sound preceded that of the shock1 the sound was loudest when the shock was strongest1 and the end of the sound followed that of the shock4 ,in other words1 the duration of the sound was in most cases greater than that of the shock4 ,,minor ,,earthquakes4 ,of the twelve undoubted minor earthquakes1 nine occurred before1 and three after1 the principal shock1 the times of the first eleven lying between limits about seven hours apart4 ,with three exceptions1 the records are insufficient to determine the positions of the epicentre with any approach to exactness4 ,the first occurred at about #aa or #aa4cj ,p4,m4 on ,december #afth4 ,the boundary of the disturbed area1 which coincides nearly with that of the fifth shock 7,e1 ,fig4 #fc71 is #ig miles long from north-west to south-east1 #hc miles wide1 and contains about #f1cjj square miles4 ,the focus was apparently situated between the two foci of the principal earthquake and partly coincided with them4 ,illustration3 ,,fig4 #fc4--,map of minor shocks of ,hereford earthquake4 7,davison47 ,then came three slight shocks 7at about #a ,a4,m4 on ,december #agth1 #a4cj or #a4de ,a4,m41 and #b ,a4,m471 about which little is known except that they probably originated somewhere near the ,ross focus4 ,the fifth shock 7,e1 ,fig4 #fc7 occurred at about #c ,a4,m41 and disturbed an area #ajd miles in length1 #gi miles in width1 and about #f1djj square miles in area4 ,its boundary occupies approximately the position that would be taken by an isoseismal of intensity between #g and #f of the principal earthquake4 ,we may therefore infer that this shock and the principal earthquake were caused by slips along the same fault and in about the same region of the fault4 ,also1 as there is no evidence of discontinuity in the vibrations of the minor shock1 it is probable that the focus was continuous1 and occupied the space between the two foci of the principal earthquake1 as well as part or the whole of both these foci4 ,the next four shocks occurred at about #c4cj1 #d1 #e1 and #e4bj ,a4,m41 and were more closely associated with the ,ross than with the ,hereford focus1 and then followed the principal earthquake at #e4cb ,a4,m4 ,a few minutes later1 at #e4dj or #e4de ,a4,m41 a very slight shock was felt1 the focus of which was possibly situated in the central region between the two foci4 ,the next1 at about #f4ae ,a4,m4 7,k1 ,fig4 #fc71 disturbed an area #da miles long1 #bg miles broad1 and containing about #hgj square miles4 ,its focus must have coincided approximately with the ,ross focus of the principal earthquake1 and this was also the case probably with the last shock of all1 which occurred on ,july #aith1 #ahig1 at #c4di ,a4,m4 ,,origin ,,of ,,the ,,earthquakes4 ,the greater part of the epicentral district is covered by a sheet of ,old ,red ,sandstone 7,fig4 #fd71 but1 just to the north-east of the position laid down for the originating fault 7indicated by the straight broken line71 is the well-known ,woolhope anticlinal1 by which ,silurian beds are brought to the surface4 ,the anticlinal axis runs approximately north-west and south-east1 and is thus roughly parallel to the earthquake-fault4 ,moreover1 the thinning-out and occasional disappearance of some of the ,silurian beds on the south-west side of the anticlinal 7as compared with those on the north-east side7 is suggestive of a north-west and south-east fault or rapid flexure at or near the south-west junction of the ,old ,red ,sandstone and the ,silurian strata4 ,if it be a fault1 it must hade to the north-east1 and would therefore satisfy two of the conditions determined by the seismic evidence4 ,it would lie1 however1 about two miles too far to the north-east1 being in fact to the north-east of the villages which suffered most from the earthquake4 ,illustration3 ,,fig4 #fd4--;,geology of meizoseismal area of ,hereford earthquake4 7,davison47 ,but only a few miles to the south-east of the ,woolhope anticlinal1 and almost in the same line with it1 there is a second anticlinal1 that of ,may ,hill4 ,this is a triangular area1 and is known to be bounded on all three sides by faults4 ,the fault on the north-east side has an average north-west and south-east direction1 and1 if it were continued through the ,old ,red ,sandstone towards the north-west1 but bending at first a few degrees more to the west1 it would pass through a point about #a-a_/b miles west of ,hereford4 ,it is worthy of notice that both this fault and another nearly parallel to it1 about half-a-mile farther north-east1 stop1 according to the ,geological ,survey map1 at the points where they enter the ,old ,red ,sandstone4 ,the latter is an area which has never been investigated with thoroughness by modern stratigraphical methods1 and in which it is difficult to trace faults4 ,it therefore appears not improbable that the earthquakes were due to slips along a continuation of this fault4 ,whether this be the case or not1 however1 it is clear that the earthquake-fault must pass between the anticlinal areas of ,woolhope and ,may ,hill1 the former being on the north-east1 and the latter on the south-west1 side of the fault4 ,at the ,hereford focus1 the fault must hade to the north-east2 and1 at the ,ross focus1 it is probable1 from the distribution of places where damage occurred to buildings1 that it hades to the south-west ,if this be the case1 the fault must change in hade between the two foci4 ,how long a time had elapsed since the last sign of growth in the earthquake-fault took place1 it is impossible to say2 but it must be many years in length4 ,during this interval1 the stresses tending to produce movement along the fault-service had been gradually increasing1 until they were sufficient to overcome the resistance opposed to them4 ,it is worthy of notice that the earliest perceptible movements were slight4 ,their function seems to have been to prepare the way for the great slips by equalising the difference between stress and resistance over a large area of the fault-surface4 ,we cannot trace with accuracy the transference of the seat of movement from one part of the fault-surface to another4 ,the first slip seems to have taken place chiefly in the region between the two foci of the principal earthquake2 possibly it overlapped both of them partly4 ,the next three slips were apparently in the neighbourhood of the ,ross focus1 and were followed by a fifth in the same area as the first4 ,then came a series of small movements that we cannot locate further than by saying that they were more closely connected with the ,ross focus than the other4 ,in consequence of the preliminary slips within and near the ,ross focus1 the effective stress in that portion of the fault was diminished2 and this may be the reason why the first great slip took place at the ,hereford focus4 ,the immediate result of such a movement would naturally be an increase of stress in and beyond the terminal regions1 and the next slip might have been expected in an area partly overlapping the ,hereford focus1 and either to the north-west or south-east of it4 ,instead of this1 for a distance of two miles in the latter direction1 there was not the least perceptible movement during the principal earthquake1 and the second great slip occurred in the region beyond occupied by the ,ross focus4 ,this second slip1 moreover1 occurred within two or three seconds after the other2 that is1 before the earth-waves had time to travel from the ,hereford to the ,ross focus4 ,in other words1 the slip at the ,ross focus was not a consequence of the slip at the ,hereford focus2 but both were due to a single generative effort4 ,now1 a section drawn parallel to the earthquake-fault and on the north-east side of it1 would show an anticline near the ,hereford focus and a corresponding syncline near the ,ross focus1 with an undisplaced portion in the intermediate region2 while a parallel section on the other side of the fault would show a syncline near the ,hereford focus1 an anticline near the ,ross focus1 and again an undisplaced portion in the intermediate region4 ,if further movements tending to accentuate such a structure were to occur 7that is1 if the anticlinals were to be made more anticlinal and the synclines more synclinal71 there would therefore be two slips1 one in each focus2 while1 along the fault-surface between1 there would be practically no displacement4 ,at any rate1 the earlier stresses in that region may have been fully relieved by two slight preliminary slips 7those causing the first and fifth minor earthquakes71 and those resulting from the great displacements by the first after-slip which followed in about ten minutes4 ,half-an-hour later1 another slip took place at the ,ross focus1 and by this the equilibrium of the rock-masses was almost completely restored2 for we have no certain evidence of any further movements until seven months have elapsed 7,july #aith1 #ahig71 when there was a final slip in the same region of the fault4 ,,the ,,inverness ,,earthquake ,,of ,,september #ah,,th1 #aija4 ,between the north-east end of ,loch ,ness and the ,moray ,firth at ,inverness1 there lies a tract of land not more than seven miles in length1 which is notable as one of those most frequently shaken by earthquakes in the ,british ,islands4 ,in the intensity of its shocks it is inferior to the south-east of ,essex and the centre of ,herefordshire1 and1 in mere number1 to the celebrated village of ,comrie in ,perthshire4 ,but1 in the interest of its seismic phenomena1 in the light which they cast on the development of the earth's crust1 the neighbourhood of ,inverness has no equal in ,great ,britain1 and not many superiors in any part of the world4 ,for this importance from a seismological point of view1 the district is indebted to the great fault which traverses ,scotland along the line of the ,caledonian ,canal1 and to the fact that this fault1 although it dates from ,old ,red ,sandstone times1 has not yet finished growing4 ,as results of its formation1 we have the almost straight cliff along the south-east coast of ,rossshire1 and the long chain of lakes1 beginning with ,loch ,dochfour and ,loch ,ness1 and ending with ,loch ,oich1 ,loch ,lochy1 and ,loch ,linnhe4 ,as evidences of its persistent though intermittent growth1 we have the slight tremors and earth-sounds occasionally observed at and near ,fort ,william1 and the much stronger shocks felt in the neighbourhood of ,inverness4 ,during the nineteenth century there were three strong earthquake shocks in this district4 ,the first and most severe occurred on ,august #acth1 #ahaf1 and was felt over the greater part of ,scotland2 the second on ,february #bnd1 #ahhh2 and the third and weakest on ,november #aeth1 #ahij4 ,this last shock was followed by several slighter ones1 the series ending with a rather smart shock on ,december #adth4 ,between this date and the summer of #aija no earthquakes seem to have been felt at or anywhere near ,inverness4 ,,preparatory ,,shocks4 ,the date of the first shock of #aija is not quite certain4 ,one is said to have been felt at ,aldourie 7see ,fig4 #ff7 some time in ,june1 and a second at ,dochgarroch in ,july4 ,these may have been succeeded by others too slight to attract much notice1 but the first to be generally observed occurred on ,september #afth at #f4d ,p4,m4 ,a weak tremor1 accompanied by a faint sound1 was perceived over a nearly circular area about #ab miles in diameter1 and with its centre about #a-a_/b miles south of ,dochgarroch4 ,on the next day1 at #aa ,p4,m41 a quivering lasting two seconds was felt at ,inverness1 and a weak tremor1 accompanied by sound1 at ,dochgarroch at #a4ae ,a4,m4 on ,september #ahth4 ,nine minutes later1 at #a4bd ,a4,m41 occurred the principal earthquake1 the shock of which would be called a strong one1 even in ,italy and ,japan4 ,,effects ,,of ,,the ,,shock4 ,in ,inverness1 the damage to buildings1 though seldom serious1 was by no means inconsiderable4 ,one brick building used as a smithy was destroyed1 several chimneys or parts of them fell1 and many chimney-cans were displaced or overthrown4 ,at ,dochgarroch and other places within the meizoseismal area1 walls were cracked1 chimneys thrown down1 and lintels loosened4 ,but1 for this country1 an unusual effect of the earthquake was a long crack made in the north bank of the ,caledonian ,canal near ,dochgarroch ,lochs4 ,it occurred in the middle of the towing-path1 and could be traced at intervals for a distance of #bjj yards to the east of the ,lochs1 and #djj yards to the west1 being often a mere thread1 and in no place more than half-an-inch wide4 ,soon after its formation1 however1 the fissure was obliterated by heavy showers of rain4 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,the map 7,fig4 #fe7 shows the area over which the earthquake was perceptible4 ,the isoseismal lines are drawn partly continuous and partly dotted--continuous where some confidence can be placed in their accuracy1 and dotted where their course must be regarded as doubtful1 owing to the rarity or absence of observations4 ,the innermost isoseismal 7shown on a larger scale in ,fig4 #ff7 corresponds to the intensity #h of the ,rossi-,forel scale1 and includes the places where the shock was strong enough to cause slight structural damage to buildings4 ,it is elliptical in form1 #ab miles long1 #g miles broad1 and #fg square mile in area1 with its centre at a point about #a-a_/b mile east-north-east of ,dochgarroch1 and its longer axis running ,n4 #cc ,e4 and ,s4 #cc ,w4 ,illustration3 ,,fig4 #fe4--;,isoseismal lines of the ,inverness earthquake4 7,davison47 ,the remaining isoseismals are less accurately drawn1 owing to the scarcity of observations made in the west of ,scotland4 ,except towards the west1 however1 the course laid down for the isoseismal #g may be trusted4 ,its length is #ec-a_/b miles1 width #ce miles1 and area #a1ejj square miles4 ,its longer axis is almost exactly parallel to that of the preceding isoseismal1 but the distance between the two curves is #i miles on the north-west1 and #ad miles on the south-east1 side4 ,the isoseismal #f is #aje miles long1 #hg miles wide1 and contains #g1cjj square miles2 and the isoseismal #e1 #aeg miles long1 #adc miles wide1 and about #ag1jjj square miles in area4 ,the isoseismal #d may be regarded as the boundary of the disturbed area of the earthquake1 for1 so far as known1 the shock was not noticed at any point outside it4 ,towards the north1 it was felt at ,wick1 ,castletown1 and other intermediate places2 towards the west at ,tobermory in the island of ,mull2 and1 towards the south1 at ,skelmorlie 7in ,ayrshire71 ,paisley1 ,belsyde 7near ,linlithgow71 ,gullane 7near ,north ,berwick71 and ,dunbar4 ,along the east coast of ,scotland1 between ,wick and ,dunbar1 there are few places of any size where the shock was not felt4 ,the disturbed area of the earthquake is thus #bae miles long from north-east to south-west1 #aih miles wide1 and contains about #cc1jjj square miles4 ,position of the ,originating ,fault4--,the only isoseismals which are drawn accurately enough to determine the earthquake-fault are the two inner ones1 those marked #h and #g2 but these are sufficient for the purpose4 ,it is clear1 from the direction of their longer axes1 that the average direction of the fault must be ,n4 #cc ,e4 and ,s4 #cc ,w4 ,again1 the isoseismals are farther apart towards the south-east than towards the north-west1 implying that the fault hades to the south-east4 ,lastly1 as the intensity of the shock is greater on the side towards which the fault hades1 it follows that the fault-line must lie a short distance 7about a mile or so7 on the north-west side of the centre of the isoseismal #h4 ,now1 the great fault alluded to above occupies almost exactly the position indicated by the seismic evidence4 ,its mean direction from ,tarbat ,ness to ,loch ,linnhe is ,n4 #ce ,e4 and ,s4 #ce ,w41 it hades to the south-east1 and the fault-line passes through a point about three-quarters of a mile to the north-west of the centre of the isoseismal #h 7,fig4 #ff74 ,there can be little doubt1 therefore1 that the earthquake was caused by a slip of this fault2 and the evidence of the after-shocks1 as will be seen1 offers additional support to this conclusion4 ,the region in which the slip took place may be determined roughly from the position and form of the innermost isoseismal4 ,its centre must have been close to the point marked ,a in ,fig4 #ff1 which corresponds to a point about #a-a_/b mile east-north-east of ,dochgarroch4 ,in a horizontal direction1 its length must have been at least five or six miles2 otherwise1 the isoseismal #h would have been less elongated4 ,it must therefore have reached from about half-a-mile north-east of ,loch ,ness to about half-a-mile south-west of ,inverness4 ,its width1 measured along the dip of the fault-surface is unknown2 but the small distance between the centre of the isoseismal and the fault-line shows that the principal movement took place at a depth which was probably under1 rather than over1 one mile4 ,,nature ,,of ,,the ,,shock4 ,we come now to the evidence afforded by the nature of the shock1 in which there was but little variation throughout the disturbed area4 ,at ,inverness1 a gentle movement was first felt1 followed by an extraordinary quivering1 which increased in force for two or three seconds1 and then decreased for two or three seconds2 just as the quivering was about to cease1 there was a distinct lurch or heave1 after which the vibration was much more severe than before and lasted several seconds longer than the first part of the shock4 ,dalarossie lies about fourteen miles south-east of ,inverness1 and here the first indication was a loud sound1 as of an express train1 coming from the east1 rushing close to1 and then under1 the house2 this lasted for a few seconds1 and towards the end of it the house vibrated4 ,then succeeded an interval of quietness for about a second1 followed by a terrific burst or crash1 not unlike the crash of a loud thunder peal1 of about two seconds' duration1 during which the house distinctly heaved up once and then sank back4 ,after another brief interval of quietness1 there was a low rumble1 like the sound of a dying peal of thunder4 ,it will be noticed1 in this account1 that the two parts of the shock were no longer consecutive4 ,there was a short interval of rest between them1 the intermediate vibrations observed at ,inverness being too weak to be felt at ,dalarossie4 ,still farther away1 the extinction became more marked4 ,at ,aberdeen1 for instance1 the shock consisted of two parts1 the first a tremble1 followed1 after an interval of a few seconds1 by a swinging movement of longer duration than the tremble4 ,in all parts of the disturbed area1 the shock maintained the same character of division into two parts1 the second of which was of greater duration and intensity than the first and consisted of vibrations of longer period4 ,a phenomenon of such wide occurrence was clearly not due to local influences4 ,it must have been caused by two separate initial impulses1 the stronger succeeding the other after an interval of a few seconds and taking place in nearly the same region of the fault4#fg ,,sound-,,phenomena4 ,outside the isoseismal #e1 there are but few records of the earthquake-sound2 but it was heard faintly at ,skelmorlie 7in ,ayrshire71 ,belsyde 7near ,linlithgow71 and ,gullane 7near ,north ,berwick74 ,towards the north1 it was not observed beyond ,wick and ,wathen 7in ,caithness74 ,the boundary of the sound-area cannot be laid down with any approach to accuracy1 but it must have included a district containing about #bg1jjj square miles4 ,throughout the whole disturbed area1 #hd per cent4 of the observers heard the sound4 ,the percentage varies in different counties1 from #ic in ,inverness-shire to #gg in the counties of ,perth and ,aberdeen2 but the records in the more distant regions are too few to allow of the construction of isacoustic lines4 ,in its character1 the sound resembled that usually heard with strong earthquakes1 #ci per cent4 of the observers having compared it to passing waggons1 traction-engines1 etc41 #be per cent4 to thunder1 #ad to wind1 #h to loads of stones falling1 #c to the fall of heavy bodies1 #d to explosions or the firing of heavy guns1 and #g per cent4 to miscellaneous sounds4 ,the intensity of the sound gradually diminished outwards from the epicentre1 and most rapidly near the isoseismal #g1 which abounds approximately the area in which the sound was very loud from that in which it was distinctly fainter1 and also includes nearly all the places at which loud explosive crashes were heard with the strongest vibrations4 ,in the time-relations of the sound and shock1 the ,inverness earthquake resembles the ,hereford earthquake of #ahif4 ,the beginning of the sound preceded that of the shock in #gb per cent4 of the records1 coincided with it in #bj1 and followed it in #h per cent42 the epoch of maximum intensity of the sound preceded that of the shock in #bj per cent4 of the records1 coincided with it in #gc1 and followed it in #g per cent42 while the end of the sound preceded that of the shock in #ae per cent4 of the records1 coincided with it in #cd1 and followed it in #eb per cent4 ,somewhat similar proportions hold over the greater part of the disturbed area1 the percentages being nearly the same in the counties of ,inverness1 ,ross1 ,nairn1 ,elgin1 ,banff1 and the most distant counties4 ,but in ,aberdeenshire an exception occurs1 the three epochs of sound and shock in most cases coinciding with one another4 ,the majority of the observations in this county come from the southern part1 and the line joining this district to the epicentre is nearly perpendicular to the line of the earthquake-fault4 ,this result has an important bearing on the origin of the sound-vibrations4 ,for1 if the general precedence of the sound with respect to the shock were due to its superior velocity1 the percentage of records in which the beginning of the sound preceded that of the shock would vary only with the distance1 and not with the direction from the origin4 ,indeed1 with increasing distance from the origin1 this percentage should continually approach #ajj2 while that in which the end of the sound followed that of the shock should diminish to zero4 ,there is1 however1 no trace of either tendency1 the sound being heard after the shock at places close to the boundary of the sound-area4 ,on the other hand1 it the sound-vibrations were to start simultaneously1 or nearly so1 from all parts of the focus1 but especially from its marginal regions1 then1 in the greater part of the disturbed area1 the sound would be heard both before and after the shock2 for the lateral margins of the focus would be the portions nearest to1 and farther from1 most observers2 while1 at places near the line through the epicentre at right angles to the earthquake-fault1 the three principal epochs of the sound and shock should approximately coincide4 ,the inference that the sound-vibrations heard before and after the shock come from the margins of the focus is also supported by the observations on the relative duration of the sound and shock4 ,if we take only those records which are free from doubt1 in #gh per cent4 of the total number1 the duration of the sound was greater than that of the shock2 while1 in ,aberdeenshire1 according to #ic per cent4 of the observers1 the durations of sound and shock were equal4 ,we may imagine1 then1 that the slip within the seismic focus would be greatest in a central region1 and that it would die outwards in all directions towards the edges4 ,the friction arising from the slipping in the central region would produce chiefly the comparatively large oscillations that formed the perceptible shock2 the evanescent creep within the marginal regions would produce the small and rapid vibrations that were sensible only as sound4 ,,origin ,,of ,,the ,,earthquake4 ,while the seismic evidence enables us to determine the surface-position and the horizontal dimensions of the seismic focus1 it unfortunately throws no light whatever on a point of some importance--namely1 the direction of the movement which caused the earthquake4 ,we cannot infer from it whether it was the rock on the south-east or north-west side of the fault that slipped or whether both sides slipped at once2 nor1 if that point had been settled1 do we know if the movement of the displaced side was upward or downward4 ,in the formation of the fault1 however1 it is clear that either the south-east side has been depressed or the north-west side elevated2 and1 as the bed of ,loch ,ness is below the level of the sea1 that the former movement has predominated4 ,if the displacements which gave rise to the earthquake were merely a continuation of the original series of movements--and this is1 to say the least1 a very probable view to take--then we may imagine that1 for a distance of five or six miles1 and at a depth of about a mile or less1 there was a sudden sag downwards of the rock on the south-east side of the fault through a distance which perhaps in no part exceeded a fraction of an inch4 ,fig4 #ff is an attempt to represent roughly the displacement which caused the principal earthquake4 ,the diagram makes no pretence to accuracy1 and the scale in the vertical direction is enormously greater1 perhaps a hundred thousand times greater1 than that in the horizontal direction4 ,the straight line is supposed to represent a straight line drawn before the earthquake on the surface of the rock adjoining the fault on the south-east side and at a depth of about a mile1 and the curve the form of the same line after the earthquake4 ,illustration3 ,,fig4 #ff4--;,diagram to illustrate supposed fault-displacement causing ,inverness earthquake4 ,the effect of this great slip would obviously be to relieve the stress in the central region ,a1 and to increase it suddenly in the parts denoted by the letters ,b and ,c4 ,it is1 therefore1 in these parts especially that we should expect future slips to occur4 ,each slip would of course give rise to an after-shock1 and would in like manner result in an increase of stress in its own terminal regions1 though chiefly on the side remote from the centre ,a4 ,,the ,,after-,,shocks ,,and ,,their ,,origin4 ,it is difficult to form any estimate of the total number of after-shocks4 ,the list1 compiled from the records of careful observers only1 includes forty-six shocks and ten earth-sounds1 the last of all occurring on ,november #bast4 ,but the list is certainly incomplete4 ,it contains1 for instance1 only one entry on ,september #ahth between #c4ef and #i ,a4,m42 whereas1 during the same interval1 no fewer than eighteen slight shocks were felt by one observer at ,dochgarroch1 while another near ,aldourie estimates the number of shocks up to ,october #bcrd at about seventy4 ,the total number probably did not fall short of one hundred4 ,the majority were certainly very slight1 and1 at another time1 would hardly have attracted any notice4 ,there were1 however1 three of much greater importance than the rest4 ,these occurred on ,september #ahth at #c4ef and #i ,a4,m41 and on ,september #cjth at #c4ci ,a4,m4 ,the isoseismal lines of all three are elongated ovals1 their longer axes are parallel to the fault1 and their centres lie on the south-east side of the fault-line4 ,the shocks were therefore evidently due to slips several miles in length along the fault4 ,at present1 we are concerned more with the position of their epicentres4 ,these are indicated by the dots lettered ,b1 ,c1 ,d in ,fig4 #fg2 the dot marked ,a denoting the centre of the principal earthquake1 and the continuous line the path of the fault4 ,thus1 within two and a half hours1 the great slip was followed by one with its centre at ,b1 near the south-west margin of the principal focus4 ,about five hours later1 the scene of action was suddenly transferred to a region with its centre at ,c on the north-east margin4 ,both slips affected a portion of the fault-surface several miles in length1 and must therefore have increased the area of displacement1 slightly towards the north-east and considerably towards the south-west4 ,only small movements occurred during the next twelve days until #c4ci ,a4,m4 on ,september #cjth1 when another long slip took place1 with its centre at ,d1 still farther to the south-west1 and therefore again extending the area and amount of displacement in this direction4 ,illustration3 ,,fig4 #fg4--,map of epicentres of after-shocks of ,inverness earthquakes4 7,davison47 ,turning now to the weaker after-shocks and earth-sounds1 we find them affecting chiefly three regions of the fault4 ,one of these is close to ,dochgarroch1 another near ,inverness1 and the third between ,aldourie and ,drumnadrochit2 the effects of the slips in the last two districts being1 as before1 to extend the area of displacement a short distance 7perhaps half a mile7 to the north-east and not less than six miles to the south-west underneath ,loch ,ness4 ,the unequal division of the after-shocks between the two sides of the principal centre 7,a1 ,fig4 #fg7 is very marked4 ,the positions of the epicentres of forty-four shocks and earth-sounds can be determined with more or less accuracy1 and1 of these1 only ten lie to the north-east of the principal centre1 while thirty-four lie to the south-west1 six or seven of the latter being beneath ,loch ,ness4 ,one other point may be referred to before leaving these minor shocks4 ,so far as regards the stronger shocks1 there was a continual decrease in the depths of the seismic foci4 ,this is shown by the progressive approach of their epicentres towards the fault-line2 the distances in the three chief after-shocks being #a4g1 #a4j1 and #j4e miles respectively2 and in one of the latest shocks 7that of ,october #acth at #d4bd ,p4,m41 ,e1 ,fig4 #fg7 the distance is no more than one-tenth of a mile4 ,the focus of this shock must1 indeed1 have been quite close to the surface near ,dochgarroch4 ,this constant diminution in the depth of the foci shows that the great slip was followed by a sudden increase of stress upwards as well as laterally1 and explains why that slip did not leave any perceptible trace1 either as fault-scarp or fissure1 at the surface4 ,,sympathetic ,,earthquakes4 ,it is remarkable that1 of the #ef recorded after-shocks1 at least six were felt or heard only at ,dalarossie and other places in the valley of the ,findhorn1 a valley which lies about #ac or #ad miles to the south-east of the great fault4 ,that they had no connection with that fault is certain1 for two of them were so strong that1 if they were so connected1 they could not have escaped the notice of one or more of the watchful observers between ,drumnadrochit and ,inverness4 ,the probable explanation of these after-shocks is that they were due to slips of a fault running along the ,findhorn valley2#fh and that the great displacement near ,inverness on ,september #ahth led to a sudden increase of stress within the rocks for many miles around1 which1 at and near ,dalarossie1 was sufficient to precipitate the slips referred to4 ,,conclusion4 ,at first sight1 two earthquakes could hardly be more unlike than the ,japanese earthquake of #ahia and the ,inverness earthquake of #aija4 ,in the rice-fields of central ,japan1 as we have seen1 the roads for many leagues were edged with ruins1 the fault-slip was prolonged up to the surface and visible as a scarp forty1 if not seventy1 miles in length1 plots of ground were compressed and their boundaries altered1 the hillsides were scored by landslips1 places can now be seen from one another that formerly were hidden by a mountain ridge1 and the total number of after-shocks within little more than two years amounted to above three thousand4 ,on the other hand1 when we examine the distribution of the after-shocks in space1 we find that1 though no part of the fault was exempt from slips1 they favoured three regions in particular--one1 the most important1 a central region1 yet not coincident with that in which the principal shock was most intense2 and the other two surrounding the extremities of the fault4 ,with the lapse of time1 the after-shocks on the whole became weaker and occurred less frequently1 and the average depth of the foci gradually diminished4 ,moreover1 in two districts distant forty-five and fifty-five miles from the fault1 the frequency of the shocks during the month succeeding the earthquake was suddenly increased to ten and sixteen times the normal rate4 ,it is interesting to notice so close a similarity in character1 subsisting with so vast a difference in the scale of intensity4 ,the identity of the powers at work in shaping the structure of both islands ,is manifest4 ,in ,japan1 we see the mountain-making forces acting with violence and producing effects that are only too apparent to the eye4 ,in ,scotland1 whatever may have happened in former geological epochs1 the changes in surface-structure are now taking place with almost infinite slowness1 and hundreds or thousands of years must elapse before ,loch ,ness makes any visible progress in its march towards the sea4 ,,references4 #a4 ,,davison1 ,c4--,the ,hereford ,earthquake of ,december #ag1 #ahif4 7,birmingham1 #ahii47 #b4 ---- ,the ,inverness ,earthquake of ,sept4 #ah1 #aija1 and its accessory shocks4 ,quart4 ,journ4 ,geol4 ,soc41 vol4 lviii41 #aijb1 pp4 #cgg-cig4 ,,footnotes3 #fa ,the study of the ,hereford earthquake is based on #b1ijb records1 coming from #a1idc places2 that of the ,inverness earthquake on #gaj records from #cha places4 #fb ,the disturbed area of the ,hereford earthquake of #ahif was probably greater than that of any other ,british earthquake of the nineteenth century2 that of the ,pembroke earthquake of #ahib being more than #ef1jjj square miles1 of the ,pembroke earthquake of #ahic about #fc1fjj square miles1 while that of the ,essex earthquake of #ahhd 7a far stronger shock in the meizoseismal area7 is estimated at about #ej1jjj square miles4 #fc ,the approximate circularity of the two outer isoseismals is due to the fact that the vibrations propagated to such great distances are those which start from the comparatively small central region of the focus4 #fd ,the above statement summarises the evidence of the majority of the observers in each portion of the disturbed area4 ,in this1 as in other similar cases1 discrepancies in the observations are unavoidable2 but it is important to notice that they are least frequent in the observations evidently made with the greatest care4 #fe ,except in the case of ,yorkshire1 where the three ,ridings are regarded as separate counties4 #ff ,the ,derby earthquake of ,march #bdth1 #aijc1 was also a twin earthquake4 ,the centres of the two foci were situated near ,ashbourne and ,wirksworth1 above eight or nine miles apart1 along a line running ,n4 #cc ,e4 and ,s4 #cc ,w4 ,the two parts of the shock coalesced along a rectilineal band about five miles wide running centrally across the lower isoseismals in a direction at right angles to their longer axes4 ,the isacoustic lines are also elongated in the direction of this band4 ,in this case1 the impulses at the two foci must have taken place at the same instant4 7,quart4 ,journ4 ,geo4 ,soc41 vol4 lx41 #aijd1 pp4 #bae-bcb47 #fg ,if the foci of the two impulses had been detached1 there would1 with so small an interval between the two parts1 have been a variation in the nature of the shock like that observed during the ,hereford earthquake4 #fh ,this part of ,inverness-shire has not yet been mapped by the ,geological ,survey1 but a fault is known to exist in the ,findhorn valley near ,drysachan ,lodge1 which lies about eleven miles down the valley from ,dalarossie4 ,,chapter ,,ix4 ,,the ,,indian ,,earthquake ,,of ,,june #ab,,th1 #ahig4 ,very different from the shocks of ,britain was the earthquake that overwhelmed so large a part of its great dependency on ,june #abth1 #ahig--;an earthquake which1 if it is not without a rival1 is certainly one of the most disastrous and most widely-felt of which we possess any record4 ,that it was of the first magnitude was evident at once in ,calcutta from the extensive injury to buildings1 and its investigation was undertaken without delay by the members of the ,geological ,survey of ,india4 ,the four officers who were at the headquarters in ,calcutta were despatched to the area of greatest damage1 letters and circulars were distributed as widely as possible1 a large number of observers were induced to co-operate by keeping records of the after-shocks1 and1 later on1 during the cold weather of #ahig-ih1 ,mr4 ,r4,d4 ,oldham1 one of the superintendents of the ,survey1 made a tour through the epicentral district4 ,to him1 moreover1 fell the much harder task of discussing the very numerous observations collected by himself and others2 and the least that can be said of the valuable report prepared by him is that it is worthy of a great subject4 ,professor ,omori also spent several months in studying the earthquake on behalf of the ,japanese ,government2 but the account1 which is written in his own language1 unfortunately remains a sealed book to western seismologists4 ,illustration3 ,,fig4 #fh4--;,isoseismal ,lines of ,indian ,earthquake4 7,oldham47 ,,isoseismal ,,lines ,,and ,,disturbed ,,area4 ,in ,fig4 #fh1 which shows the area disturbed by the earthquake1 ,mr4 ,oldham has drawn two series of curves4 ,in the absence of detailed records of the intensity--records that could not have been obtained from some parts of the disturbed area1 and would have been difficult to procure in sufficient number from others--he has represented by the dotted curves a group of isoseismals in the form which he believes they would have assumed had the earth-waves been propagated in a homogeneous medium4 ,the first includes all places1 such as ,shillong and ,goalpara1 where the destruction of brick and stone buildings was practically universal2 the second1 those1 like ,darjiling1 in which damage to buildings was universal and often serious2 the third1 places1 like ,calcutta1 where the earthquake was strong enough to injure all or nearly all brick buildings4 ,inside the fourth isoseismal1 the shock was strong enough to disturb furniture and loose objects1 but not to cause more than slight damage2 within the fifth1 it was generally noticed2 and1 beyond this1 and as far as the sixth isoseismal1 the earthquake was perceived only by a small number of sensitive persons at rest4 ,the approximation of the curves towards the east and south-east1 ,mr4 ,oldham believes to be partly real1 and not due to imperfect information4 ,the continuous curves represent more closely the actual variation of intensity4 ,the innermost curve ,a indicates the probable boundary of the epicentral tract1 which is about #bjj miles in length and more than #fjjj square miles in area4 ,this will be referred to afterwards in greater detail4 ,the next curve ,b bounds the region within which serious damage to brick houses was common4 ,its irregular course is closely connected with the geological structure of the country1 and is due to the fact1 of which we have already met with several examples1 that earthquakes are more destructive to houses built on alluvial ground than to those founded on rock4 ,the area included within this curve is not less than #ade1jjj square miles2 and1 if we include the parts from which reports were not obtainable1 it must amount to about #afj1jjj square miles4 ,the curve ,c represents the boundary of the disturbed area1 so far as known1 for about one-third of the area lies in regions from which no information was procurable1 while another third is inhabited by ignorant and illiterate tribes4 ,but1 notwithstanding this1 the shock is known to have been felt over an area of at least #a1bjj1jjj square miles4 ,if we include the detached region to the west1 near ,ahmedabad1 the portion of the ,bay of ,bengal in which the shock would have been felt had the sea been replaced by land1 and a large part of ,thibet or ,western ,china1 from which no reports have come1 but in which the shock was certainly sensible1 this estimate1 great as it is1 must be raised to about #a1gej1jjj square miles4#fi ,figures1 such as those given above1 convey but little idea of the vastness of the area concerned4 ,transferring them to countries with which we are more familiar1 we may say that the disturbed area was only a little less than half the size of ,europe2 the region in which serious damage occurred to masonry was more than twice as large as the whole of ,great ,britain2 while1 if the centre of the epicentral tract had been in ,birmingham1 nearly every brick and stone building between ,york and ,exeter would have been levelled with the ground4 ,,nature ,,of ,,the ,,shock4 ,few and slight were the forerunners of the greatest of modern earthquakes4 ,early in ,june1 faint tremors were felt by sensitive persons at ,shillong4 ,others at the same place heard a rumbling sound for ten or fifteen seconds before the shock began1 and at ,silchar birds were seen to rise suddenly from trees before the movement became sensible to man4 ,except for these almost imperceptible warnings1 the earthquake broke abruptly over the whole district4 ,at #e4ae1 writes one observer at ,shillong1 a deep rumbling sound1 like near thunder commenced1 apparently coming from the south or south-west4444 ,the rumbling preceded the shock by about two seconds 444 and the shock reached its maximum violence almost at once1 in the course of the first two or three seconds4 ,the ground began to rock violently1 and in a few seconds it was impossible to stand upright1 and ,i had to sit down suddenly on the road4 ,the shock was of considerable duration1 and maintained roughly the same amount of violence from the beginning to the end4 ,it produced a very distinct sensation of sea-sickness4444 ,the feeling was as if the ground was being violently jerked backwards and forwards very rapidly1 every third or fourth jerk being of greater scope than the intermediate ones4 ,the surface of the ground vibrated visibly in every direction1 as if it was made of soft jelly2 and long cracks appeared at once along the road4444 ,the road is bounded here and there by low banks of earth1 about two feet high1 and these were all shaken down quite flat4 ,the school building1 which was in sight1 began to shake at the first shock1 and large slabs of plaster fell from the walls at once4 ,a few moments afterwards the whole building was lying flat1 the walls collapsed1 and the corrugated iron roof lying bent and broken on the ground4 ,a pink cloud of plaster and dust was seen hanging over every house in ,shillong at the end of the shock4444 ,my impression at the end of the shock was that its duration was certainly under one minute1 and that it had travelled from south to north4444 ,the violence of the shock may be imagined when it is stated that the whole of the damage done was completed in the first ten or fifteen seconds of the shock4 ,other estimates of the duration are generally higher than that given above1 ranging from three to five or even more minutes at ,tura1 ,dhubri1 ,silchar1 ,calcutta1 and other places4 ,in some cases1 it is possible that the immediately succeeding tremors were included as part of the great shock2 but1 in the central area1 it is probable that the average duration of the shock did not differ much from three or four minutes4 ,in this district1 the movement was most complicated4 ,changes of direction were frequently noticed4 ,at ,silchar1 for instance1 the earthquake began with an undulatory movement from north to south1 like the swinging of a suspension bridge2 it closed with a motion like that of a boat tossed in a choppy sea1 or by the crossing of great waves which1 whatever their dominant direction may have been1 certainly did not travel from north to south4 ,the vertical component of the motion must have been considerable2 for1 at ,shillong1 loose stones lying on the roads were tossed in the air like peas on a drum1 ,but this was even less pronounced than the horizontal movement1 the range of which was at least eight or nine inches1 and during which people felt as if they were being shaken like a rat by a terrier4 ,the period of these vibrations was estimated at about a second4 ,as they left the central region1 the period of the waves lengthened1 so that1 at a distance1 the shock no longer consisted of short jerks1 but became a gentle rocking motion1 causing in some people a sensation of nausea4 ,at ,calcutta1 the undulations were regular and resembled the rolling of a mighty ship1 the period being between one and two seconds4 ,at ,balasor1 the motion was a long rolling one1 such as would be felt on the deck of a ship in a fairly heavy sea2 and1 farther to the south as far as the limit of the disturbed area1 the same undulatory movements were observed1 gradually decreasing in intensity1 and usually compared to the easy motion of a ship in a gentle sea4 ,visible ,earth-,waves4--,a few examples have already been given of the observation of visible waves on the surface of the ground4 ,they were seen at ,charleston during the earthquake of #ahhf 7p4 #aaj71 and at ,akasaka and other places in the meizoseismal area during the ,japanese earthquake of #ahia 7p4 #ahf74 ,but they were more than usually prominent in the ,indian earthquake2 indeed1 much of the difficulty experienced in standing during the shock seems to have been due to the passage of these surface-waves4 ,at ,shillong1 according to an observer quoted above 7p4 #bff71 the surface of the ground vibrated visibly in every direction1 as if it were made of soft jelly4 ,another describes it as presenting the aspect of a storm-tossed sea1 with this difference that the undulations were infinitely more rapid than any seen at sea4 ,near ,maimansingh1 earth-waves were watched approaching1 exactly like rollers on the sea-coast1 and1 as they passed1 the observers had a difficulty in standing4 ,at ,nalbari1 the rice in the fields could be seen rising and falling at intervals during the transit of the waves4 ,in the ,assam valley1 near ,mangaldai1 there were seen waves coming from opposite directions and meeting in a great heap and then falling back2 each time the waves seemed to fall back the ground opened slightly1 and each time they met1 water and sand were thrown up to a height of about #ah inches or so4 ,even as far as ,midnapur1 the ground was distinctly billowy1 and at ,allahabad a series of waves was observed to cross the ground from south-south-west to north-north-east4 ,it is obviously difficult to judge in any case of the magnitude of such waves4 ,in the epicentral area1 ,mr4 ,oldham believes that1 on an average1 they were probably about thirty feet long and one foot in height1 though some may have been both shorter and higher4 ,these movements must have been comparatively slow1 for their progress could be