,illustration3 ,many buildings in ,charleston1 ,south ,carolina1 were damaged or destroyed by the large earthquake that occurred ,august #ca1 #ahhf4 ,u4,s4 ,department of the ,interior _/ ,u4,s4 ,geological ,survey ,earthquakes by ,kaye ,m4 ,shedlock and ,louis ,c4 ,pakiser ,for sale by the ,u4,s4 ,government ,printing ,office ,superintendent of ,documents ,mail ,stop3 ,,ssop1 ,washington1 ,,dc #bjdjb-icbh ,one of the most frightening and destructive phenomena of nature is a severe earthquake and its terrible aftereffects4 ,an earthquake is a sudden movement of the ,earth1 caused by the abrupt release of strain that has accumulated over a long time4 ,for hundreds of millions of years1 the forces of plate tectonics have shaped the ,earth as the huge plates that form the ,earth's surface slowly move over1 under1 and past each other4 ,sometimes the movement is gradual4 ,at other times1 the plates are locked together1 unable to release the accumulating energy4 ,when the accumulated energy grows strong enough1 the plates break free4 ,if the earthquake occurs in a populated area1 it may cause many deaths and injuries and extensive property damage4 ,today we are challenging the assumption that earthquakes must present an uncontrollable and unpredictable hazard to life and property4 ,scientists have begun to estimate the locations and likelihoods of future damaging earthquakes4 ,sites of greatest hazard are being identified1 and definite progress is being made in designing structures that will withstand the effects of earthquakes4 ,illustration3 ,,usgs scientist uses portable seismic recording equipment near ,mount ,st4 ,helens1 ,washington4 ,earthquakes in ,history ,the scientific study of earthquakes is comparatively new4 ,until the #ahth century1 few factual descriptions of earthquakes were recorded1 and the natural cause of earthquakes was little understood4 ,those who did look for natural causes often reached conclusions that seem fanciful today2 one popular theory was that earthquakes were caused by air rushing out of caverns deep in the ,earth's interior4 ,the earliest earthquake for which we have descriptive information occurred in ,china in #aagg ,b4,c4 ,the ,chinese earthquake catalog describes several dozen large earthquakes in ,china during the next few thousand years4 ,earthquakes in ,europe are mentioned as early as #ehj ,b4,c41 but the earliest for which we have some descriptive information occurred in the mid-#afth century4 ,the earliest known earthquakes in the ,americas were in ,mexico in the late #adth century and in ,peru in #adga1 but descriptions of the effects were not well documented4 ,by the #agth century1 descriptions of the effects of earthquakes were being published around the world--although these accounts were often exaggerated or distorted4 ,the most widely felt earthquakes in the recorded history of ,north ,america were a series that occurred in #ahaa-ab near ,new ,madrid1 ,mo4 ,a great earthquake1 whose magnitude is estimated to be about #h1 occurred on the morning of ,december #af1 #ahaa4 ,another great earthquake occurred on ,january #bc1 #ahab1 and a third1 the strongest yet1 on ,february #g1 #ahab4 ,aftershocks were nearly continuous between these great earthquakes and continued for months afterwards4 ,these earthquakes were felt by people as far away as ,boston and ,denver4 ,because the most intense effects were in a sparsely populated region1 the destruction of human life and property was slight4 ,if just one of these enormous earthquakes occurred in the same area today1 millions of people and buildings and other structures worth billions of dollars would be affected4 ,illustration3 ,the great #aijf ,san ,francisco earthquake and fire destroyed most of the city and left #bej1jjj people homeless4 ,the ,san ,francisco earthquake of #aijf was one of the most destructive in the recorded history of ,north ,america--the earthquake and the fire that followed killed nearly #gjj people and left the city in ruins4 ,the ,alaska earthquake of ,march #bg1 #aifd1 was of greater magnitude than the ,san ,francisco earthquake2 it released perhaps twice as much energy and was felt over an area of almost #ejj1jjj square miles4 ,the ground motion near the epicenter was so violent that the tops of some trees were snapped off4 ,one hundred and fourteen people 7some as far away as ,california7 died as a result of this earthquake1 but loss of life and property would have been far greater had ,alaska been more densely populated4 ,where ,earthquakes ,occur ,the ,earth is formed of several layers that have very different physical and chemical properties4 ,the outer layer1 which averages about #gj kilometers in thickness1 consists of about a dozen large1 irregularly shaped plates that slide over1 under1 and past each other on top of the partly molten inner layer4 ,most earthquakes occur at the boundaries where the plates meet4 ,in fact1 the locations of earthquakes and the kinds of ruptures they produce help scientists define the plate boundaries4 ,illustration3 ,diagram of plate boundaries and earthquake locations4 ,there are three types