easily followed by the eye2 indeed1 their rate1 as one witness remarks1 though decidedly faster than a man could walk1 was not so fast as he could run4 ,,elements ,,of ,,the ,,wave-,,motion4 ,in his study of the ,neapolitan earthquake1 ,mallet showed how the amplitude and maximum velocity of the vibrations could be determined roughly from the displacement1 projection1 or overthrow of various bodies by the earthquake4 ,somewhat similar methods were employed by ,mr4 ,oldham in the absence of seismographs from the epicentral area4 ,his results are of course only approximate1 but they lead nevertheless to a conclusion of great value and interest4 ,illustration3 ,,fig4 #fi4--,section of ,tombs in the ,cemetery at ,cherrapunji4 7,oldham47 ,amplitude4--,the best measure of the amplitude was obtained at the cemetery at ,cherrapunji1 situated near the southern margin of the epicentral area4 ,here were two oblong masonry tombs 7,fig4 #fi71 standing close together with their longer axes pointing north and south4 ,their inner sides were partially destroyed4 ,on the outer sides1 they are almost intact1 but the tombs have been driven bodily down into the ground1 and on either side to east and west1 there is a depression with a vertical side parallel to the outer surface of the tomb and a smooth flat bottom over which the base of the tomb has slid4444 ,the edge of the western depression has the grass growing undisturbed up to the edge of it1 and along the edge small fragments of lime and plaster show that this was originally in contact with the edge of the tomb1 which has now moved away to a distance of #ah inches4 ,on the east the edge of the depression is raised and the grass and earth forced upwards by the thrust of the tomb against it2 the breadth of this depression is #aj inches4 ,during the movement of the ground1 the tombs1 owing to their inertia1 remained comparatively stationary1 and the depressions were formed by the backward and forward movement of the ground against them4 ,the movement on the east side was clearly arrested in some manner1 and the range therefore cannot have been less than #aj inches4 ,it may have been as much as #ah inches1 and was probably1 in ,mr4 ,oldham's opinion1 the mean of these two amounts--namely1 #ad inches4 ,this would give an amplitude of about #g inches1 a value which may be in excess of the average amount elsewhere in the district1 as the cemetery is situated near the edge of a high sandstone scarp4 ,at ,tura1 also within the epicentral area1 a range of not less than #aj inches was given by the sliding of a wooden house over the posts on which it rested4 ,six months after the shock1 ,mr4 ,oldham frequently noticed vacant spaces four or five inches across by the side of large boulders scattered over the ,khasi hills1 and he infers that throughout the whole tract lying west of ,shillong and ,gauhati1 as far as the hills extend1 and probably over a large area of the plains besides1 the amplitude of the wave-motion was nowhere less than #c inches1 while in many places it was over #f inches4 ,maximum ,velocity4--,the most trustworthy measure of the maximum velocity are those obtained from the projection of bodies4 ,mr4 ,oldham selects the following as most deserving of notice3--,at ,goalpara1 an obelisk surmounting a tomb was broken off and thrown to one side1 giving a maximum velocity of not less than #aa feet per second4 ,at ,gauhati1 the coping of a small gate-pillar was shot off and fell at a distance of #d feet #d inches from the centre of the pillar2 in this case the maximum velocity must have exceeded #h feet per second4 ,the highest velocity1 of more than #af feet per second1 was measured at ,rambrai1 where a small group of monoliths were shot out of the ground1 one of them to a distance of #f-a_/b feet4 ,lastly1 at ,silchar1 a bullet was projected from the corner of a wooden post1 acting as a rough form of seismometer1 from which a maximum velocity of at least #a-a_/b feet per second was deduced4 ,maximum ,acceleration4--,estimates of the maximum horizontal acceleration were made from #bh overthrown pillars by means of ,professor ,west's formula 7p4 #ahd1 footnote74 ,the measures obtained at the same place show some variation1 but ,mr4 ,oldham considers as fair average values those of #ad feet per second per second at ,goalpara1 #ab at ,gauhati1 ,shillong1 and ,sylhet1 #aj at ,cherrapunji1 #i at ,dhubri1 and #d feet per second per second at ,silchar4 ,of the vertical component of the acceleration1 not even the roughest numerical estimate can be formed4 ,we know1 however1 that at ,shillong1 ,gauhati1 and indeed throughout the epicentral area1 stones were projected upwards1 and this is evidence that the vertical component was greater than that of gravity--namely1 #cb feet per second per second4 ,violent as the shock was at the places just mentioned1 it must have been still greater in certain parts of the epicentral area4 ,at ,dilma1 in the ,garo hills1 the shock seems to have been strong enough to disable men2 and1 in the neighbourhood of the faults that will be described in a later section1 forest trees were snapped in two4 ,fortunately1 as ,mr4 ,oldham remarks1 there were in these districts no towns or populous settlements to feel the full power of the earthquake to destroy4 ,anomalies in the above ,measurements4--,if the movements of the ground followed the law of simple harmonic motion1 any two of the four elements 7period1 amplitude1 maximum velocity1 and maximum acceleration7 would suffice to determine the others 7p4 #d74 ,applying the usual formul to the quantities obtained at ,gauhati--namely1 #h feet per second for the maximum velocity and #ab feet per second per second for the maximum acceleration1 it follows that the amplitude would be #e feet and the period #d seconds--values1 which are evidently inadmissible4 ,or1 taking the maximum vertical component at #cb feet per second per second1 the corresponding values would be #b feet and #a-a_/b seconds1 that of the amplitude being still too great4 ,again1 at ,rambrai1 the maximum velocity was found to exceed #af feet per second4 ,the other elements are unknown1 but1 if the amplitude were one foot1 ,mr4 ,oldham shows that the maximum acceleration would be #bef feet per second per second2 or1 taking the amplitude at the impossible amount of two feet1 that the maximum acceleration would be #abh feet per second per second4 ,it follows1 therefore1 that only part of the high velocities at ,rambrai and elsewhere can be due to the elastic waves provoked by the initial disturbances4 ,the remaining portion must be attributed to a bodily displacement of the earth's crust within the epicentral area--a displacement of which the fault-scarps and other distortions of that region furnish ample evidence4 ,,sound-,,phenomena4 ,in the epicentral area1 the sound that accompanied the earthquake was remarkable for its extraordinary loudness4 ,at ,shillong1 the crash of houses falling within thirty yards was completely drowned by the roar of the earthquake4 ,the sound was generally compared to distant thunder1 the passage of a train or cart1 etc42 but1 whatever the type may be1 it always implies a sound of deep pitch1 close to the lower limit of audibility--a continuous rumbling or rattling noise1 as a rule gradually becoming louder and then dying away4 ,there was the usual conflict in the evidence of different observers due to the depth of the sound4 ,in ,calcutta1 which lies well within the sound-area1 some persons asserted that they heard a rumbling noise2 others were positive that the only noise was that caused by falling buildings and furniture4 ,some1 again1 noticed that the shock was preceded by a loud roar2 while others were certain that there was no sound of any kind until the earthquake had become severe4 ,as in the case of the disturbed area1 it is impossible to define the boundary of the region over which the sound was heard4 ,like the shock1 also1 it seems to have been observed farther to the west than towards the east4 ,leaving out of account records that are possibly doubtful1 the sound was heard for a distance of #ccj miles to the west and south-west1 and #bij miles to the east of the epicentral area--that is1 allowing for the dimensions of that area1 it must have been perceptible over a region measuring not less than #hjj miles from east to west4 ,,velocity ,,of ,,the ,,earth-,,waves4 ,it is somewhat doubtful whether a more accurate estimate of the velocity is to be obtained from a violent earthquake or from one of moderate intensity4 ,in the former case1 the vast distances to which the shock is noticed lessen the effects of errors in the time-determinations1 but this advantage is to a great extent compensated by the considerable duration of the shock and the consequent uncertainty whether all observers have timed the same phase of the movement4 ,also1 in the ,indian earthquake1 there are further sources of error in the variety of standard times employed throughout the country and in the magnitude of the epicentral area4 ,of the numerous time-records collected by ,mr4 ,oldham1 the best are those which were obtained from a few self-recording instruments1 from the more busy telegraph offices1 from the larger railway stations1 and in some cases from private individuals4 ,all records were in the first place subjected to a rigid process of selection2 a large number were rejected on various grounds1 and those only were retained which bore internal evidence of accuracy1 due either to the conditions of the reporter's occupation or to the care taken by him to ensure exactness4 ,to guard against any unconscious bias in making the selection1 this process was carried out before the distances were calculated1 and even before the position of the epicentral area was known4 ,the boundary of this area is shown by the continuous line ,a in ,fig4 #fh4 ,its greatest length being about #bjj miles from east to west1 it is necessary in the first place to fix upon an equivalent centre within it1 which may be regarded for this special purpose as the point of departure of the earth-waves4 ,the more natural course perhaps would be to assume this point to coincide with the centre of the area4 ,but1 as the rate at which the initial movement spread over that area would probably differ little from the velocity of the earth-wave1 and as all the time-stations lie towards the west1 ,mr4 ,oldham regards a point near the western boundary of the area 7in lat4 #be #de' ,n4 and long4 #ij #ae' ,e47 as a sufficiently exact approximation to the position of the equivalent centre4 ,the nearest place at which good time-observations were made is ,calcutta1 distant #bee4e miles from the assumed centre4 ,one is indicated on the recording tide-gauge by a sudden rise of the water1 while the others were obtained from the central telegraph office1 the terminal railway stations1 and from two careful readings by interested observers4 ,they vary from #d;h4 #bgm4 #js4 to #d;h4 #bhm4 #cgs4 ,p4,m41 all being liable to an error of half-a-minute4 ,the arithmetic mean for the beginning of the shock is #d;h4 #bgm4 #dis41 and this is probably as accurate an estimate as the conditions allow4#gj ,bombay lies outside the disturbed area1 #abjh4c miles from the equivalent centre2 and1 for the time of arrival in that city1 we have to depend on the records of the barograph and the three magnetographs4 ,the horizontal force magnet was set in motion two and a half minutes before the others1 no doubt by the advance tremors4 ,the times given by the barograph and the vertical force-instrument differ by only one minute1 and the best result seems to be that obtained by taking their mean--namely1 #d;h4 #cem4 #dcs41 which is probably accurate to within a minute4 ,assuming1 then1 that the time-interval between ,calcutta and ,bombay does not err by more than half-a-minute1 it follows that the intervening velocity must lie between #b4h and #c4b kilometres per second1 its probable value being #c kilometres1 or #b miles1 per second4 ,the remaining records1 which are of less value than those obtained in these cities1 fall into two groups1 the first consisting of a number of stations along a line running north and south between ,calcutta and ,darjiling or within a hundred miles on either side of the same1 and the second a long series of stations crossing ,northern ,india in a nearly westerly direction4 ,the observations made at the ,burmese stations were unfortunately affected by an error arising from the retardation of the ,madras time-signals through frequent repetition along the line4 ,illustration3 ,,fig4 #gj4--,time-curve of ,indian earthquake4 7,oldham47 ,individually1 these records are not exact enough to be used in determining the velocity1 but they may be employed collectively for the construction of the time-curve in ,fig4 #gj4 ,in this diagram1 distances in hundreds of miles from the equivalent centre are represented along the horizontal line1 and the time of occurrence in minutes past #d ,p4,m4 along the perpendicular line4 ,the small circles represent the observations at ,calcutta and ,bombay1 the dots those at places lying nearly west of the origin1 and the crosses those at places situated to the south or north-west4 ,the continuous curve passes in an average manner through the series of points1 and probably does not differ much from the true curve of the time of arrival of the shock at different places4 ,the curve1 it will be noticed1 is at first concave1 and afterwards convex1 upwards2 indicating that the times required to traverse successive equal distances at first increased1 and then decreased4 ,thus1 if the curve is an accurate representation of the facts1 it would follow that the surface-velocity was subject to a continual decrease outwards from the centre1 until it was a minimum at a distance of about #bhj miles1 after which it increased4 ,the deviation of the curve from a straight line is1 however1 so slight that we cannot feel much confidence in this conclusion4 ,if we join the points corresponding to ,calcutta and ,bombay by a straight line 7drawn dotted in ,fig4 #gj71 it does not in any part vary from the continuous line by a distance equivalent to more than half-a-minute4 ,indeed1 if a very few discordant records are excluded1 and if less weight is given to those times which are multiples of five minutes1 the straight line represents the mean quite as fairly as the curved line does2 and that this is the more probable interpretation will appear from the observations on the unfelt earthquake described in the next section4 ,we may therefore conclude that the earth-waves travelled along the surface at an approximately uniform rate of #c kilometres per second1 or about #abj miles a minute--a result which ,mr4 ,oldham considers may be accepted as accurate to within five per cent4 ,if the two time-curves in ,fig4 #gj are continued to the right until they meet the time-scale1 it will be seen that they intersect it near the point corresponding to #d4bf ,p4,m41 implying that this would be approximately the time at which the shock was felt within the epicentral area4 ,this agrees closely with the observed times of about #d4be at ,parbatipur and ,kuch ,bihar1 #d4bf at ,siliguri1 and #d4bg at ,shillong and ,goalpara2 and it is probable that the error is not more than a quarter of a minute in defect or half-a-minute in excess4 ,thus1 the time of arrival of the first sensible waves at the surface would lie between #d;h4 #bem4 #des41 and #d;h4 #bfm4 #cjs4 ,p4,m41 ,madras time1 or between #aa;h4 #dm4 #des4 and #aa;h4 #em4 #cjs4 ,a4,m41 ,greenwich mean time4 ,,the ,,unfelt ,,earthquake4 ,of the crowd of vibrations that agitate the ground during an earthquake1 part only combine to form the perceptible shock4 ,some are insensible owing to their small amplitude1 others to the slowness of the motion4 ,an interesting observation belonging to the latter class was made by an engineer near ,midnapur1 a place which lies just within the area of damage4 ,at the time of the earthquake1 he was taking levels on a railway bank1 and was about to take a reading when he noticed the bubble of the level oscillating4 ,in five or ten seconds the shaking began and appeared to last three or four minutes2 but1 for more than five minutes after it had apparently ceased1 the level showed that the ground continued to rock4 ,again1 in ,burmah1 at a place nineteen miles east of ,tagaung and close to the border of the disturbed area1 the water in a shallow tank1 about #cjj yards in length1 was seen lapping up against the side in a manner that was at first attributed to elephants bathing4 ,no shock was felt1 but the shaking of the trees at the same time showed that the disturbance was due to the earthquake4 ,far beyond the limits of the disturbed area1 however1 the earthquake was recorded by many of the delicate instruments which have been employed during the last few years for the registration of distant shocks4 ,among the more important of these instruments are long vertical pendulums1 horizontal pendulums of various forms1 and magnetographs4 ,in the vertical1 and some of the horizontal1 pendulums1 especially in those used in the ,italian observatories1 the masses carried are heavy1 and the movements of the ground are magnified by lightly-balanced levers ending in points which trace their records on bands of smoked paper driven by clockwork4 ,in the other horizontal pendulums and in the magnetographs1 the method of registration is photographic4 ,the paper required for the mechanical records being inexpensive1 a high velocity 7half-an-inch or more per minute7 can be given to it1 and the resulting diagrams are open and detailed4 ,the ,italian instruments also respond more readily than the others to the earlier and slighter tremors3 while the apparatus in which photographic methods are used are sometimes so violently disturbed by the later undulations that the spot of light fails to leave any trace on the photographic paper4 ,it is therefore from the ,italian observatories that the more interesting records come4 ,one of these1 given by a horizontal pendulum at ,rocca di ,papa near ,rome1 is reproduced in ,fig4 #ga2 while the curve of the bifilar pendulum at ,edinburgh 7,fig4 #gb7 is a good example of those obtained by the photographic method of registration4#ga ,all over ,italy1 from ,ischia and ,catania in the south to ,pavia in the north1 the different instruments employed began1 one after the other1 to write their records of the movement as the unfelt earth-waves sped outwards from the centre4 ,italy passed1 the tale was taken up by magnetographs at ,potsdam and ,wilhelmshaven1 ,pawlovsk 7near ,st4 ,petersburg71 ,copenhagen1 ,utrecht1 and ,parc ,st4 ,maur 7near ,paris72 by horizontal pendulums at ,strassburg and ,shide 7in the ,isle of ,wight71 and by a bifilar pendulum at ,edinburgh4 ,shide is #d1hia miles from the centre of disturbance1 but1 as we shall see1 the movement could be traced for a distance greater even than this4 ,illustration3 ,,fig4 #ga4--,seismographic ,record of ,indian ,earthquake at ,rocca di ,papa4 7,cancani47 ,in the more complete records1 and especially in those given by the ,italian apparatus1 ,mr4 ,oldham distinguishes three phases of motion4 ,the first consists of rapid and nearly horizontal movements of the ground4 ,in ,italy1 it begins at about #aa4ag ,a4,m4--that is1 about #ab-a_/b minutes after the commencement of the shock at the epicentre 7,fig4 #ga1 a74 ,without any break in the movement1 and after a further interval of about #h-a_/b minutes1 the second phase begins2 the vibrations are similar to the preceding1 but they are larger and more open1 and are accompanied by an unmistakable tilting of the surface of the ground 7,fig4 #ga1 b74 ,lastly1 after the lapse of about twenty minutes more1 the second phase gives place1 without interruption1 to the third 7,fig4 #ga1 c71#gb consisting of well-marked slow undulations1 which have been aptly compared by ,professor ,milne to the movements caused by an ocean-swell4 ,as they travelled across ,europe1 the surface of the ground was thrown into a series of flat waves1 #cd miles in length1 and #bj inches in maximum height1 the complete period of each wave being #bb seconds4 ,this phase is by far the longest of the three2 in the more sensitive instruments1 two or three hours elapsed before their traces ceased to show any sign of movement4 ,illustration3 ,,fig4 #gb4--,seismographic ,record of ,indian ,earthquake at ,edinburgh4 7,heath47 ,knowing the distances of the different observatories from the epicentre1 and the times taken by each phase to reach them1 we can form some idea of the rates at which they travelled4 ,if the early tremors moved in straight lines1 their mean velocity for the first phase was #i4j1 and for the second #e4c1 kilometres per second2 but1 if they moved along curved paths through the body of the earth1 their mean velocities must have exceeded these amounts4 ,for the first undulations of the third phase1 the velocity would be #b4i kilometres per second if they travelled along straight lines1 and #c4j kilometres per second if they were confined to the surface of the earth4 ,the existence of the second phase was noticed for the first time by ,mr4 ,oldham in the records of the ,indian earthquake1 but he has since detected it in those of other shocks4 ,he believes1 in common with most seismologists1 that the first phase corresponds to waves of elastic compression1 or longitudinal waves1 travelling through the body of the earth2 and the second phase he attributes to waves of elastic distortion1 or transversal waves1 travelling in the same way1 in which the particles move at right angles to the direction in which the wave travels1 thus causing a slight tilting of the surface4 ,it is probable that the waves of both phases move along curved1 rather than straight1 lines through the earth1 that the curves are concave towards the surface1 and that the velocity of the waves increases with the depth of their path below the surface4 ,on the other hand1 the surface-velocity of the first undulations of the third phase is practically constant for all distances from the epicentre1 and1 in the case of the ,indian earthquake1 it agrees almost exactly with that obtained for the velocity within the disturbed area1 and as far as ,bombay4 ,it is therefore difficult to resist the conclusion that the third phase consists of undulations which travel along the surface of the earth4 ,diverging in two dimensions only1 they fade away much more slowly than the vibrations of the other two phases4 ,we may thus imagine these surface-undulations speeding outwards from the epicentre in ever-widening circles until they have passed over a quarter-circumference of the earth1 when they should begin to converge towards the antipodes4 ,here they should cross each other1 and again spread out as circular waves1 once more in their course passing the same observatories where they were first recorded1 but in the opposite order4 ,it has been reserved for the most violent earthquake of modern times to verify this interesting conclusion4 ,faint1 but decided1 are the traces of the second crossing4 ,at ,edinburgh1 they occur at #b4f ,p4,m41 at about the same time at ,shide1 at ,leghorn #b4aj1 ,catania #b4ab-c_/d1 while at ,ischia there are several movements between #b and #c ,p4,m4 ,at ,rocca di ,papa1 near ,rome1 the time is slightly earlier1 but the undulations1 like those at the first crossing1 have a complete period of about #bj seconds4 ,the distances traversed by the waves are more than #bj1jjj1 instead of less than #ejjj miles2 but the mean velocity with which they travelled is almost exactly the same as at first--namely1 #b4ie kilometres per second4 ,,earth-,,fissures1 ,,sand-,,vents1 ,,etc4 ,earth-,fissures4--,among the superficial effects of the earthquake1 none take a more important place than the fissures formed in alluvial plains4 ,not only were they remarkably abundant1 more so than in any other known earthquake1 but they occurred over an unusually wide area4 ,wherever the necessary conditions prevailed1 they were found to be numerous over a district bounded approximately by the isoseismal #a 7,fig4 #fh71 and measuring about #djj miles from east to west1 and about #cjj miles from north to south2 and they were present1 though in smaller numbers1 over an area nearly #fjj miles long in an east-north-east and west-south-west direction4 ,they were naturally more frequent near river-channels and reservoirs1 on account of the absence of lateral support1 and as a rule were parallel to the edge of the bank1 a few hundred yards in length1 and in width varying from some inches to four or five feet4 ,fissures in such positions are formed with every violent earthquake1 and even with some of those more moderate shocks that visit the ,british ,islands 7see p4 #bdg74 ,but an interesting point established by the ,indian earthquake is that they also occurred at a distance from any water-channel or excavation1 often running parallel to1 and along either side of1 a road or embankment4 ,in other situations1 they showed a distinct tendency to range themselves parallel to one another2 and1 in these cases1 it is possible that their formation was connected with the passage of the visible surface-waves4 ,in an account already quoted 7p4 #bdg71 it is stated that these waves came from opposite directions and that1 as they separated after meeting1 the ground opened slightly4 ,among the ,khasi and ,garo hills 7see ,fig4 #ge71 wherever the alluvium of the plains runs up to the foot of the hills1 another form of fissure1 represented in ,fig4 #gc1 was constantly noticed4 ,close to the junction1 there was a sudden drop1 as at a1 of from one to five feet1 the vertical face having the appearance of a fault1 but distinguished from one by following the windings of the hills4 ,then came a depressed band b1 from ten to twenty feet wide1 and outside this a low rounded ridge c raised above its former level1 and merging beyond at d into the undisturbed plain4 ,when ,mr4 ,oldham visited the district in ,march #ahih1 the natives had flooded the rice-fields1 and the features described were clearly depicted by the gathering of the water in the depression and the isolation of the ridge4 ,illustration3 ,,fig4 #gc4--;,displacement of alluvium at foot of a hill4 7,oldham47 ,the explanation of these peculiarities is evidently that given by ,mr4 ,oldham4 ,during the passage of repeated waves of compression1 the thrust of the hill and plain against one another caused the heaping up of the alluvium in the ridge c2 while the return movements resulted in the tearing of the alluvium away from the hillside1 leaving the scarp a and the depression b4 ,displacements of ,alluvium4--,many other remarkable evidences of compression were observed4 ,telegraph posts1 originally set up in a straight line1 were displaced1 occasionally as much as ten or fifteen feet2 sometimes without any apparent connection with neighbouring river-channels4 ,in one part of the ,assam-,bengal ,railway1 for nearly half a mile1 the whole embankment1 including borrow-pits and trees on either side1 was shifted laterally without any sign of wrenching from the adjoining ground1 the maximum distance amounting to #f-c_/d feet4 ,as the displacement took place parallel to the only river-course in the neighbourhood1 ,mr4 ,oldham attributes it to the sliding of the surface-layers over some yielding bed beneath4 ,again1 throughout large areas of ,northern ,bengal1 ,lower ,assam1 and ,maimansingh1 rice-fields1 which had been carefully levelled so that they might be uniformly flooded1 were thrown into gentle undulations1 the crests of which were occasionally two or three feet above the hollows4 ,the piers of bridges were also moved parallel to1 as well as towards1 the streams1 showing that the displacements extended to the depth of the foundations4 ,the buckling of railway lines was often violent and took place over a large area4 ,in the ,charleston earthquake1 every such bend was accompanied by a corresponding extension elsewhere 7p4 #aac72 but1 in the ,baluchistan earthquake of #ahib1 the neighbouring fish-joints were jammed up tight4#gc ,in the one case1 there was merely local compression2 in the other1 a permanent displacement of the earth's crust4 ,the distortion of the ,indian lines seems to belong to the former class4 ,repairs were of course generally made without delay2 but all the information that could be obtained on this point showed that the compression causing the crumpling of the lines was accompanied by a compensating expansion1 generally at a distance of about #cjj yards4 ,sand-,vents4--,shortly after the earthquake1 large quantities of water and sand issued from fissures in the ground4 ,at ,dhubri1 innumerable jets of water1 like fountains playing1 spouted up to heights varying from #ah inches to quite #c-a_/b or #d feet4 ,wherever this had occurred1 the land was afterwards seen to occupy a sandy circle with a depression in its centre4 ,these circles ranged from #b to #f and #h feet in diameter1 and were to be seen all over the country4 ,in some places1 several were quite close together2 in others they were at a distance of several yards4 ,near ,maimansingh1 they seem to have been almost as numerous1 fifty-two1 of four feet and less in diameter1 being counted within an area #ajj yards long and about #bj feet wide4 ,the sand and water were ejected from the vents with some force4 ,a few observers estimated the height of the spouts at about #ab feet1 but this probably refers to stray splashes4 ,it is clear1 however1 that the sand and water were forced not only up to the surface1 but even in a continuous stream to heights of from two to ten feet above it4 ,in many districts1 trunks of trees or lumps of coal and fossil resin were washed up with the water1 and even1 in one or two cases1 pebbles of hard rock weighing as much as half-a-pound4 ,the origin of the sand-vents is to be sought in the presence of a water-bearing bed situated not far below the surface4 ,in the central area1 where there was a marked vertical component in the motion1 this bed during the earthquake was compressed between those above and below it1 and the resulting pressure was in places sufficient to force the water and sand1 through the fissures formed by the earthquake1 up to and beyond the surface4 ,the gradual settling of the upper layer1 cut up by the fissures1 into the underlying quicksand1 prolonged the process for some time after the shock was over2 and1 when the pressure was at last relieved1 some of the water was sucked back and so produced the crateriform hollows4 ,rise of ,river-,beds1 etc4--,over a large area1 river-channels1 tanks1 wells1 etc41 were filled up1 partly by the outpouring of the sand from vents1 but chiefly1 as shown by the forcing up of the central piers of bridges1 by the elevation of the beds of the excavations4 ,in the lowlands which lie between the ,garo hills and the ,brahmaputra1 there were numerous channels from #ae to #bj feet in depth1 the beds of which were pressed up until they became level with the banks1 while a compensating subsidence took place close to the streams on either side4 ,the general tendency of the earthquake was thus to obliterate the surface inequalities1 so that1 when the rivers rose later on1 the district was extensively flooded4 ,besides these deferred floods1 there occurred immediately after the earthquake a sudden rise in many rivers1 amounting to from two to ten feet1 followed by a gradual decline to the former state in two or three days4 ,at ,gauhati1 for instance1 the river-gauge showed that1 at about three-quarters of an hour after the earthquake1 the water stood #g feet #g inches higher than on the morning of ,june #abth2 at #g ,a4,m4 on ,june #acth it had fallen to #e feet #h inches1 and at the same time on the two following days to #b feet #g inches and #f inches1 showing that the water had returned nearly to its original level after the lapse of two and a half days4 ,in most of the large rivers1 the rise of water was due to the formation of partial dams formed by the local elevation of the river-beds described above4 ,as the barriers were composed of loose sand1 they were gradually scoured away and the material was spread over the bottom so as to leave the water at a level slightly higher than that which it maintained before the earthquake4 ,,landslips4 ,the distribution of landslips shows that their formation depends almost as much on local conditions as on the violence of the shock4 ,the effect of the latter is manifested by their limitation to a certain central area4 ,to the east of the ,north ,cachar hills1 few1 if any1 were to be seen2 but1 as far as ,kohima1 cracks or incipient landslips were formed on the hillsides4 ,the ,sylhet valley and a line to the west of ,darjiling form the southern and western boundaries of the landslip area1 which was therefore not less than #cjj miles in length from east to west4 ,within this area1 however1 local conditions asserted their superiority4 ,among the more important may be mentioned the constitution of the hills and the presence of a thick superficial layer of subsoil or rock with an inner bounding surface of weak cohesion1 the slope of the hillsides1 and their height from base to crest4 ,thus1 though the epicentral area was situated chiefly to the south of the ,brahmaputra valley 7,fig4 #ge71 the east and west range of the landslips was more extensive in the ,himalayas on the north side than in the ,garo and ,khasi hills on the south4 ,in many places1 the steep sides of the ,himalayan valleys exist always in a critical condition of repose1 and the effect of the ,indian earthquake was such that all along the north side of the ,brahmaputra valley1 the range is scarred by landslips1 even to the east of ,tezpur4 ,again1 along the southern edge of the ,garo and ,khasi hills1 landslips were unusually prevalent4 ,viewed from the deck of a steamer sailing up to ,sylhet1 says ,mr4 ,oldham1 the southern face of these hills presented a striking scene4 ,the high sandstone hills facing the plains of western ,sylhet1 usually forest-clad from crest to foot1 were stripped bare1 and the white sandstone shone clear in the sun1 in an apparently unbroken stretch of about #bj miles in length from east to west4 ,at ,cherrapunji1 also1 the deep valleys were so scored that1 from a distance1 there appeared to be more landslip than untouched hillside4 ,but in no part1 probably1 were landslips more strikingly developed than in the small valley of the ,mahdeo1 which forms an amphitheatre about four miles long from east to west1 and a mile and a half across1 lying to the south of the ,blpakrm and ,pundengru hills4 ,here1 remarks ,mr4 ,oldham1 everything combined to favour the formation of landslips4 ,the hills were composed of soft sandstone1 they were steep-sided1 high1 and narrow from side to side1 and consequently were doubtless thrown into actual oscillation as a whole2 while the range of motion of the wave particle was not less than eight inches near the edge of the precipices4 ,the result 444 has been to produce an indescribable scene of desolation4 ,everywhere the hillsides facing the valley have been stripped bare from crest to base1 and the seams of coal and partings of shale could be seen running in and out of the irregularities of the cliffs with a sharpness and distinctness which recalled the pictures of the caons of ,colorado4 ,at the bottom of the valley was a piled-up heap of dbris and broken trees1 while the old stream had been obliterated and the stream could be seen flowing over a sandy bed1 which must have been raised many feet above the level of the old watercourse4 ,in the sandstone districts of the area here considered1 the landslips had some important secondary effects4 ,along the southern edge of the ,garo and ,khasi hills1 great sand-fans spread over the fields1 and the exposure of the hillsides formerly protected by forest left free scope for future denudation4 ,every stream of any size has in this way devastated many square miles of country4 ,among the hills themselves1 more sand was brought down than the streams could carry away1 and everywhere their beds were raised4 ,ordinarily1 the beds of these rivers1 which are raging torrents when in flood1 consist of a succession of deep pools separated by rocky rapids4 ,after the rains of #ahig1 it was found that the pools had been filled up1 and the rapids obliterated by a great deposit of sand1 over which the rivers flowed in a broad and shallow stream4 ,a few valleys were for a short time barred across by landslips4 ,in one1 on the northern foot of the ,garo hills1 a landslip crossed the drainage channel and formed a shallow pond1 which was not filled up by sand until the end of ,january #ahih4 ,near ,sinya1 in the northern ,khasi hills1 an unusually large landslip formed a barrier1 of which the remains are more than #bjj feet above the level of the river-bed4 ,behind this1 the water accumulated in a great lake until the beginning of ,september #ahig1 when the barrier burst and a flood of water rushed down the valley4 ,,rotation ,,of ,,pillars1 ,,etc4 ,a curious effect of earthquakes strong enough to damage buildings is that pillars1 monuments1 etc41 may be fractured and the upper part rotated over the lower without being overthrown4 ,even in ,hereford and the surrounding villages1 several pinnacles and chimney-stacks were twisted by the earthquake of #ahif4 ,the interest of the phenomenon1 which has been known1 since #agee1gd is mainly historical1 for the endeavour to discover its cause was the origin of ,mallet's views on the dynamics of earthquakes4 ,partly1 also1 it lies in the difficulty of finding a satisfactory explanation1 or rather in deciding which of three or four possible explanations is the true one in any particular case4 ,illustration3 ,,fig4 #gd4--,twisting of monument at ,chhatak4 7,oldham47 ,the ,indian earthquake offered exceptional opportunities for studying the phenomenon in the large number of examples observed and the variety of objects rotated4 ,none could be more striking than the twisted monument to ,george ,inglis1 represented in outline in ,fig4 #gd4 ,chhatak1 where this is situated1 lies close to the southern boundary of the epicentral area4 ,the monument is an obelisk1 built of broad flat bricks or tiles on a base of #ab feet square1 and originally more than #fj feet high4 ,it was split by the earthquake into four portions4 ,the two upper1 about six and nine feet long1 were thrown down2 while the third1 #bb feet high1 remains standing1 but is twisted through an angle of #cj with respect to the lowest part1 which is unmoved4 ,the upper of these two parts had evidently rocked on the lower1 as the corners and edges were splintered1 and below the fracture a slice of masonry about #ae inches thick1 which was not bonded into the main mass1 was split off by the pressure on its upper end4 ,the plan of the parts still standing is shown in the lower part of ,fig4 #gd4 ,the possible explanations of the phenomenon are at least three in number4 ,according to the first1 which was given by ,mallet in #ahdf1 the adhesion of the twisted portion to its base is not uniform1 and the resultant resistance to motion is not in the same vertical plane as the wave-movement4#ge ,some years later1 ,mallet offered another explanation4 ,the body1 he imagined1 might be tilted on one edge by the earthquake1 and1 while still rocking1 a second shock oblique to the first might twist it about that edge4#gf ,in #ahhj1 ,professor ,t4 ,gray suggested that the column might be tilted on one corner and then twisted round it by later vibrations of the same shock4#gg ,none of these theories1 ,mr4 ,oldham argues1 can give by itself a complete explanation of the phenomena observed in the central district of the ,indian earthquake2 and he therefore favours an extension of the second theory1 which1 though first proposed in #ahhb1gh was thought out independently and in greater detail by himself4 ,when the focus is of considerable dimensions1 the shock at neighbouring places is constantly varying in direction1 owing to the arrival of vibrations from different parts of the focus4 ,thus1 instead of the two separate shocks required by ,mallet's second explanation1 we have a number of closely successive impulses frequently changing in direction and giving rise to what is known in the ,south of ,europe as a vorticose shock4 ,and1 instead of a single twist of the pillars about one centre only1 we have a series of small twists round a number of different centres1 accompanied in consequence by a much smaller displacement of the centre of gravity than would have occurred had the same rotation been accomplished in one operation4 ,the theory1 it will be seen1 accounts for the twisting of the pillar without overthrow1 and for the splintering of the edges during the rocking of the column4 ,it explains why in any district a number of similarly placed objects are generally twisted in the same direction4 ,moreover1 a low column rocks to and fro more rapidly than a tall one similar in form and position1 so that1 at the instant when a later impulse comes from a different direction1 two such columns might happen to be tilted on opposite edges1 and would then be twisted in opposite directions4 ,in certain cases1 then1 as occurred at several places during the ,indian earthquake1 an object may rotate in one direction1 while others1 similar in every respect but size1 may be twisted in the opposite direction4 ,,after-,,shocks4 ,frequency of ,after-,shocks4--,for some days after the great earthquake1 the after-shocks by their very frequency and by their wide distribution baffled close inquiry4 ,during the first #bd hours1 hundreds were felt at all points of the epicentral area2 indeed1 it is not too much to say that for several days the ground was never actually at rest4 ,at the ,bordwar tea-estate1 which is traversed by one of the great fractures to be described in the next section1 the surface of a glass of water on a table was for a whole week in a constant state of tremor2 and at ,tura a hanging lamp was kept continually swinging for the first three or four days4 ,most of these shocks were1 of course1 very slight2 but1 interspersed among them1 were others of greater strength1 and a few of considerable violence4 ,one1 on ,june #acth1 about eight hours after the earthquake1 was sensible beyond ,allahabad--that is1 for more than #ebj miles from the epicentre2 and another on the same day was felt in ,calcutta1 distant #bee miles4 ,on ,june #adth1 #bbnd1 and #bith1 and again on ,august #bnd and ,october #ith1 shocks were noticed in that city2 but1 after the latter date1 the disturbed area of no shock reached to so great a distance4 ,to form any estimate of the total number of after-shocks is impossible1 even for any one station4 ,at first1 lists were kept at isolated places1 such as ,shillong1 ,maimansingh1 ,dhubri1 and a few others4 ,then1 from ,july #aeth1 through ,mr4 ,oldham's efforts1 the records became more numerous until the end of the year1 after which interest in the subject declined4 ,mr4 ,oldham's catalogue closes with the year #ahih2 but the register of a roughly-constructed seismograph1 erected at ,shillong in ,july #ahig1 continues to the present day4 ,imperfect as all non-instrumental registers must be1 they nevertheless furnish some idea of the frequency of the after-shocks4 ,thus1 until the end of ,june1 #fgi shocks were recorded at ,rangmahal 7,north ,gauhati71 #ace at ,maimansingh1 #hi at ,kuch ,bihar1 and #hc at ,kaunia 7omitting those on ,june #abth74 ,again1 from ,august #ast to #aeth1 #ahb were felt at ,goalpara1 #aea at ,darangiri1 #abd at ,tura1 #aje at ,bijni1 #id at ,lakhipur1 #id at ,krishnai1 #dh at ,dhubri1 #bh at ,rangpur1 and #ab at ,kuch ,bihar2 while at ,borpeta1 #aac shocks were reported during the first nine days of ,august4 ,turning to the registers of longer duration1 we find that at ,maophlang 7near ,shillong7 #a1aid shocks were felt by one observer from ,september #abth1 #ahig1 to ,october #gth1 #ahih2 at the neighbouring station of ,mairang1 #a1jfe from ,september #gth1 #ahig1 to ,december #cast1 #ahih2 and at ,tura1 in the ,garo hills1 #a1ade shocks from ,july #bast1 #ahig1 to ,december #cast1 #ahih4 ,the total number of earthquakes registered by the seismograph at ,shillong from ,august #ahig to the end of #aija amounts to #a1bgd1 and all of these were probably strong enough to arouse the observer from sleep4 ,outside the epicentral area1 ,mr4 ,oldham's list includes #hh shocks from ,june #abth to ,july #aeth1 about #iej from ,july #afth to ,december #cast 7the period when the after-shocks were most carefully observed71 and #bif shocks during the year #ahih4 ,geographical ,distribution of ,after-,shocks4--,when we endeavour to compare the lists of after-shocks at different places1 we are at once met by two serious difficulties1--the imperfection of the records and the approximate character of the times of occurrence4 ,making every allowance1 however1 for these deficiencies1 it is evident that very few of the shocks felt at any one station were perceptible at its neighbours2 in other words1 that the shocks originated in a large number of foci scattered over a very wide area4 ,for instance1 two of the most carefully kept registers of after-shocks are those compiled at ,maophlang 7near ,shillong71 and at ,mairang1 only #aa miles to the north-west4 ,now1 between ,september #abth and ,september #bhth1 #ahig 7both dates inclusive71 #ib shocks were felt at ,maophlang and #hc at ,mairang4 ,of the former1 #cg were described as smart1 #de slight1 and #aj feeble2 of the latter1 #f as smart1 #i slight1 #fe feeble1 and #c very feeble4 ,but1 of the total number1 only #bj were felt at both places at recorded times that were not more than fifteen minutes apart2 #ac being described as smart--one at both places1 one at ,mairang alone1 and the remaining #aa at ,maophlang alone4 ,when shocks occur so frequently1 as in these cases1 it is inevitable that1 even if all were independent1 some should coincide approximately in time of occurrence4 ,it is therefore probable that only one in every eight shocks1 and possibly only one in every twelve1 was felt at both places4 ,the actual numbers of shocks felt within stated periods at different places are perhaps hardly comparable1 owing to the obvious imperfection of the records and the probably varying standards adopted by the reporters4 ,but there can be little doubt that certain districts were more subject to after-shocks than others1 especially such places as ,north ,guahati1 ,shillong1 and neighbouring villages1 ,tura1 ,darangiri1 ,goalpara1 ,bijni1 ,borpeta1 ,kaunia1 and ,rangpur4 ,on the other hand1 they seem to have been unusually scarce at ,dhubri and in the district to the north-west1 and they became rare at ,gauhati long before they ceased to be frequent at ,borpeta4 ,in the plain to the south of the ,garo and ,khasi hills1 they were also uncommon1 the combined records for ,sylhet and ,sonamganj for ,august #a-ae giving only #bj shocks1 and1 neither to the east nor to the west of these places1 is there any sign of greater frequency4 ,sound-,phenomena of ,after-,shocks4--,many of the after-shocks were accompanied by sound1 or else consisted of sound-vibrations only2 and ,mr4 ,oldham notices that such sounds were equally frequent both on the rocky ground of the hills and on alluvial plains nearly all the shocks that originated under the ,borpeta plain being attended by distinctly audible rumblings4 ,during his tour in the epicentral area in the winter of #ahig-ih1 ,mr4 ,oldham had many opportunities for observing these earth-sounds4 ,they were1 he says1 close to the lower limit of audibility1 less a note than a rumble1 and very like distant thunder1 though sometimes they consisted of a rapid succession of short sounds1 such as is caused by a cart when driven rapidly over a rough pavement4 ,as a rule1 they began as a low1 almost inaudible rumble1 gradually increasing in loudness1 though to a very varying degree1 and then gradually dying out after having lasted anything from #e to #ej seconds4 ,it cannot be said that there was any connection between the duration and the loudness of the sounds1 some of the most prolonged never becoming loud1 and some of those which lasted a shorter period being as loud as ordinary thunder at a distance of two or three miles4 ,mr4 ,oldham records an interesting fact in connection with the distribution of the earth-sounds4 ,at ,naphak1 in the ,garo hills and about five miles south of ,samin1 #dh distinct rumbles were heard during #bc hours on ,january #ba-bc1 #ahih1 only seven of them being accompanied by a perceptible shock4 ,at ,samin1 which was visited next1 they were much less frequent1 not more than #h or #aj a day1 and most of them attended by tremors4 ,at ,damra1 a few miles to the north-east1 they again became frequent2 while1 in the ,chedrang valley1 very few were heard1 and only a small proportion of them were unaccompanied by sensible shocks4 ,in the next section1 it will be seen that the most conspicuous fault-scarps known in the epicentral area pass close by ,samin and along the ,chedrang valley4 ,thus1 though the statement perhaps requires further confirmation1 it would appear that earth-sounds were more common where the surface of the ground had been merely bent than where fractures extended right up to the surface4 ,,structural ,,changes ,,in ,,the ,,epicentral ,,area4 ,we come now to the important features which assign the ,indian earthquake to a small class apart from nearly every other shock4 ,most earthquakes are due to movements that are entirely deep-seated4 ,if strong enough1 they may precipitate landslips or fissure the alluvial soil near river-channels4 ,in the ,neapolitan1 ,andalusian1 and ,charleston earthquakes1 there were many such effects of the shock within the meizoseismal areas4 ,in all three1 however1 the disturbances produced were superficial2 no structural change1 no fissuring that did not die out rapidly downwards1 was in any place perceptible4 ,in the ,riviera earthquake1 the seismic sea-waves point to a small displacement of the ocean-bed2 but it is only in the long fault-scarp of the central ,japanese plain that we find a rival of the mountain-making movements that gave rise to the ,indian earthquake4 ,the boundary of the epicentral area1 to the growth of which these distortions contributed1 is represented by the curve marked ,a in ,fig4 #fh1 and on a larger scale by the continuous line ,a in ,fig4 #ge4 ,a great part of the district is occupied by a group of hills known by various names locally1 but which are conveniently included under the general term of the ,assam range4 ,to avoid the confusion of hill-shading1 only the boundary of the range is indicated 7by the broken line7 in the map in ,fig4 #ge4 ,the ,garo hills form the western part1 and the ,khasi and ,jaintia hills the central and western parts1 of the range as there depicted4 ,they are formed chiefly of crystalline gneissic and granitic rocks and some metamorphic schists and quarzite1 with cretaceous and tertiary rocks of varying thickness along its southern edge4 ,three stages have been distinguished in the history of the range4 ,during the earliest1 an old land-surface was worn down by rain and rivers till they were almost incapable of producing any further change4 ,traces of this surface are still visible in the plateau character of the mass4 ,it was then elevated1 not uniformly1 but along a series of faults1 so that it now consists of a succession of ranges1 the face of each range being a fault-scarp1 and its crest the edge of an adjoining plateau sloping away from the summit4 ,with this elevation began the third and last stage4 ,the streams were able to work again1 and deep gorges were cut out of the range1 so that in parts its original character was nearly effaced4 ,but the retention of that character in other districts is of course evidence of the comparatively recent date of the final elevation4 ,illustration3 ,,fig4 #ge4--;,epicentral ,area of ,indian ,earthquake4 7,oldham47 ,owing to the great size of the epicentre and to the thickness of the forests which cover so much of its area1 a comparatively small part of it could be traversed by ,mr4 ,oldham during his tour in the winter of #ahig-ih4 ,the positions of the more important structural changes are indicated in ,fig4 #ge4 ,of these1 the fault-scarps are represented by continuous straight lines1 the ,bordwar fracture by the dotted straight line1 pools and lakes not due to faulting by black ovals1 reported changes in the aspects of the hills by circles1 and the principal stations of the revised trigonometrical survey by crosses4 ,fault-,scarps4--,the most important fault-scarp is that called by ,mr4 ,oldham the ,chedrang fault1 after the stream which coincides roughly with a great part of its course4 ,the longer straight line in ,fig4 #ge represents its position and general direction1 and the sketch-map in ,fig4 #gf gives the plan of its southern half4 ,from these1 it will be seen that the fault follows on the whole a nearly straight path from south-south-east to north-north-west for not less than twelve miles1 and that its throw1 as indicated by the numbers to the right in ,fig4 #gf2 is very variable1 being zero in some places1 and in one as much as #ce feet or more4 ,the upthrow is uniformly on the eastern side of the fault4 ,at its southern end1 as mapped in ,fig4 #gf1 there is no perceptible throw at the surface1 but various marks of violence are manifested in the fissuring of the hillside and the snapping of small trees4 ,about a quarter of a mile from this point1 the fault crosses a tributary stream1 where the throw amounts to two feet1 and the same distance farther on it meets the ,chedrang river1 the bed of which it crosses many times in its short course4 ,illustration3 ,,fig4 #gf4--,plan of ,chedrang fault4 7,oldham47 ,mr4 ,oldham describes the fault in detail1 as observed by him in ,february #ahih4 ,here1 it will be sufficient to refer to its more important features1 and to its effects on the superficial drainage of the district4 ,at the spot marked a 7,fig4 #gf7 the river1 after running on the west or down-throw side of the fault for nearly half a mile1 meets the scarp1 and is ponded back by it for about a quarter of a mile upstream4 ,for the next half-mile1 the river keeps to the upthrow side of the fault1 the scarp of which blocks the tributary streams from the west1 forming a number of small pools4 ,at the last of these1 the total throw is not less than #be feet4 ,a little farther on1 the fault crosses the ,chedrang and causes the waterfall at b1 the height of which1 owing to the fall of dislodged fragments1 does not exceed nine feet4 ,the fault then runs along the old and now dry bed of the river1 while the stream itself flows in a depression on the down-throw side4 ,about a quarter of a mile below the waterfall1 the fault crosses the river1 and soon after enters a large sheet of water at c1 half a mile long1 from #cjj to #djj yards wide1 and with a maximum depth of #ah feet4 ,at first1 the pool spreads on both sides of the fault1 but the inequalities due to the scarp are evidenced by soundings4 ,at the point where the fault leaves the pool1 its throw is reduced to nothing1 and it is just here that the water attains its greatest depth4 ,to the north the throw increases rather rapidly1 to #be feet in a quarter of a mile4 ,but the peculiarity of this pool is that it is not1 like the others mentioned above1 dammed back by the fault-scarp4 ,there is no barrier at its northern end1 where the river escapes1 except that formed by the gradually increasing throw of the fault4 ,the pool is simply due to the reversal of the natural slope of the river-bed1 caused by the formation of a roll or undulation in the ground on the upthrow side of the fault4 ,its recent origin is evident from the number of dead trees and bamboo clumps still standing in the water4 ,for a mile after the fault leaves the pool1 its throw varies considerably4 ,it rises1 as already mentioned1 from zero to #be feet4 ,a little farther on1 the fault runs up the side of a spur1 the throw increasing to #ca feet2 and1 in this part1 the violence of the shock was shown by the dislodgment of blocks of granite as much as #bj feet in diameter1 and by the overthrow or destruction of many trees4 ,after crossing the spur1 the fault returns to the neighbourhood of the river1 and crosses its bed four times1 forming pools 7e1 g7 or waterfalls 7d1 f7 according as the scarp occurs on the downstream or upstream side4 ,the throw of the fault then changes considerably within little more than half a mile1 from #ah feet to zero and again to #bj feet1 the undulation so formed producing a large pool 7h7 entirely on the upthrow side of the fault4 ,at the point marked i on the map1 the river once more crosses the fault2 but the bottom of the valley is filled with alluvium1 and1 instead of a waterfall1 a large sandy delta spreads down the stream4 ,the scarp is1 however1 readily traced on the east side of the river1 a throw of #cb feet being measured4 ,after this1 the alluvium becomes of considerable thickness1 and the continuation of the fault is marked by a short slope1 which tilts over the trees when it traverses forest-land4 ,leaving the valley of the ,chedrang1 the fault crosses an open plain1 and is followed with some difficulty to the neighbourhood of ,jhira1 where1 owing to the thick bed of alluvium1 it forms a gentle roll or undulation of the surface1 crossing the main channel of the ,krishnai to the north-east of ,jhira4 ,on the west side of this barrier a large sheet of water1 a mile and a half in length1 three-quarters of a mile wide1 and #ab feet in depth1 gathered over the village of ,jhira4 ,on the east side of the ,jhira lake1 says ,mr4 ,oldham1 there is ample evidence of change of level1 for part of the dry land was formerly 444 perpetually under water1 and at one place the remains of an old irrigation channel can be seen4444 ,at the northern end of the lake the drainage now makes its escape in a broad and shallow sheet of water over what was once high land covered with sal forest4 ,this is the last marked feature due to the ,chedrang fault4 ,beyond the north of ,jhira the throw rapidly diminishes1 and perhaps dies out altogether before reaching the low hills lying to the north of that village4 ,in several ways1 this fault-scarp differs from that formed with the ,japanese earthquake of #ahia4 ,throughout its course the down-throw1 wherever it is perceptible1 is invariably to the west2 in no place could any trace of horizontal shifting be detected2 and the plane of the fault1 when it traversed rock1 is practically vertical4 ,whether the scarp was formed by the elevation of the rock to the east of the fault1 or by the depression of that to the west1 or by both such movements at once1 there is no decisive evidence2 but there are very good reasons for believing the first alternative to be the true one4 ,the undulations in the ground which gave rise to the large pools at c and h 7,fig4 #gf7 occur on the east side of the fault4 ,also1 between the outlet of the lake at ,jhira and the point where the ,krishnai rejoins its original channel1 the gradient of the river approaches that of a mountain stream1 although the new bed consists of alluvium1 and not of rock4 ,now1 the alluvial plain of this district is raised so slightly above the sea-level that no subsidence great enough to have caused the existing gradient could have occurred without the depressed area being flooded with water4 ,though some movements may have taken place on the west side of the fault1 it seems clear1 then1 that elevation of the rock on the east side was the predominant1 if not the sole1 cause of the fault-scarp4 ,as the ,chedrang fault has been described somewhat fully1 a brief reference to the rest will be sufficient ,the only other known scarp of any consequence lies about ten miles to the south of the ,chedrang fault1 and runs by the village of ,samin1 with an average course from ,e4 #cj ,s4 to ,w4 #cj ,n4 ,its total length does not exceed #b-a_/b miles4 ,the down-throw is uniformly to the north1 and the throw1 which amounts to ten feet near its centre1 gradually diminishes to zero at either end4 ,several pools are formed along the course of the fault-scarp by the blocking of small streams4 ,the ,bordwar ,fracture4--,in the map of the epicentral area 7,fig4 #ge71 this remarkable fracture is represented by a dotted straight line4 ,it is apparently an incipient fault4 ,though traceable for a distance of about seven miles1 at no point is there any decisive evidence of either vertical or horizontal displacement2 and1 even if some doubtful indications of a change of level should be real1 the throw must certainly be less than one foot4 ,yet1 in the immediate neighbourhood of the fracture1 the violence of the shock was extreme4 ,trees have been overthrown or killed as they stood2 a huge mass of rock1 dislodged from near the crest of the hills1 has rolled down the slope1 scoring the side of the hill4 ,on the opposite side an equally large block has been dislodged1 and in its downward course cleared a straight track down the hill2 and on the summit a gap has been cleared by the overthrow of trees along the line of fracture4 ,being only a few inches in width where it has rent the solid rock1 the fracture was difficult to follow in many parts of its course4 ,but1 through forest-clad land1 its track was marked by a well-defined band of about half a mile broad1 in which overturned trees are much more abundant than on either side1 and towards the centre of this band the overturned trees are not only more numerous1 but many of the smaller ones1 up to six inches in diameter1 have been snapped across by the violence of the shock4 ,lakes and ,pools not due to ,faulting4--,a few miles to the south of the ,chedrang and ,samin faults1 and also of the ,bordwar fracture1 occurs a group of lakes or pools1 represented on the map of the epicentral area 7,fig4 #ge7 by small black ovals4 ,in the gradual increase in depth from either end1 they resemble the two large sheets of water along the course of the ,chedrang fault 7c and h1 ,fig4 #gf71 but they differ from them in having no direct connection with any apparent fault4 ,one of these pools lies in the valley of the ,rongtham river1 to the south of the ,samin fault4 ,it seemed1 at first sight1 to be nothing more than an ordinary pool1 such as may be seen on any mountain stream4 ,on the bottom1 and close to the outlet1 however1 are coarse1 partially rounded boulders1 exactly resembling those farther down the river2 and1 as the old bed was followed up1 these became coated with a slight deposit of sand and mud1 pointing clearly to a change in the conditions under which they were formed4 ,the water gradually deepened1 until trees were met standing in the water1 but killed by the recent submergence of their roots4 ,the pool is nearly a quarter of a mile long1 and its greatest depth 7#ab feet7 occurs near the middle1 just where the former stream1 with an average depth of about a foot1 was crossed by the track from ,darangiri4 ,towards the upper end1 the water shallows as gradually as it deepens at the other1 and ends in a delta of boulders brought down by the stream above4 ,as no fault could be discovered in the neighbourhood of the pool1 it is evident that its formation was due to a bend of the river-bed1 the maximum change of level1 taking into account the river-slope1 being not less than #bd feet4 ,similar features characterise the other pools that were examined1 some of which are smaller1 and others larger1 than that described above4 ,one1 higher up the valley of the ,rongtham1 has a length of about #a-a_/b mile and a maximum depth of #ah feet4 ,others of the same type1 but of smaller size1 were observed among the ,khasi hills1 about fifteen miles south of the ,bordwar fissure2 and it is probable that many others would have been found in the intermediate district1 which ,mr4 ,oldham was unable to visit4 ,changes in the ,aspects of the ,hills4--,there are1 again1 other facts of considerable interest which point to changes of level over a wide area2 the places where they were noticed being indicated by small circles in ,fig4 #ge4 ,for instance1 from ,maophlang1 near ,shillong1 a road leads to the neighbouring station of ,mairang4 ,before the earthquake1 only a short stretch of this road could be seen from the former place1 as it rounded a spur about three miles away4 ,now1 a much longer stretch is visible1 and it can also be seen passing round the next1 and previously hidden1 spur4 ,in this district the movements seem to have continued with the after-shocks2 for1 before the earthquake1 the crest only of a ridge about a mile and a half to the west was visible2 while1 after it1 a considerable portion could be seen1 and much more some months later than immediately after the shock4 ,again1 from a spot near the southern end of the ,chedrang fault1 it used to be only just possible to see the ,brahmaputra over an intervening hill2 whereas1 now1 the whole width of the river has come into view4 ,lastly1 at ,tura1 which is #ie miles west of ,maophlang1 a battalion of military police were accustomed to signal by heliograph with another station1 ,rowmari1 #ae miles farther to the west4 ,this1 formerly1 could just be done by means of a ray which grazed a hill between the two places2 it can now be done quite easily1 and1 in addition1 a broad stretch of the plains east of the ,brahmaputra is visible from the same spot4 ,revision of the ,trigonometrical ,survey4--,the movements described in the preceding pages are of course referred to points which may themselves have been displaced1 and only a revision of the trigonometrical survey of the epicentral area and of part of the surrounding district could determine their absolute magnitude4 ,during the cold weather of #ahig-ih1 some of the triangles were re-measured by a member of the trigonometrical survey2 but1 as the time at his disposal was short1 they were confined to the eastern part of the epicentral area1 as the focus at that time was supposed to lie under the ,khasi hills4 ,the positions of some of these stations are indicated by crosses in ,fig4 #ge2 and in ,fig4 #gg the more important triangles are shown4 ,in the revised work1 all tower stations1 consisting of brick towers built on alluvium1 were omitted1 as it could not be assumed that they had been undisturbed by displacements of the superficial beds4 ,in re-calculating the lengths of the sides1 the side ,rangsanobo-,taramun ,tila was adopted as the initial base1 and the height of ,rangsanobo as the initial height2 a choice which later experience showed to be unfortunate1 for ,taramun ,tila probably lies just outside1 and ,rangsanobo within1 the epicentral area4 ,of the #af sides1 whose old and new lengths were compared1 only one was found to be apparently unchanged1 two were shortened by an inch or two1 while the others were all lengthened by amounts varying from one to eight or nine feet1 the numbers affixed to the sides in ,fig4 #gg denoting the calculated increases in feet4 ,assuming the new base-line to be unaltered by the earthquake movements1 these changes imply the following displacements of the principal stations3--,thanjinath #f feet1 ,mun #d1 and ,laidera #b1 feet to the north2 ,mopen #e1 ,dinghei #i1 ,landau ,modo #ab1 and ,umter #aa1 feet to the north-west2 and ,mosingi #c1 and ,mautherrican #e1 feet to the west4 ,at the same time1 the height of most of the stations was found to be increased with reference to that of ,rangsanobo3 ,mun by #b feet1 ,thanjinath and ,umter by #c1 ,mosingi by #d1 ,taramun ,tila and ,laidera by #f1 ,dinghei by #g1 ,landau ,modo by #ag1 and ,mautherrican by #bd1 feet2 while the height of ,mopen seems to have been diminished by #d feet4 ,thus1 at first sight1 these calculations appear to indicate a general elevation and extension of the hills1 such as might follow on a bulging upwards of the surface due to the extension of a large mass of molten matter underground4 ,illustration3 ,,fig4 #gg4--,re-triangulation of ,khasi hills4 7,oldham47 ,unfortunately1 as ,mr4 ,oldham shows1 a very different1 and more probable1 interpretation may be given of these results2 for all the calculated changes are rendered uncertain by the choice of the two stations which form the ends of the new base-line4 ,one at least may have been displaced by the structural movements within the epicentral area2 and1 moreover1 the line joining them runs nearly north and south4 ,as compression in this direction is to be expected1 it is probable that this line was shortened2 and the assumption that its length was unchanged would therefore lead to an apparent expansion of all the other sides4 ,the calculated changes seem to favour this explanation to a great extent4 ,the sides joining ,mopen1 ,rangsanobo1 and ,thanjinath run nearly east and west1 and are apparently lengthened by #d4i and #c4d feet respectively2 while1 of the four sides joining these stations to ,mosingi and ,mun1 lying next to the north1 two are nearly or quite unchanged1 and the others increased by #b4c and #c4b feet4 ,again1 the estimated increase of the ,mosingi-,mun line is #d4d feet2 while the four sides joining these stations to the next northerly group are increased by small amounts--namely1 #a4b1 #b4f1-j4c1 and #b4d feet4 ,thus1 the apparent expansion that should have occurred in these more or less northerly sides is lessened1 or roughly compensated1 probably by a compression of the whole region in a meridianal direction4 ,for a similar reason1 the slight general upheaval of the hills indicated by the repeated calculations1 must be regarded as doubtful1 for it depends on the assumed fixity of the station of ,rangsanobo1 whereas it is more probable that it was the height of ,taramun ,tila that remained unchanged4 ,reducing the calculated heights of all the other stations by six feet 7the assumed rise of the latter71 it follows that1 on the whole1 the height of the ,khasi hills underwent but little change1 except at ,mautherrican and ,landau ,modo1 and the secondary stations of ,mairang and ,kollong ,rock1 near ,maonoi4 ,the apparent elevations of #bd1 #ag1 #aa1 and #ae feet at these places exceed the probable error of the observations2 and it is worthy of notice that all four stations lie close to the edge of fault-scarps1 while ,landau ,modo is not far from two of the pools formed by distortion of the surface unaccompanied by faulting4 ,if1 then1 the revised triangulation of the ,khasi hills has failed to provide absolute measures of the displacements in the epicentral area1 it has1 nevertheless1 proved that important movements1 both horizontal and vertical1 have taken place4 ,distribution of the ,structural ,changes4--,the boundary of the epicentral area1 as drawn in ,figs4 #fh and #ge1 lays no claim to great accuracy2 but its departure from the true line is probably in no place considerable4 ,it must evidently include all the districts where marked structural changes occurred1 and must therefore extend east of ,maophlang and west of ,tura4 ,towards the north1 these changes have been traced to the foot of the ,garo hills1 and there is some1 though not very certain1 evidence of alterations of level along the course of the ,brahmaputra4 ,the very large number of after-shocks recorded at ,borpeta and ,bijni also points to an extension of the epicentral area beyond these places4 ,to the east1 the course of the boundary becomes doubtful1 but it must pass close to ,gauhati and east of ,shillong1 and probably ends a short distance beyond ,jaintiapur4 ,the southern boundary must coincide nearly with the north edge of the alluvial plains of ,sylhet1 for there is no evidence of its intrusion into the plains4 ,on the west side1 the epicentral area includes the ,garo hills and part of the alluvial plain to the west2 and1 from the large number of after-shocks felt at ,rangpur and ,kaunia1 and the great violence of the shock at the former1 we may infer that both places lie within the boundary-line4 ,if1 then1 there is no great error in the mapping of this line1 it follows that the epicentre was about #bjj miles long from east to west1 not less than #ej1 and possibly as much as #ajj1 miles in maximum width1 and contained an area of at least #fjjj square miles4 ,near the boundary1 the permanent displacements must have been comparatively small2 but they were certainly marked in the northern part of the ,assam hills for a distance of #ajj miles from east to west4 ,at the limits of the latter area1 as ,mr4 ,oldham remarks1 the evidence points to the changes being of the nature of long1 low rolls1 the change of slope being insufficient to cause any appreciable change in the drainage channels4 ,then comes a zone in which the surface changes are more abrupt1 the slopes of the stream beds have been altered so as to cause conspicuous changes in the nature of the streams1 but any fracture or faulting which may have taken place has died out before the surface was reached4 ,and north of this1 close to the edge of the hills1 the rocks have been fractured and faulted right up to the surface4 ,,origin ,,of ,,the ,,earthquake4 ,almost every feature of the great earthquake points to an origin very different from that of the others described in this volume4 ,the suddenness with which the shock began1 its unusual duration1 and the occurrence of many maxima of intensity1 are inconsistent with a simple fault-displacement4 ,again1 the excessive velocities of projection at ,rambrai and elsewhere1 the existence of isolated fault-scarps and fractures1 the local changes of level1 the compression indicated by the revised trigonometrical survey1 the wide area over which these structural changes took place1 and the numerous distinct centres of subsequent activity1 all these phenomena demonstrate the intense and complex character of the initial disturbances1 as well as the widespread bodily displacement of the earth's crust within the epicentral area4 ,there may1 it is conceivable1 have been a number of foci1 nearly or quite detached from one another1 and giving rise to a group of nearly concurrent shocks4 ,or--and this is a far more probable supposition--there may have been one vast deep-seated centre1 from which off-shoots ran up towards the surface1 each partaking to a greater or less degree in the movement within the parent focus4 ,as ,mr4 ,oldham points out1 we have recently become acquainted with a structure exactly corresponding to that which is here inferred4 ,the great thrust-planes1 so typically developed in the ,scottish ,highlands1 are only reversed faults which are nearly horizontal instead of being highly inclined2 and they are accompanied by a number of ordinary reversed faults running upwards to the surface4 ,in ,fig4 #gh1 the main features of a section drawn by the ,geological ,survey of ,scotland are reproduced2 ,t1 ,t1 representing thrust planes1 and t1 t1 minor thrusts or faults4 ,a great movement along one of the main thrust-planes would carry with it dependent slips along many of the secondary planes4 ,direct effects of the former might be invisible at the surface1 except in the horizontal displacements that would be rendered manifest by a renewed trigonometrical survey2 whereas the latter might or might not reach the surface1 giving rise in the one case to fissures and fault-scarps1 in the other to local changes of level1 and in both to regions of instability resulting in numerous after-shocks4 ,illustration3 ,,fig4 #gh4--;,diagram of ,thrust-planes4 ,the enormous dimensions of the parent focus will be obvious from the phenomena that have been described above4 ,mr4 ,oldham has traced the probable form of the epicentre4 ,it may in reality be neither so simple nor so symmetrical as is represented in ,fig4 #ge1 but there are good reasons for thinking that it does not differ sensibly either in size or form from that laid down4 ,the part of the thrust-plane over which movement took place must therefore have been about #bjj miles long1 not less than #ej miles wide1 and between #fjjj and #gjjj square miles in area4 ,with regard to its depth1 we have no decisive knowledge4 ,it may have been about five miles or less2 it can hardly have been much greater4 ,it is a strain on the imagination to try and picture the displacement of so huge a mass4 ,we may think1 if we will1 of a slice of rock three or four miles in thickness and large enough to reach from ,dover to ,exeter in one direction and from ,london to ,brighton in the other2 not slipping intermittently in different places1 but giving way almost instantaneously throughout its whole extent2 crushing all before it1 both solid rock and earthy ground alike2 and1 whether by the sudden spring of the entire mass or by the jar of its hurtling fragments1 shattering the strongest work of human hands as easily as the frailest4 ,such a thrust might well be sensible over half a continent1 and give rise to undulations which1 unseen and unfelt1 might wend their way around the globe4 ,,references4 #a4 ,,agamennone1 ,g4--,notizie sui terremoti osservati in ,italia durante l'anno #ahig 7,terremoto dell' ,india poco dopo il mezzogiorno del #ab giugno74 ,ital4 ,sismol4 ,soc4 ,boll41 vol4 iii41 pte4 ii41 #ahig1 pp4 #bdi-bic4 #b4 ---- ,il terremoto dell' ,india del #ab giugno #ahig4 ,ibid41 vol4 iv41 #ahih1 pp4 #cc-dj4 #c4 ---- ,eco in ,europa del terremoto indiano del #ab giugno #ahig4 ,ibid41 vol4 iv41 #ahih1 pp4 #da-fg4 7,see also the same volume1 pp4 #afg-agb47 #d4 ,,baratta1 ,m4--,il grande terremoto indiano del #ab giugno #ahig4 ,ital4 ,soc4 ,geogr4 ,boll41 vol4 x41 #ahig1 fasc4 viii4 #e4 ,,cancani1 ,a4--,i pendoli orizzontali del ,r4 ,osservatorio geodinamico di ,rocca di ,papa1 ed il terremoto indiano del #ab giugno #ahig4 ,ital4 ,sismol4 ,soc4 ,boll41 vol4 iii41 #ahig1 pp4 #bce-bdj4 #f4 ,,heath1 ,t4--,note on the ,calcutta ,earthquake 7,june #abth1 #ahig7 as recorded by the bifilar pendulum at the ,edinburgh ,royal ,observatory4 ,edinb4 ,roy4 ,soc4 ,proc41 #ahig1 pp4 #dha-dhh4 #g4 ,,oldham1 ,r4,d4--,report on the ,great ,earthquake of #abth ,june #ahig4 ,mems4 ,geol4 ,surv4 of ,india1 vol4 xxix41 #ahii1 pp4 i4-xxx41 #a-cgi1 with #dd plates and #c maps4 #h4 ---- ,list of ,after-shocks of the ,great ,earthquake of #abth ,june #ahig4 ,ibid41 vol4 xxx41 pt4 i41 #aijj1 pp4 #a-ajb4 #i4 ---- ,on ,tidal ,periodicity in the ,earthquakes of ,assam4 ,journ4 ,asiat4 ,soc41 vol4 lxxi41 #aijb1 pp4 #aci-aec4 ,,footnotes3 #fi ,according to some reports1 the earthquake was felt in ,italy4 ,at ,livorno1 the first movements were registered by seismographs at #aa4ag ,a4,m4 7,g4,m4,t471 and tremors were noticed by some persons at rest at about #aa4ae ,a4,m4 ,at ,spinea1 a sensible undulatory shock from south-east to north-west1 and lasting about four seconds1 was felt at the moment when all the seismographs were set in motion by the ,indian earthquake4 ,in spite of the great distance1 the perception of the earthquake in ,italy is not impossible1 but the records seem to me to refer to local tremors rather than to the very slow evanescent oscillations of a very distant earthquake4 #gj ,all the times in this section are referred to ,madras mean time1 which is #e;h4 #bjm4 #ei4bs4 in advance of ,greenwich mean time4 ,in the next section it will be found convenient to use the latter standard4 #ga ,it may be useful to give references to works in ,english in which the principal instruments for registering distant earthquakes are described4 ,for ,cancani's vertical pendulum1 see ,brit4 ,assoc4 ,rep41 #ahif1 pp4 #df-dg2 ,darwin's bifilar pendulum1 ,brit4 ,assoc4 ,rep41 #ahic1 pp4 #bia-cjc1 and ,nature1 vol4 #a41 #ahid1 pp4 #bdf-bdi2 ,milne's horizontal pendulum1 ,seismology1 pp4 #eh-fa2 ,rebeur-,paschwitz's horizontal pendulum1 ,brit4 ,assoc4 ,rep41 #ahic1 pp4 #cjc-cjh4 #gb ,the beginnings of the second and third phases are shown more clearly in the record of the vertical pendulum at ,catania1 a record1 however1 that will not bear the reduction necessary for these pages4 #gc ,geol4 ,mag41 vol4 x41 #ahic1 pp4 #cef-cfj4 #gd ,irish ,acad4 ,trans41 vol4 xxi1 #ahdh1 p4 #eb4 #ge ,irish ,acad4 ,trans41 vol4 xxi41 #ahdh1 pp4 #ee-eg4 #gf ,neapolitan ,earthquake of #aheg1 vol4 i41 #ahfb1 pp4 #cgf-cgh4 #gg ,japan ,seismol4 ,soc4 ,trans41 vol4 i41 pt4 ,,ii41 #ahhj1 pp4 #cc-ce4 #gh ,geol4 ,mag41 vol4 ix41 #ahhb1 pp4 #beg-bfe4 ,,chapter ,x4 ,,conclusion4 ,in this concluding chapter1 ,i propose to give a summary of the results at which we have arrived from the study of recent earthquakes1 and this can1 ,i think1 be done best by describing what may be regarded as an average or typical earthquake1 though it may be convenient occasionally to depart slightly from such a course4 ,few shocks have contributed more to our knowledge than the majority of those described in this volume2 but1 on certain points1 we gain additional information from the investigation of other earthquakes1 and these are referred to when necessary for the purpose in view4 ,,fore-,,shocks4 ,at the outset1 we are met by a question of some interest and great practical importance--namely1 whether there are any constant signs of the coming of great earthquakes by means of which their occurrence might be predicted and their disastrous effects mitigated4 ,excluding the ,ischian earthquakes1 which belong to a special class1 it is evident that there is generally some slight preparation for a great earthquake4 ,for a few hours or days beforehand1 weak shocks and tremors are felt or rumbling noises heard within the future meizoseismal area4 ,but1 unfortunately1 it has not yet been found possible to distinguish these disturbances from others of apparently the same character which occur alone1 so that for the present they fail to serve as warnings4 ,in ,japan1 where the organisation of earthquake-studies is more complete than elsewhere1 it is possible that a vague forecast might be made1 if the distribution of the fore-shocks of the earthquake of #ahia should prove to be a general feature of all great earthquakes4 ,it was at first supposed that this earthquake occurred without preparation of any kind2 but a closer analysis of the records shows that during the previous two years there was a very decided increase in the seismic activity of the district1 and also that the distribution of the epicentres marked out the future fault-scarp1 and at the same time exhibited a tendency to comparative uniformity over the whole fault-region4 ,for the present1 then1 the only warning available is that given by the preliminary sound1 which may precede the strongest vibrations by as much as five or ten or even more seconds4 ,though two or three seconds may elapse before its character is recognised1 the fore-sound thus allows time for many persons to escape from their falling houses4 ,some races1 however1 are less capable of hearing the sound than others1 and this may be one reason why ,japanese earthquakes are so destructive of human life4 ,,disturbed ,,area4 ,it is usual with some investigators to measure the intensity of an earthquake roughly by the extent of its disturbed area4 ,the depth of the seismic focus must of course have some influence on the size of this area1 and this condition is only neglected because we have no precise knowledge of the depth in any case4 ,thus1 ,mr4 ,oldham regards the ,indian earthquake of #ahig as rivalling the ,lisbon earthquake of #agee1 which is generally considered to hold the first place1 because its disturbed area was not certainly exceeded by that of the latter4 ,that disturbed area is1 however1 an untrustworthy measure of intensity will be evident from the following table1 in which the earthquakes described in this volume 7omitting those of ,ischia7 are arranged as nearly as may be in order of intensity1 beginning with the strongest3-- ,earthquake4 ,disturbed ,area in ,sq4 ,miles4 ,indian #a1gej1jjj ,japanese #ccj1jjj ,neapolitan #ci1bjj ,charleston #b1hjj1jjj ,riviera #bai1jjj ,andalusian #agd1jjj ,hereford #ih1jjj ,inverness #cc1jjj ,here we see that the ,charleston earthquake was perceptible over a greater area than the ,indian earthquake1 while the ,neapolitan earthquake was inferior to that of ,hereford in this respect4 ,the explanation of course is that the boundaries of the disturbed areas are isoseismal lines corresponding to different degrees of intensity1 the inhabitants of ,great ,britain and the ,united ,states being evidently more sensitive to weak tremors1 or more observant1 than those of ,italy1 ,spain1 or ,central ,asia4 ,the only disturbed areas that are bounded by isoseismals of the same intensity are the two last4 ,very roughly1 then1 we may say that the intensity of the ,hereford earthquake was three times as great as that of the ,inverness earthquake4 ,,position ,,of ,,the ,,epicentre4 ,one of the first objects in the investigation of an earthquake is to determine the position and form of the epicentre4 ,in a few rare cases1 as in the ,japanese and ,indian earthquakes1 when the fault-scarp is left protruding at the surface1 only careful mapping is required to ascertain both data4 ,but1 in the great majority of earthquakes1 the fault-slip dies out before reaching the surface and the position of the epicentre is then inferred by methods depending chiefly on the time of occurrence or on the direction or intensity of the shock4 ,at first sight1 methods that involve the time of occurrence at different places seem to be of considerable promise4 ,no scientific instruments are so widely diffused as clocks and watches2 but1 on the other hand1 few are so carelessly adjusted4 ,it is the exception1 rather than the rule1 to find a time-record accurate to the nearest minute2 and1 as small errors in the time may be of consequence1 methods depending on this element of the earthquake are seldom employed4 ,if1 however1 the number of observations is large for the size of the disturbed area1 the construction of coseismal lines may define approximately the position of the epicentre4 ,in the ,hereford earthquake of #ahif1 the centre of the innermost coseismal line 7,fig4 #fb7 is close to the region lying between the two epicentres4 ,the method of locating the epicentre by means of the intersection of two or more lines of direction of the shock was first suggested by ,michell in #agfj1gi and has been employed by ,mallet in investigating the ,neapolitan earthquake1 by ,professors ,taramelli and ,mercalli in their studies of the ,andalusian and ,riviera earthquakes1 as well as by other seismologists4 ,the diversity of apparent directions at one and the same place caused its temporary neglect1 until ,professor ,omori showed in #ahid that the mean of a large number of measurements gives a trustworthy result 7p4 #ai74 ,his interesting observations should reinstate the method to its former place among the more valuable instruments at the disposal of the seismologist4 ,no observations1 however1 are at present so valuable for the purpose in view as those made on the intensity of the shock4 ,for many years1 it has been the custom to regard the epicentre as coincident with the area of greatest damage to buildings2 and1 when the area is small1 the assumption cannot be much in error4 ,it is of course merely a rough way of obtaining a result that is generally given more accurately by means of isoseismal lines2 but there are exceptional cases1 such as the ,neapolitan and ,ischian earthquakes1 when the destruction wrought by the earthquake furnishes evidence of the greater value4 ,a single isoseismal accurately drawn not only gives the position of the epicentre with some approach to exactness1 but also by the direction of its longer axis determines that of the originating fault4 ,when two or three such lines can be traced1 the relative position supplies in addition the hade of the fault 7p4 #bai74 ,the successful application of the method requires1 it is true1 a large number of observations1 and these cannot as a rule be obtained except in districts that are somewhat thickly and uniformly populated1 such as those surrounding the cities of ,hereford and ,inverness4 ,in the ,charleston earthquake1 also1 the position and form of the epicentres were deduced from the trend of isoseismal lines based on the damage to railway-lines and various structures within a sparsely inhabited meizoseismal area4 ,in a few cases1 of which the ,indian earthquake may be regarded as typical1 a fourth method has recently been found of service4 ,the numerous after-shocks which follow a great earthquake originate for the most part within the seismic focus of the latter2 and1 as they usually disturb a very small area1 it is not difficult to ascertain approximately the positions of their epicentres4 ,some1 as in the ,inverness after-shocks of #aija1 result from slips in the very margin of the principal focus2 but1 as a rule1 the seat of their activity tends to contract towards a central region of the focus4 ,bearing in mind1 then1 that some of the succeeding shocks originate at and beyond the confines of the focus1 and that others may be sympathetic shocks precipitated by the sudden change of stress1 it follows that the shifting epicentres of the true after-shocks map out1 in part at any rate1 the epicentral area of the principal earthquake4 ,,depth ,,of ,,the ,,seismic ,,focus4 ,it is much to be regretted that we have no satisfactory method of determining so interesting an element as the depth of the seismic focus4 ,that it amounts to but a few miles at the most is certain from the limited areas within which slight shocks are felt or disastrous ones exhibit their maximum effects4 ,nor can we suppose that the rocks at very great depths are capable of offering the prolonged resistance and sudden collapse under stress that are necessary for the production of an earthquake4 ,the problem is evidently beyond our present powers of solution1 and its interest is therefore mainly historical4 ,all the known methods are vitiated by our ignorance of the refractive powers of the rocks traversed by the earth-waves4 ,but1 even if this ignorance could be replaced by knowledge1 most of the methods suggested are open to objection4 ,falb's method1 depending on the time-interval between the initial epochs of the sound and shock1 is of more than doubtful value4 ,dutton's1 based on the rate of change of surface-intensity1 is difficult to apply1 and in any case gives only an inferior limit to the depth4 ,time-observations have been employed1 especially in ,new ,zealand2 but the uncertainty in selecting throughout the same phase of the movement1 and the large errors in the estimated depth resulting from small errors in the time-records1 are at present most serious objections4 ,there remains the method devised by ,mallet1 and1 though he claimed for it an exaggerated accuracy1 it still1 in my opinion1 holds the field against all its successors4 ,when carefully applied1 as it has been by ,mallet himself1 by ,johnston-,lavis and ,mercalli1 we probably obtain at least some conception of the depth of the seismic focus4 ,professor ,omori and ,mr4 ,k4 ,hirata have recently#hj lessened the chief difficulty in the application of ,mallet's method4 ,they have deduced the angle of emergence from the vertical and horizontal components of the motion as registered by seismographs1 instead of from the inclination of fissures in damaged walls4 ,in two recent earthquakes recorded at ,miyako in ,japan1 they find the angle of emergence to be #g4b and #i respectively1 the corresponding depths of the foci being #e4f and #i4c miles4 ,these are probably the most accurate estimates that we possess1 and it will be noted that they differ little from the mean values obtained for the ,neapolitan1 ,andalusian1 and ,riviera earthquakes--namely1 #f4f1 #g4f1 and #aj4h miles4 ,,nature ,,of ,,the ,,shock ,in one respect1 the earthquakes described above fail to represent the progress of modern seismology4 ,they furnish no diagrams made by accurately constructed seismographs within their disturbed areas4 ,the curve reproduced in ,fig4 #cf1 as already pointed out1 is no exception to this statement4 ,for another reason1 the records that were obtained in ,japan of the earthquake of #ahia are trustworthy for little more than the short-period initial vibrations2 for1 owing to the passage of the surface-waves1 visible in and near the meizoseismal area1 the ,japanese seismographs registered the tilting of the ground rather than the elastic vibrations that traversed the earth's crust4 ,notwithstanding this defect1 personal impressions of an earthquake-shock give a fairly accurate1 if incomplete1 idea of its nature4 ,nearly all observers placed under favourable conditions agree that an earthquake begins with a deep rumbling sound1 accompanied1 after the first second or two1 by a faint tremor which gradually1 and sometimes rapidly1 increases in strength until it merges into the shock proper1 which consists of several or many vibrations of larger amplitude and longer period1 and during which the attendant sound is generally at its loudest2 the earthquake dying away1 as it began1 with tremors and a low rumbling sound4 ,illustration3 ,,fig4 #gi4--,seismographic ,record of ,tokio ,earthquake of #ahid4 7,omori47 ,the vibrations that produce the sensible shock are by no means all that are present during an earthquake4 ,the ,indian earthquake1 for instance1 seemed to last about three or four minutes at ,midnapur2 but the movements of the bubble of a level showed that the ground continued to oscillate for at least five minutes longer 7p4 #bhj74 ,many of these unfelt waves are rendered manifest by seismographs1 although there are still others that elude registration either from the extreme shortness or the great length of their periods4 ,in ,fig4 #gi is shown the principal part of a diagram obtained at ,tokio during the ,japanese earthquake of ,june #bjth1 #ahid 7p4 #ah71 the curve representing the ,n4,e4-,s4,w4 component of the horizontal motion during the first #be seconds of the record4 ,the instrument employed is one specially designed for registering strong earthquakes1 and is unaffected by very minute tremors4 ,those which formed the commencement of this earthquake lasted for about #aj seconds1 as shown by ordinary seismographs1 and the vibrations had attained a range of a few millimetres before they affected the instrument in question4 ,for the first #b-a_/b seconds1 they occurred at the rate of four or five a second4 ,the motion then suddenly became violent1 and the ground was displaced #cg mm4 in one direction1 followed by a return movement of #gc mm41 and this again by one of #db mm41 the complete period of the oscillation being #a4h seconds4 ,the succeeding vibrations were of smaller amplitude and generally of shorter period for a minute and a half1 then dying out during the last three minutes as almost imperceptible waves with a period of two or more seconds4#ha ,though incomplete in some respects1 this diagram illustrates clearly the division of the earthquake-motion into three stages--namely1 the preliminary tremors1 the principal portion or most active part of an earthquake1 and the end-portion or gradually evanescent slow undulations4 ,in all three stages1 however1 both tremors and slow undulations may be present2 and1 as the latter1 owing to their long period1 are more or less insensible to human beings1 the ripples of the final stage give the impression of a tremulous termination as described above4 ,the duration of each stage varies considerably in different earthquakes4 ,thus1 in a valuable study of #bg earthquakes recorded at ,miyako1 in ,japan1 during the years #ahif-ih1 ,messrs4 ,omori and ,hirata show#hb that the duration of the preliminary stage varies from #j to #bf seconds1 with an average of about #aj seconds2 that of the principal portion from #j4g to #bf seconds1 also with an average of about #aj seconds2 and that of the end portion from #bh and #aje seconds1 with an average of about one minute4 ,the total apparent duration1 however1 depends on the instrument employed2 one of the earthquakes1 that of ,april #bcrd1 #ahih1 disturbing the seismograph at ,miyako for two minutes2 while1 at ,tokio1 a horizontal pendulum designed by ,professor ,omori oscillated for at least two hours4 ,the periods of both ripples and slow undulations1 again1 vary from one earthquake to another2 but it is worthy of notice that the average period of the undulations is almost constant in all three stages of the motion1 being #a4a1 #a4c1 and #a4c seconds1 respectively1 for the east-west component of the horizontal motion1 and #a4j second throughout for the north-south component4 ,for the ripples1 the average period is 4#jh second in the preliminary stage1 4#aj second in the principal portion1 and 4#jh second again in the end portion2 those of the principal portion being slightly larger in amplitude1 as well as longer in period1 than the ripples of the first and third stages4 ,,sound-,,phenomena4 ,besides the ripples already mentioned1 there are others of still smaller amplitude and shorter period that are sensible1 but as a rule only just sensible1 to us as sounds4 ,all the known evidence points to the extraordinary lowness of the earthquake-sound4 ,according to some observers1 it seems as if close to their lower limit of audibility2 while others1 however intently they may listen1 are unable to hear the slightest noise4 ,in other words1 the most rapid vibrations present in an earthquake do not recur at a rate of much more than about #cj to #ej per second2 or1 if they do1 they are not strong enough to impress the human ear4 ,to most observers1 the sound seems to increase and decrease in intensity with the shock1 and so gradually and smoothly does this change take place that the sound is frequently mistaken for that of an underground train approaching the observer's house1 passing beneath it1 and receding in the opposite direction4 ,some persons1 especially if situated within the meizoseismal area1 hear also loud crashes in the midst of the rumbling sound and simultaneously with the strongest vibrations4 ,at a moderate distance1 say from #cj to #dj miles1 the sound becomes more harsh and grating while the shock is felt2 and1 at a greater distance1 even this change disappears1 and nothing is heard but an almost monotonous sound like the low roll of distant thunder4 ,the explanation of this is that the sound-vibrations are of different periods and varying amplitude1 and the limiting vibrations tend to become inaudible with increasing distance1 the lower on account of their long period1 the higher owing to their small amplitude4 ,the magnitude of the sound-area depends1 even more than that of the disturbed area1 on the personal equation of the observers4 ,the lower limit of audibility varies not only in different individuals1 but also in different races4 ,in ,great ,britain1 it is doubtful whether an earthquake ever occurs unaccompanied by sound2 and in the meizoseismal area the noise is heard by nearly all observers4 ,with ,italians1 the average lower limit of audibility is higher than with the ,anglo-,saxon race2 slight shocks frequently occur without noticeable sound1 but with strong ones1 the larger number of observers is sure to include one or more capable of hearing the rumbling noise4 ,the ,japanese are1 however1 seldom affected by the most rapid earthquake-vibrations1 and the strongest shocks may be unattended by any recorded sound4 ,the result is manifest in the size of the sound-area in different countries4 ,in the ,hereford earthquake1 the sound-area contained #gj1jjj square miles2 in the ,neapolitan earthquake1 about #c1cjj square miles2 while1 in ,japanese earthquakes1 the sound is rarely heard more than a few miles from the epicentre4 ,another effect of this personal equation of the observers is that the sound-vibrations apparently outrace those of longer period4 ,the ,italians1 for instance1 generally hear the sound that precedes the shock1 and more rarely the weaker sound that follows it4 ,in ,japan1 only the earlier sound-vibrations1 if any1 seem to be audible4 ,in ,great ,britain1 on the contrary1 the fore-sound is perceptible to four1 and the after-sound to three1 out of every five observers2 and these proportions are maintained roughly to considerable distances from the epicentre4 ,it follows1 therefore1 that the sound-vibrations and those which constitute the shock must travel with nearly1 if not quite1 the same velocity2 and that the greater duration of the sound is due either to the prolongation of the initial movement or to the overlapping of the principal focus by the sound-focus4 ,neither alternative can be regarded as improbable1 but observations made on ,british earthquakes point to the latter explanation as the true one4 ,it will be sufficient to refer to two phenomena in support of this statement4 ,in the first place1 the percentage of observers who hear the fore-sound varies with the direction from the epicentre4 ,thus1 during the ,inverness earthquake of #aija1 the majority of observers in ,aberdeenshire regarded the sound as beginning and ending with the shock2 while1 in counties lying more nearly along the course of the great fault1 the sound was generally heard both before and after the shock 7p4 #bec74 ,in this case1 then1 the initial and concluding sound vibrations must have come chiefly from the margins of the seismic focus2 and those from the margin nearest to an observer would be more sensible than those from the farther margin4 ,again1 in slight earthquakes1 such as the ,cornwall earthquake of ,april #a1 #ahih1hc the curves of equal sound intensity1 while their axes are parallel to those of the isoseismal lines1 are displaced laterally with respect to these curves1 owing to the arrival of the strongest sound-vibrations from the upper margin of an inclined seismic focus4 ,when a fault-slip occurs1 the displacement is obviously greatest in the central region1 and dies out gradually towards the margins of the focus4 ,the phenomena described above show that the evanescent displacement within these margins generate sound-vibrations only2 and that the greater slip within the central region produces also the more important vibrations that compose the shock4 ,as the former are perceptible over a limited district1 while the latter may be felt through half a continent1 it is clear that the sound-area should bear no fixed relation in point of size to the disturbed area1 but should be comparatively greater for a slight shock than for a strong one4 ,,velocity ,,of ,,the ,,earth-,,waves4 ,if we consider only the earthquakes here described1 we see at once how great is the diversity in the estimated velocity of the earth-waves4 ,on the one hand1 we have a value as high as #e4b kms4 per sec4 for the ,charleston earthquake1 and1 at the other end of the scale1 a value of #j4i km4 per sec4 for the ,hereford earthquake4 ,between them1 and equally trustworthy1 lie the estimates of #c4j km4 per sec4 for the ,indian earthquake1 and #b4a kms4 per sec4 for the ,japanese earthquake and its immediate successors4 ,it is difficult to account entirely for such discordance4 ,errors of observation may be responsible for a small part of the differences4 ,the initial strength of the disturbance appears to have some effect1 and the nature of the rocks traversed must be a factor of consequence when the distances in question are not very great4 ,in the ,japanese and ,hereford earthquakes1 all three may have combined to produce the divergent results1 the distance in these cases being only #bge and #adb kms4 respectively4 ,in the ,indian and ,charleston earthquakes1 the distances are much greater 7#aidd and #adhg kms471 and the variety of rocks traversed must tend to give a truer average4 ,in the former1 the result obtained 7#c4j kms4 per sec47 agrees so closely with the velocity of the long-period undulations of distant earthquakes as to suggest that it was these waves that were timed at the stations west of ,calcutta and disturbed the magnetographs at ,bombay4#hd ,omitting1 then1 the ,indian estimate1 we find that1 for the ,japanese and ,charleston earthquakes1 the velocity increases with the distance as measured along the surface4 ,to a certain extent1 such a result might have been expected1 had we assumed the earthquake-waves to travel along the chords joining the focus to very distant places of observation4 ,the wave-paths that penetrate the earth are straight lines1 however1 only when the conditions that determine the velocity are uniform throughout1 and such uniformity we have no reason to expect4 ,from what we know of the earth's interior1 there can1 indeed1 be little doubt that the velocity of earthquake-waves increases with the depth below the surface1 and that the wave-paths in consequence are curved lines with their convexity downwards4 ,it would be out of place to state more than the principal result of the recent investigations by ,dr4 ,a4 ,schmidt#he and ,prof4 ,p4 ,rudzki#hf on this subject4 ,these are based on the assumptions that the velocity increases with the depth below the surface1 and that it is always the same at the same depth4 ,from the focus of the earthquake1 wave-paths diverge in all directions4 ,those which start horizontally curve upwards1 and intersect the surface of the earth in a circle dividing the whole surface into two areas of very unequal size4 ,within the small area1 the surface-velocity is infinite at the epicentre1 and decreases outwards until it is least on the boundary-circle4 ,in the larger region beyond1 the surface-velocity increases with the distance from the epicentre1 until1 at the antipodes of that point1 it is again infinite4 ,but1 as the depth of the focus is always slight compared with the radius of the earth1 the small circular area surrounding the epicentre is practically negligible1 and we may regard the surface-velocity of the waves that traverse the body of the earth as a quantity that continually increases with the distance from the epicentre4 ,how fully this interesting theoretical result has been confirmed is well shown in ,mr4 ,oldham's recent and very valuable investigation on the propagation of earthquake-motion to great distances4#hg ,a study of the records of the ,indian earthquake revealed the existence of three series of waves1 the first two consisting in all probability of longitudinal and transversal waves travelling through the body of the earth1 and the third of undulations spreading over its surface 7pp4 #bhb-bhe74 ,extending his inquiries to ten other earthquakes originating in six different centres1 ,mr4 ,oldham distinguishes the same three phases in their movements2 the third phase being the most constantly recorded1 the second less so1 while the first phase is the most frequently absent4 ,with the exception of a few very divergent records1 the initial times of these phases and the maximum epoch of the third phase are plotted on the accompanying diagram 7,fig4 #hj71 in which distances from the epicentre in degrees of arc are represented along the horizontal line and the time-interval in minutes along the perpendicular line4 ,the dots near the two lower curves refer to the records of the heavily weighted ,italian instruments1 and the crosses to those of the light horizontal pendulums1 which respond somewhat irregularly to the motion of the first two phases 7p4 #bhb74 ,in the third phase1 there is less divergence between the indications of the two classes of instruments1 and dots are used in each case for the initial1 and crosses for the maximum epoch4 ,illustration3 ,,fig4 #hj4--,time-curves of principal epochs of earthquake-waves of distant origin4 7,oldham47 ,of the smoothed curves drawn between these series of points1 those marked ,a1 ,b1 and ,c represent the time-curves of the beginnings of the first1 second1 and third phases respectively1 while ,d is the time-curve for the maximum of the third phase4 ,the concavity of the two lower lines towards the horizontal base-line shows that the surface-velocity of the corresponding waves increases rapidly with the distance1 far more so than would be possible with rectilinear motion4 ,the rates at which these waves travel through the earth therefore increase with the depth1 and the wave-paths must in consequence be curved lines convex towards the centre of the earth4 ,if the time-curves ,a and ,b were continued backwards to the origin1 their inclinations at that point to the horizontal line give the initial velocities of the corresponding waves1 which prove to be about #e and #c kms4 per sec4 respectively4 ,now1 according to recent experiments made by ,mr4 ,h4 ,nagaoka on the elastic constants of rocks1#hh the mean velocity of seven archaean rocks is #e4a kms4 per sec4 for the longitudinal waves1 and #b4h kms4 per sec4 for the transversal waves--values which agree so closely with those obtained for the first two series of earthquake-waves as to leave little doubt with regard to their character4 ,the other time-curves1 ,c and ,d1 corresponding to the initial and maximum epochs of the third phase1 are practically straight lines4 ,some of the records are slightly discordant for the average curve1 especially for the initial epoch2 but it is often difficult to define the commencement of this phase with precision4 ,at any rate1 the observations show no distinct sign of an increase in the surface-velocity of these waves with the distance from the origin4 ,it may therefore be concluded that they travel along the surface with velocities which are practically constant for each individual earthquake1 the largest waves at the rate of about #b4i kms4 per sec41 and the advance waves with a velocity of about #c4c kms4 per sec41 rising occasionally to over #d4j kms4 per sec4 ,,structural ,,changes ,,in ,,the ,,epicentral ,,area4 ,changes of elevation have long been known as accompaniments of great earthquakes1 though many of the earlier observations and measurements left much to be desired in accuracy and completeness4 ,the ,japanese earthquake of #ahia1 however1 placed the reality of such movements beyond doubt1 and revealed the existence of a fault-scarp1 with a height in one place of #ah or #bj feet1 and a length of at least #dj1 if not of #gj1 miles4 ,in the ,indian earthquake of #ahig1 the fault-scarps were shorter1 though more pronounced in character1 the largest known 7the ,chedrang fault7 being about #ab miles long1 and having a maximum throw at the surface of #ce feet4 ,in some other recent earthquakes1 also1 remarkable fault-scarps have been developed4 ,after the great shocks felt in ,eastern ,greece on ,april #bjth and #bgth1 #ahid1 a fissure was traced for a distance of about #cd miles1 running in an east-south-east and west-north-west direction through the epicentral district1 and varying in width from an inch or two to more than three yards4 ,that it was a fault1 and not an ordinary fissure1 was evident from its great length1 its uniform direction1 and its independence of geological structure4 ,the throw was generally small1 in no place exceeding five feet4#hi ,again1 in ,british ,baluchistan1 after the severe earthquake of ,december #bjth1 #ahib1 a fresh crack was observed in the ground running for several miles in a straight line parallel to the axis of the ,khojak range4 ,it coincided almost exactly with a line of springs1 and was clearly produced by a fresh slip along an old line of fault1 for before the earthquake it had the appearance of an old road1 and the natives assert that the ground has always cracked along this line with every severe shock4 ,in #ahib1 the change in relative height of the two sides of the fault was small1 in one place where it was measured being only two inches4#ij ,but other changes1 besides those in a vertical direction1 occasionally take place2 though1 owing to their recent discovery1 comparatively few examples are as yet known4 ,while the throw of the ,japanese fault varied greatly in amount1 and once even in direction1 there was also a constant shift towards the northwest of the ground on the north-east side of the fault1 the displacement at one spot being as much as #ac feet4 ,in the fault-scarp formed in #ahid in ,eastern ,greece1 a similar shift took place1 though to what extent is unknown4 ,there is1 moreover1 evidence of actual compression of the earth's crust at right angles to the fault-line4 ,the ,neo valley1 traversed by the ,japanese fault1 was apparently narrower after the earthquake than it was before1 and plots of ground were reduced from #dh to #cj feet in length--i4e41 by nearly #dj per cent4 ,in ,british ,baluchistan1 the formation of the fissure referred to above was accompanied both by compression perpendicular1 and by shifting parallel1 to the fault4 ,the actual displacement in each direction is unknown1 but the resultant was not less than #bg inches4 ,there can be no doubt that a fault-scarp is formed in the first place with great rapidity4 ,so abrupt1 indeed1 were the structural displacements in the epicentral area of the ,indian earthquake1 that they contributed very materially to the intensity of the shock1 giving rise to the excessive velocities observed at ,rambrai and elsewhere 7p4 #bgc74 ,the growth of the scarp does not1 however1 always cease with the first great earthquake1 though it may take place in a contrary sense1 as in the elevation connected with the ,conception earthquake of #ahce4 ,the principal shock1 according to ,darwin1 was followed during the few succeeding days by some hundred minor ones 7though of no inconsiderable violence71 which seemed to come from the same quarter from which the first had proceeded2 whilst1 on the other hand1 the level of the ground was certainly not raised by them2 but1 on the contrary1 after an interval of some weeks1 it stood rather lower than it did immediately after the great convulsion4#ia ,,after-,,shocks4 ,a series of after-shocks1 more or less long1 is a constant attendant on every great tectonic earthquake1 and few are the earthquakes of any degree of strength that can be regarded as completely isolated4 ,even in those which visit this country1 after-shocks are seldom absent4 ,for instance1 confining ourselves to the last few years1 the ,pembroke earthquake of #ahib was followed by #h shocks1 the ,inverness earthquake of #ahij by at least #aj1 and possibly by #ai shocks1 and that of the same district in #aija by #ae well-defined after-shocks in addition to many others recorded by one observer4 ,of #cjj ,italian earthquakes strong enough to cause some damage to buildings1 ,dr4 ,cancani finds that every one was either preceded or followed1 and chiefly followed1 by its own train of minor shocks4 ,for some hours1 and even for days1 after a great earthquake1 the shocks are so numerous that it is often impossible to keep count of them4 ,many local centres spring into activity in different parts of the epicentral area2 and1 though only the strongest shocks can be identified elsewhere1 it is clear that as a rule the shocks felt at any one station are quite distinct from those observed at another4 ,the enormous number of after-shocks that follow some earthquakes can only be realised when they are subjected to continuous seismographic registration2 and1 even then1 countless earth-sounds and the slightest tremors must escape detection4 ,the shocks may1 indeed1 succeed one another so rapidly that one begins before another ends1 and the result is an almost incessant tremulous motion rendered manifest by the quivering of water-surfaces or the swinging of chandeliers4 ,of the total number of after-shocks1 we may form some idea from recent records in ,japan4 ,after the ,mino-,owari earthquake of #ahia1 #c1cfe shocks were recorded within little more than two years at ,gifu1 and #a1bih at ,nagoya1 but neither of these figures includes the shocks felt within the first few hours4 ,of the ,kumamoto earthquake of ,july #bhth1 #ahhi1 the after-shocks recorded at ,kumamoto until the end of #ahic amount to #ibb2 and those of the ,kagoshima earthquake of ,september #gth1 #ahic1 recorded at ,chiran until the end of ,january #ahid1 to #dhj4 ,during the first #cj days1 the numbers recorded were #a1gdf at ,gifu1 #cdj at ,kumamoto1 and #bgh at ,chiran2 showing1 as ,professor ,omori remarks1 that the after-shocks diminish in frequency with the size of the disturbed areas1#ib--;i4e41 roughly with the initial intensity of the shocks4 ,next to absolute number1 the rapid decline in general frequency is the most marked characteristic of after-shocks4 ,professor ,omori has shown that1 excluding minor oscillations1 it follows the law represented geographically by the curves in ,fig4 #ea1 and algebraically by the equation y k _/ 7h x71 where y is the frequency at time x and h and k are constants for one and the same earthquake4 ,by means of this formula1 it is possible to estimate roughly the interval of time that must elapse before the seismic activity of the central district resumes its normal value4 ,for the ,mino-,owari earthquake1 this proves to be about forty years1 for the ,kumamoto earthquake about seven or eight years1 and for the ,kagoshima earthquake about three or four years4 ,in a recent memoir on ,italian after-shocks1#ic ,dr4 ,cancani has urged that other factors besides initial intensity determine the duration of a seismic period1 and prominently among these he places the depth of the seismic focus4 ,when the depth is very small1 the duration of the period is short1 not much more than ten days2 when the depth is moderate1 the duration may extend to three months2 and1 when great1 it may amount to several years4 ,the principal law that governs the distribution of after-shocks in time may be regarded as well-established4 ,it is otherwise with regard to their distribution in space4 ,this has been examined only in the cases of the ,japanese earthquake of #ahia and the ,inverness earthquake of #aija4 ,so far as we can judge from the evidence which they furnish1 after-shocks appear to be most numerous within and near the central portion of the seismic focus2 though the area of maximum activity is subject to continual oscillation4 ,in this region1 also1 there is evidence of a gradual decrease in the depths of the after-shock foci2 while1 near the extremities of the epicentral area1 there occur districts of slightly greater frequency than elsewhere4 ,with the lapse of time1 there seems therefore to be a constant extension1 both upwards and longitudinally1 of the area over which the principal fault-slip took place4 ,,origin ,,of ,,earthquakes4 ,in the introductory chapter1 a brief sketch is given of the different causes to which earthquakes are assigned4 ,with those due to rock-falls in subterranean channels1 we need have little to do4 ,the shocks are invariably slight1 and the part they play in the shaping of the earth's crust is insignificant4 ,volcanic earthquakes possess a higher degree of interest4 ,they represent1 no doubt1 incipient or unsuccessful attempts to produce an eruption4 ,they may be the forerunners of a great catastrophe4 ,of far higher importance in the history of our globe is the third class of earthquakes1 including all those connected with the manifold changes which the crust has undergone4 ,in the slow annealing process1 to which it has been subjected from the earliest times1 the crust has been crumpled and fractured1 elevated into the loftiest mountain ranges or depressed below the level of the sea4 ,every sudden yielding under stress is the cause of an earthquake4 ,it is chiefly1 perhaps almost entirely1 in the formation of faults that this yielding is manifested4 ,the initial fracturing may be the cause of one or many shocks1 but infinitely the larger number must be referred to the slow growth of the fault1 the intermittent slips1 now in one part1 now in another1 which1 after the lapse of ages1 culminate in a great displacement4 ,of the length of time occupied in the formation of a single fault1 we can make no estimate in years4 ,the anticlinal fault of ,charnwood ,forest dates from a pre-carboniferous period4 ,in #ahic it had not ceased to grow4#id ,still less can we conceive1 however faintly1 the number of elemental slips that constitute the history of a single fault4 ,we may think1 if we please1 of the #adc tremors and earth-sounds noted at ,comrie in ,perthshire during the last three months of #ahci1 of the #cjf earthquakes felt in the ,island of ,zante during the year #ahif1 or the #a1gdf shocks recorded at ,gifu during thirty days in #ahia2 but we shall be as far as ever from realising the vast number of steps involved in the growth of a fault1 let alone a mountain-chain4 ,yet1 all over the land-surface of the globe1 the crust is intersected by numberless faults1 and hardly any portion is there in which some or many of these faults are not growing4 ,one country1 indeed1 such as ,great ,britain1 may have reached a condition of comparative stagnancy2 the fault-slips are few and slight1 and earthquakes in consequence are rare and generally inconspicuous4 ,in another1 like ,eastern ,japan and the adjoining ocean-bed1 the movements are frequent1 occasionally almost incessant1 and few years pass without some great convulsion by which cities are wrecked and hundreds of human lives are lost4 ,at such times1 we magnify the rle of earthquakes1 and are in some danger of forgetting that1 in the formation of a mountain-chain or continent1 they serve no higher purpose than the creaking of a wheel in the complex movements of a great machine4 ,,footnotes3 #gi ,phil4 ,trans41 vol4 li41 pt4 ii41 #agfa1 pp4 #fbe-fbf4 #hj ,journ4 ,sci4 ,coll4 ,imp4 ,univ41 ,tokyo1 vol4 xi41 #ahii1 pp4 #aid-aie4 #ha ,journ4 ,coll4 ,sci4 ,imp4 ,univ41 ,tokyo1 vol4 vii41 pt4 v41 #ahid1 pp4 #a-d2 ,ital4 ,sismol4 ,soc4 ,boll41 vol4 ii41 #ahif1 pp4 #ahj-ahh4 #hb ,journ4 ,coll4 ,sci4 ,imp4 ,univ41 ,tokyo1 vol4 xi41 #ahii1 pp4 #afa-aie4 #hc ,quart4 ,journ4 ,geol4 ,soc41 vol4 lvi41 #aijj1 pp4 #a-g4 #hd ,there is no reason why the surface-undulations of the ,indian earthquake should not have produced a sensible shock even as far as ,italy4 ,taking their amplitude in that country at #ejh mm4 and their period at #bb sec4 7p4 #bhc71 the maximum acceleration would be about #dj mm4 per sec41 corresponding to the intensity #b of the ,rossi-,forel scale4 7,amer4 ,journ4 ,sci41 vol4 xxxv41 #ahhh1 p4 #dbi47 #he ,nature1 vol4 lii41 #ahie1 pp4 #fca-fcc4 #hf ,gerland's ,beitrge zur ,geophysik1 vol4 iii41 pp4 #dhe-eah4 #hg ,phil4 ,trans41 #aijj;,a1 pp4 #ace-agd4 #hh ,publ4 of ,earthq4 ,inves4 ,com4 in ,for4 ,langs4 7,tokyo71 ,no4 #d1 #aijj1 pp4 #dg-fg4 #hi ,s4,a4 ,papavasiliou1 ,paris1 ,acad4 ,sci41 ,compt4 ,rend41 vol4 cxix41 #ahid1 pp4 #aab-aad1 #chj-cha4 #ij ,geol4 ,mag41 vol4 x41 #ahic1 pp4 #cef-cfj4 #ia ,geol4 ,soc4 ,trans41 vol4 v41 #ahdj1 pp4 #fah-fai4 #ib ,the disturbed areas of these earthquakes contained1 respectively1 #bba1jjj1 #ci1jjj1 and #cj1jjj square miles4 #ic ,boll4 ,sismol4 ,soc4 ,ital41 vol4 viii41 #aijb1 pp4 #ag-dh4 #id ,roy4 ,soc4 ,proc41 vol4 lvii41 #ahie1 pp4 #hg-ie4 ,,index4 ,acceleration1 maximum1 of wave-motion in ,japanese earthquake1 #ahd1 #ahe2 in ,indian earthquake1 #bgb ,after-shocks1 definition1 #d2 frequency1 #aih1 #bef1 #bif1 #cdd2 distribution in space1 #bjj1 #bjc1 #bih1 #cbf1 #cde2 sound-phenomena1 #bjg1 #cjj2 connection with fault-scarps1 #cjj2 outlining of epicentre by1 #cbf2 origin of1 #beg2 of ,neapolitan earthquake1 #dj2 of ,ischian earthquakes1 #ef1 #fe2 of ,andalusian earthquake1 #ig2 of ,charleston earthquake1 #acc2 of ,riviera earthquake1 #afg2 of ,japanese earthquake1 #aih2 of ,hereford earthquake1 #bdj2 of ,inverness earthquake1 #bef2 of ,indian earthquake1 #bif2 of ,british earthquakes1 #cdc2 of ,italian earthquakes1 #cdc2 of ,japanese earthquakes1 #cdd ,agamennone1 ,g41 #ic1 #id1 #aja1 #cai ,alluvium1 displacement of1 by ,indian earthquake1 #bhg ,amplitude of wave-motion1 definition1 #d2 in ,neapolitan earthquake1 #cd2 in ,japanese earthquake1 #ahe2 in ,indian earthquake1 #bgj ,andalusian earthquake1 preparation for1 #ge2 investigation of1 #gf2 damage caused by1 #gg2 isoseismal lines and disturbed area1 #gh2 the unfelt earthquake1 #hb2 position of epicentre1 #hd2 depth of focus1 #he2 nature of shock1 #hg2 sound-phenomena1 #ia2 velocity of earth-waves1 #ib2 connection between geological structure and intensity of shock1 #ie2 fissures1 #if2 landslips1 #ig2 effect on underground water1 #ig2 after-shocks1 #ig2 origin of1 #ii2 bibliography1 #aja ,animals1 effects of earthquakes on1 #adc ,baluchistan earthquake of #ahib1 #bhh1 #cda ,baldacci1 ,l41 #gj1 #gc ,baratta1 ,m41 #cbj ,barrois1 ,c41 #gf ,bergeron1 ,c41 #gf ,bertelli1 ,t41 #age ,bertrand1 ,m41 #gf ,birds1 effects of earthquakes on1 #adc ,bordwar1 crust-fracture at1 #cji ,bron1 ,r41 #gf ,burton1 ,w4,k41 #bad ,cancani1 ,a41 #bha1 #bhb1 #cbj1 #cdc1 #cde ,castro1 ,m4,f4 de1 #gf1 #aja ,charleston earthquake1 investigation of1 #ajb2 damage caused by1 #ajc2 isoseismal lines and disturbed area1 #ajd2 preparation for1 #ajg2 nature of shock1 #ajh2 double epicentre1 #aaa2 origin of double shock1 #abj2 depth of foci1 #abb2 velocity of earth-waves1 #abf2 fissures1 #acj2 sand-craters1 #acj2 effects on human beings1 #aca2 feeling of nausea1 #acb2 after-shocks1 #acc2 origin of1 #acd2 bibliography1 #acg ,charlon1 ,e41 #age ,chedrang1 fault-scarp at1 #cjd ,clocks1 untrustworthiness of time-records of stopped1 #ci1 #id1 #aba1 #abg ,conder1 ,j41 #agg1 #bac ,coseismal lines1 #bbg1 #cbd ,covelli1 ,n41 #fg1 #fi ,damage caused by ,neapolitan earthquake1 #aj1 #bd2 by ,ischian earthquakes1 #ej1 #ef2 by ,andalusian earthquake1 #gg2 by ,charleston earthquake1 #ajc2 by ,riviera earthquake1 #aci2 by ,japanese earthquake1 #aha2 by ,hereford earthquake1 #bag2 by ,inverness earthquake1 #bdg ,darwin1 ,h41 #bha ,daubre1 ,a41 #gc ,davison1 ,c1 #bjb-bjf1 #bjh1 #baj1 #bac1 #bae-bfa1 #bie ,death-rate of ,neapolitan earthquake1 #bd2 of ,ischian earthquakes1 #ej1 #ef2 of ,andalusian earthquake1 #gg2 of ,charleston earthquake1 #ajd2 of ,riviera earthquake1 #adj2 of ,japanese earthquake1 #ahb ,denza1 ,f41 #aee1 #age ,depth of seismic focus1 methods of determining1 #be1 #hf1 #abb1 #cbf2 of ,neapolitan earthquake1 #bh2 of ,ischian earthquakes1 #ed1 #fa2 of ,andalusian earthquake1 #hf2 of ,charleston earthquake1 #abb1 #abe2 of ,riviera earthquake1 #aej2 of ,japanese earthquakes1 #cbh ,derby earthquake of #aijc1 #bcf ,direction of shock1 #bb1 #cc1 #ahf1 #bbe1 #cbe ,disturbed area1 definition of1 #c2 of ,neapolitan earthquake1 #aj2 of ,ischian earthquakes1 #ea1 #eh2 of ,andalusian earthquake1 #hj2 of ,charleston earthquake1 #ajg2 of ,riviera earthquake1 #ade2 of ,japanese earthquake1 #ahc2 of ,hereford earthquake1 #bai2 of ,inverness earthquake1 #bdi2 of ,indian earthquake1 #bfe2 connection between intensity of shock and1 #cbc ,dolomieu1 #aa ,du ,bois1 ,f41 #gc ,dutton1 ,c4,e41 #ajc-acg ,dutton's method of determining depth of seismic focus1 #abb1 #cbg ,earthquake-motion1 nature of1 #bhj1 #bhb1 #cbh1 #ccj1 #ccg2 propagation of1 to great distances1 #ccg ,earth-sound1 definition of1 #d ,edinburgh1 record of ,indian earthquake at1 #bha1 #bhc1 #bhe ,ellis1 ,w41 #hc ,emergence1 angle of1 #ac ,epicentre1 definition of1 #c2 methods of determining position of1 #ad1 #eb1 #fj1 #cbd2 of ,neapolitan earthquake1 #bb1 #bc2 of ,ischian earthquakes1 #ec1 #fj1 #fg2 of ,andalusian earthquake1 #hd2 of ,charleston earthquake1 #aaa2 of ,riviera earthquake1 #adf2 of ,hereford earthquake1 #bbd2 of ,inverness earthquake1 #bdh2 of ,indian earthquakes1 #bfd1 #bgf1 #cjb ,epomeo1 #de1 #fa1 #ga ,falb's method of determining depth of seismic focus1 #hf1 #cbg ,fallen pillars1 evidence of1 #ag1 #ai ,fault1 originating1 of ,hereford earthquake1 #bai2 of ,inverness earthquake1 #bdi ,fault-scarp of ,japanese earthquake1 #ahi2 general appearance1 #ahi2 length1 #aib2 throw1 #aic2 horizontal shift1 #aic2 course1 #aic2 swamp formed by it1 #aid ,fault-scarps of ,indian earthquakes1 #bgc1 #cjd2 ,chedrang fault1 #cjd2 ,samin fault1 #cjh2 of ,greek earthquake of #ahid1 #cdj1 #cda2 of ,baluchistan earthquake of #ahic1 #cda1 #cdb2 formation and growth of1 #cdb ,fault-slips1 tectonic earthquakes due to1 #e1 #dc1 #ajj1 #ace1 #agd1 #baa1 #bai1 #bbd1 #bda1 #bdi1 #bee1 #cag1 #cdf ,fishes1 destruction of1 by ,riviera earthquake1 #afb ,fissures1 caused by ,andalusian earthquake1 #if2 by ,charleston earthquake1 #acj2 by ,inverness earthquake1 #bdg2 by ,indian earthquake1 #bhe ,focus1 seismic1 definition of1 #c ,focus1 seismic1 depth of1 methods of determining1 #be1 #hf1 #abb1 #cbf2 of ,neapolitan earthquake1 #bh2 of ,ischian earthquakes1 #ed2 of ,andalusian earthquake1 #hf2 of ,charleston earthquake1 #abb1 #abe2 of ,riviera earthquake1 #aej2 of ,japanese earthquakes1 #cbh ,focus1 dimensions of seismic1 of ,hereford earthquake1 #bbd2 of ,inverness earthquake1 #bej ,fore-shocks1 #cba2 of ,neapolitan earthquake1 #dj2 of ,ischian earthquake1 #eg2 of ,andalusian earthquake1 #gf2 of ,charleston earthquake1 #ajg2 of ,riviera earthquake1 #adb2 of ,japanese earthquake1 #bja2 of ,hereford earthquake1 #bci2 of ,inverness earthquake1 #bdf ,fouqu1 ,f41 #gf1 #hd1 #aja ,fracture1 crust-1 at ,bordwar1 #cji ,fractures in buildings1 evidence of1 #ad1 #ae1 #bf ,fuchs1 ,c4,w4,c41 #ajb ,galli1 ,i41 #hb ,geological structure and intensity of shock1 connection between1 #ie1 #ajf1 #aac1 #aae1 #ace1 #afd1 #bfe ,gifu1 records of ,japanese after-shocks at1 #ahc1 #aig ,gray1 ,t41 #bie ,great ,glen fault and ,inverness earthquakes1 connection between1 #bde ,greek earthquake of #ahid1 fault-scarp of1 #cdj ,hayden1 ,e41 #ajc ,heath1 ,t41 #bhc1 #cbj ,hereford earthquake1 investigation of1 #bae2 preparation for1 #bae1 #bch2 isoseismal lines and disturbed area of1 #baf2 damage caused by1 #bag1 #bid2 position of originating fault1 #bai2 nature of shock1 #bbj2 origin of double series of vibrations1 #bbc2 position and dimensions of the two foci1 #bbd2 direction of the shock1 #bbe2 coseismal lines and velocity of earth-waves1 #bbg2 sound-phenomena1 #bbi2 isacoustic lines and sound-area1 #bcd2 fore-shocks1 #bch2 after-shocks1 #bdj2 origin of earthquake1 #bdj2 bibliography1 #bfa ,hills1 changes in aspect of1 after ,indian earthquake1 #caa ,hirata1 ,k41 #cbg1 #cca ,human beings1 effects of ,charleston earthquake on1 #aca ,hypocentre1 #c ,iberian peninsula1 earthquakes of1 #ge ,indian earthquake1 investigation of1 #bfb2 isoseismal lines and disturbed area1 #bfd2 nature of shock1 #bff2 visible earth-waves1 #bfh2 elements of wave-motion1 #bgj2 sound-phenomena1 #bgd2 velocity of earth-waves1 #bge2 the unfelt earthquake1 #bhj2 earth-fissures1 #bhe2 displacements of alluvium1 #bhg2 sand-vents1 #bhh2 rise of river-beds1 etc41 #bij2 landslips1 #bia2 rotation of pillars1 #bic2 after-shocks1 #bif2 structural changes in epicentral area1 #cja1 #cae2 structure of epicentral district1 #cjb2 fault-scarps1 #cjd2 crust-fractures1 #cji2 lakes and pools not due to faulting1 #caj2 changes in aspects of hills1 #caa2 revision of trigonometrical survey1 #cab2 origin of earthquake1 #cag2 bibliography1 #cai ,inverness earthquake1 preparation for1 #bdf2 damage caused by1 #bdg2 fissure in ground1 #bdg2 isoseismal lines and disturbed area1 #bdg2 position of originating fault1 #bdi2 nature of shock1 #bej2 sound-phenomena1 #bec2 origin of earthquake1 #bee2 after-shocks and their origin1 #bef2 sympathetic earthquakes1 #bei2 comparison with ,japanese earthquake1 #bfj2 bibliography1 #bfa4 ,investigation1 ,mallet's methods of1 #ab1 #ba ,isacoustic lines1 #bcd2 of ,hereford earthquake1 #bce2 of ,derby earthquake1 #bcf ,ischia1 volcanic history of1 #de1 #gj2 characteristics of eruptions1 #di2 seismic history1 #di ,ischian earthquake of #ahha1 investigation of1 #ej2 isoseismal lines and disturbed area1 #ea2 position of epicentre1 #eb2 depth of focus1 #ed2 nature of shock1 #ee2 after-shocks1 #ef2 origin of1 #gj2 bibliography1 #gc ,ischian earthquake of #ahhc1 investigation of1 #ef2 preparation for1 #eg2 isoseismal lines and disturbed area1 #eh2 position of epicentre1 #fj2 depth of focus1 #fa2 nature of shock1 #fd2 landslips1 #fd2 after-shocks1 #fe2 origin of1 #gj2 bibliography1 #gc ,ischian earthquakes1 characteristics of1 #ff2 origin of1 #gj ,isoseismal lines1 definition of1 #c2 their use in determining position of epicentre1 #bai1 #bdi1 #cbe2 of ,neapolitan earthquake1 #i2 of ,ischian earthquakes1 #ea1 #eh2 of ,andalusian earthquake1 #gh2 of ,charleston earthquake1 #ajd2 of ,riviera earthquake1 #adc2 of ,japanese earthquake1 #agh1 #ahb2 of ,hereford earthquake1 #baf2 of ,inverness earthquake1 #bdg2 of ,indian earthquake1 #bfd ,issel1 ,a41 #aci1 #afc1 #afd1 #aff1 #age ,japanese earthquake of #ahhg1 #ah ,japanese earthquake of #ahia1 investigation of1 #agg2 structure of meizoseismal area1 #agi2 damage caused by1 #aha2 isoseismal lines and disturbed area1 #ahb2 nature of shock1 #ahd2 the great fault-scarp1 #ahi2 minor shocks1 #aig2 distribution of after-shocks in time1 #aih2 preparation for1 #bja2 distribution of after-shocks in space1 #bjc2 sound-phenomena of after-shocks1 #bjg2 sympathetic earthquakes1 #bji2 origin1 of1 #baa2 bibliography1 #bac ,japanese earthquake of #ahid1 #ah1 #cbi ,johnston-,lavis1 ,h4,j41 #ej-gb1 #cbg ,kilian1 ,w41 #gf ,koto1 ,b41 #agg1 #ahj1 #aha1 #ahd1 #aij-aif1 #bji1 #bab1 #bac ,lakes formed by bending of river-bed during ,indian earthquake1 #caj ,lakes formed by fault-scarp of ,japanese earthquake1 #aid2 of ,indian earthquake1 #cje ,landslips caused by ,ischian earthquake1 #fd2 by ,andalusian earthquake1 #ig2 by ,indian earthquake1 #bia ,lvy1 ,m41 #gf ,lisbon earthquake of #agee1 #ge1 #hb ,mc,gee1 ,w4,j41 #acd ,macpherson1 ,j41 #aja ,magnetographs1 earthquakes recorded by1 #hb1 #aeg1 #afj1 #ahi1 #bgg1 #bhb ,mallet1 ,r41 #g-dd1 #he1 #ajb1 #abd1 #aej1 #bid-bif1 #cbe ,mallet's method of determining depth of focus1 #be1 #cbg ,masato1 ,h41 #agh1 #bac ,mascart1 ,e41 #aei1 #afj ,may ,hill anticlinal and ,hereford earthquake1 connection between1 #bdb ,meizoseismal area1 definition of1 #c2 of ,andalusian earthquake1 #ii2 of ,japanese earthquake1 #agi ,mercalli1 ,g41 #aa1 #eg1 #eh1 #fj1 #fa1 #fc1 #fg1 #gj-gc1 #gf1 #hj1 #hd1 #he1 #hh1 #ij1 #aja1 #ach-age1 #cbe1 #cbg ,michell1 ,j41 #cbe ,milne1 ,j41 #ce1 #agg1 #aha1 #ahb1 #ahf1 #ahi1 #bjj1 #bac1 #bha1 #bhc ,minor shocks of ,neapolitan earthquake1 #dj2 of ,japanese earthquake1 #aig ,mountain ranges1 effect of1 on intensity of shock1 #ie1 #ajf ,moureaux1 ,t41 #afa ,nagaoka1 ,h41 #agg1 #bad1 #cci ,nagoya1 records of ,japanese after-shocks at1 #ahc1 #aig ,nature of shock1 ,neapolitan earthquake1 #cj2 ,ischian earthquakes1 #ee1 #fd2 ,andalusian earthquake1 #hg2 ,charleston earthquake1 #ajh2 ,riviera earthquake1 #aej2 ,japanese earthquake1 #ahd2 ,hereford earthquake1 #bbj2 ,inverness earthquake1 #bej2 ,indian earthquake1 #bff ,nausea1 feeling of1 caused by ,charleston earthquake1 #acb ,neapolitan earthquake1 investigation of1 #g1 #ab2 isoseismal lines and disturbed area1 #i2 damage caused by1 #aj2 position of epicentre1 #ad2 depth of focus1 #be2 nature of shock1 #cj2 sound-phenomena1 #cg2 velocity of earth-waves1 #ci2 minor shocks1 #dj2 origin1 #da2 bibliography1 #dd ,ness1 ,loch1 connection between ,inverness earthquakes and formation of1 #bee1 #beg1 #bfa ,nogus1 ,a4,f41 #aja ,oddone1 ,e41 #age ,offret1 ,a41 #gf1 #aeh1 #aei1 #age ,oglialoro1 ,a41 #gc ,oldham1 ,r4,d41 #bfb-cbj1 #ccg1 #cdj ,omori1 ,f41 #ai1 #bj1 #agg1 #ahc-ahf1 #ahh1 #aig-aii1 #bjg1 #bad1 #bfb1 #cbe1 #cbg1 #cbi1 #cca ,origin of earthquakes1 #b1 #e1 #cde2 of ,neapolitan earthquake1 #da2 of ,ischian earthquakes1 #gj2 of ,andalusian earthquake1 #aja2 of ,charleston earthquake1 #acd2 of ,riviera earthquakes1 #agd2 of ,japanese earthquake1 #baa2 of ,hereford earthquake1 #bdj2 of ,inverness earthquake1 #bee2 of ,indian earthquake1 #cag ,overturned bodies1 maximum acceleration deduced from1 #ahd1 #bgb ,palmieri1 ,l41 #eg1 #gb1 #gc ,periodicity of ,japanese after-shocks1 #aii ,perrey1 ,a41 #g ,potenza1 evidence of damaged church at1 #ae1 #bf ,prediction of earthquakes1 possible1 #cbb ,preparation for earthquakes1 #dj1 #eg1 #gf1 #ajg1 #adb1 #bja1 #bch1 #bdf1 #cba ,rails1 flexure of1 by ,charleston earthquake1 #aab2 by ,japanese earthquake1 #ahb2 by ,indian earthquake1 #bhh ,railway-tunnels1 observations of ,riviera earthquake in1 #aff ,rebeur-,paschwitz1 ,e4 von1 #bha ,river-beds1 rise of1 caused by ,indian earthquake1 #bij ,riviera earthquake1 investigation1 #ach2 damage caused by1 #aci2 preparation for1 #adb2 isoseismal lines and disturbed area1 #adc2 position of epicentre1 #adf2 depth of principal focus1 #adi2 nature of shock1 #aej2 sound-phenomena1 #aef2 the unfelt earthquake1 #aeg2 effects of earthquake at sea1 #afb2 destruction of fishes1 #afb2 seismic sea-waves1 #afc2 connection between geological structure and intensity of shock1 #afd2 observations in railway-tunnels1 #aff2 after-shocks1 #afg2 recent movements in the ,riviera1 #agj2 seismic history of the ,riviera1 #aga2 origin of1 #aga2 bibliography1 #age ,rocca di ,papa1 record of ,indian earthquake at1 #bha1 #bhb1 #bhe ,rossi1 ,m4,s4 de1 #eg1 #gd1 #hb1 #aja1 #age ,rossi-,forel scale of seismic intensity1 #ajd1 #baf1 #bdg ,rotation of pillars1 caused by ,hereford earthquake1 #bid2 by ,indian earthquake1 #bic2 explanation of1 #bie ,rudzki1 ,p41 #ccf ,rumi1 ,prof41 #afi ,samin1 fault-scarp at1 #cjh ,sand-craters caused by ,charleston earthquake1 #acj2 by ,indian earthquake1 #bhh ,schmidt1 ,a41 #ccf ,seismic sea-waves of ,riviera earthquake1 #adb1 #afc ,seismic vertical1 #ab1 #bi1 #fb ,seismographic records of ,riviera earthquake1 #aed2 of ,japanese earthquake of #ahid1 #cbi ,sekiya1 ,s41 #ah1 #ai ,serpieri1 ,a41 #gd ,shillong1 nature of ,indian earthquake at1 #bff ,sloan1 ,e41 #ajc1 #aag-aai1 #acd1 #ace ,sound-area1 definition of1 #c2 of ,neapolitan earthquake1 #ch2 of ,andalusian earthquake1 #ib2 of ,hereford earthquake1 #bcd2 of ,inverness earthquake1 #beb2 of ,indian earthquake1 #bge ,sound-phenomena1 nature of sound1 #ch1 #bbi1 #beb1 #ccb2 inaudibility to some observers1 #bca1 #bgd2 its cause1 #bcc2 isacoustic lines1 #bcd-bcf2 variations in nature of sound throughout sound-area1 #bcg2 time-relation of sound and shock1 #bch1 #bec2 origin of earthquake-sounds1 #ccd2 sound-phenomena of ,neapolitan earthquake1 #cg2 of ,andalusian earthquake1 #ia2 of ,charleston earthquake1 #acc2 of ,riviera earthquake1 #aef2 of ,japanese after-shocks1 #bjg2 of ,hereford earthquake1 #bbi2 of ,inverness earthquake1 #beb2 of ,indian earthquake1 #bgd ,structural changes1 distribution of1 in ,indian earthquake1 #cae ,subsultory shock1 #e ,sympathetic earthquakes of ,japanese earthquake1 #bji2 of ,inverness earthquake1 #bei ,tanakadate1 ,a41 #agg1 #bad ,taramelli1 ,t41 #gf1 #hd1 #he1 #hh1 #ij1 #aja1 #ach1 #aej1 #afe1 #age1 #cbe ,tectonic earthquakes1 #e ,thrust-plane1 ,indian earthquake due to movement along1 #cah ,time-curve of ,indian earthquake1 #bgh2 of principal epochs of earthquake-waves of distant origin1 #cch ,time-records1 general inaccuracy of1 #cbd ,time-relations of sound and shock in ,hereford earthquake1 #bch2 in ,inverness earthquake1 #bec ,trigonometrical survey1 revised1 of ,khasi hills after ,indian earthquake1 #cab2 interpretation of results1 #cad ,twin earthquakes1 origin of1 #cb1 #hi1 #abj1 #aec1 #agd1 #bbc2 ,neapolitan earthquake1 #ca2 ,andalusian earthquake1 #hg2 ,charleston earthquake1 #ajh2 ,riviera earthquake1 #adi1 #aej2 ,hereford earthquake1 #bba ,undulatory shock1 #e ,unfelt earth-waves1 ,andalusian earthquake1 #hb2 ,riviera earthquake1 #aeg2 ,indian earthquake1 #bhj ,uzielli1 ,g41 #adc1 #agf ,velocity1 maximum1 of wave-motion1 in ,neapolitan earthquake1 #ce2 in ,indian earthquake1 #bgb ,velocity of earth-waves1 methods of determining1 #ci1 #ic1 #abg1 #bbi2 variation with depth1 #ccf2 form of wave-paths1 #ccf2 velocity of different phases1 #cci2 of ,neapolitan earthquake1 #ci2 of ,andalusian earthquake1 #ib2 of ,charleston earthquake1 #abf2 of ,japanese earthquakes1 #ahh2 of ,hereford earthquake1 #bbi2 of ,indian earthquake1 #bge1 #bgi1 #bhd ,visible earth-waves in ,charleston earthquake1 #aaj2 in ,japanese earthquake1 #ahf2 in ,indian earthquake1 #bfh ,volcanic earthquakes1 #e1 #gj ,vorticose shock1 #e ,water1 effect of ,andalusian earthquake on underground1 #ig ,waterfalls caused by fault-scarps of ,indian earthquake1 #cje ,wave-path1 #ac ,west1 ,c4,d41 #bgb ,woolhope anticlinal and ,hereford earthquake1 connection between1 #bda 99 99 99 99 99 ---------------------------------------- ------------------- ,typographical errors corrected in text3 ,page #ed3 ,casamenello replaced with ,casamenella ,page #aag3 ',captain ,dutton' replaced with ',major ,dutton' ,page #aai3 ',capt4 ,dutton' replaced with ',major ,dutton' ,page #cae3 ,rangsonobo replaced with ,rangsanobo ,page #ccf3 'per sec4 per sec4' replaced with 'per sec4' ,page #ccg3 negligeable replaced with negligible ---------------------------------------- ------------------- 99 99 99 99 99