of plate boundaries3 spreading zones1 transform faults1 and subduction zones4 ,at spreading zones1 molten rock rises1 pushing two plates apart and adding new material at their edges4 ,most spreading zones are found in oceans2 for example1 the ,north ,american and ,eurasian plates are spreading apart along the mid-,atlantic ridge4 ,spreading zones usually have earthquakes at shallow depths 7within #cj kilometers of the surface74 ,transform faults are found where plates slide past one another4 ,an example of a transform-fault plate boundary is the ,san ,andreas fault1 along the coast of ,california and northwestern ,mexico4 ,earthquakes at transform faults tend to occur at shallow depths and form fairly straight linear patterns4 ,illustration3 ,map explanation3 yellow lines1 plate ,boundary4 ,subduction zones are found where one plate overrides1 or subducts1 another1 pushing it downward into the mantle where it melts4 ,an example of a subduction-zone plate boundary is found along the northwest coast of the ,united ,states1 western ,canada1 and southern ,alaska and the ,aleutian ,islands4 ,subduction zones are characterized by deep-ocean trenches1 shallow to deep earthquakes1 and mountain ranges containing active volcanoes4 ,earthquakes can also occur within plates1 although plate-boundary earthquakes are much more common4 ,less than #aj percent of all earthquakes occur within plate interiors4 ,as plates continue to move and plate boundaries change over geologic time1 weakened boundary regions become part of the interiors of the plates4 ,these zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust4 ,the ,new ,madrid earthquakes of #ahaa-ab and the #ahhf ,charleston earthquake occurred within the ,north ,american plate4 ,illustration3 ,damaged house4 ,how ,earthquakes ,happen ,an earthquake is the vibration1 sometimes violent1 of the ,earth's surface that follows a release of energy in the ,earth's crust4 ,this energy can be generated by a sudden dislocation of segments of the crust1 by a volcanic eruption1 or even by manmade explosions4 ,most destructive quakes1 however1 are caused by dislocations of the crust4 ,the crust may first bend and then1 when the stress exceeds the strength of the rocks1 break and 8snap0 to a new position4 ,in the process of breaking1 vibrations called 8seismic waves0 are generated4 ,these waves travel outward from the source of the earthquake along the surface and through the ,earth at varying speeds depending on the material through which they move4 ,some of the vibrations are of high enough frequency to be audible1 while others are of very low frequency4 ,these vibrations cause the entire planet to quiver or ring like a bell or a tuning fork4 ,a fault is a fracture in the ,earth's crust along which two blocks of the crust have slipped with respect to each other4 ,faults are divided into three main groups1 depending on how they move4 ,normal faults occur in response to pulling or tension2 the overlying block moves down the dip of the fault plane4 ,thrust 7reverse7 faults occur in response to squeezing or compression2 the overlying block moves up the dip of the fault plane4 ,strike-slip 7lateral7 faults occur in response to either type of stress2 the blocks move horizontally past one another4 ,most faulting along spreading zones is normal1 along subduction zones is thrust1 and along transform faults is strike-slip4 ,illustration3 ,normal ,fault4 ,blocks are pulled apart ,illustration3 ,thrust ,fault4 ,blocks are pushed together ,illustration3 ,strike-,slip ,fault4 ,blocks slide past each other ,geologists have found that earthquakes tend to reoccur along faults1 which reflect zones of weakness in the ,earth's crust4 ,even if a fault zone has recently experienced an earthquake1 however1 there is no guarantee that all the stress has been relieved4 ,another earthquake could still occur4 ,in ,new ,madrid1 a great earthquake was followed by a large aftershock within #f hours on ,december #af1 #ahaa4 ,furthermore1 relieving stress along one part of the fault may increase stress in another part2 the ,new ,madrid earthquakes in ,january and ,february #ahab may have resulted from this phenomenon4 ,illustration3 ,diagram of ,earth's layers and seismic wave propagation4 ,the focal depth of an earthquake is the depth from the ,earth's surface to the region where an earthquake's energy originates 7the focus74 ,earthquakes with focal depths from the surface to about #gj kilometers 7#dc4e miles7 are classified as shallow4 ,earthquakes with focal depths from #gj to #cjj kilometers 7#dc4e to #ahf miles7 are classified as intermediate4 ,the focus of deep earthquakes may reach depths of more than #gjj kilometers 7#dce miles74 ,the focuses of most earthquakes are concentrated in the crust and upper mantle4 ,the depth to the center of the ,earth's core is about #f1cgj kilometers 7#c1ifj miles71 so even the deepest earthquakes originate in relatively shallow parts of the ,earth's interior4 ,the epicenter of an earthquake is the point on the ,earth's surface directly above the focus4 ,the location of an earthquake is commonly described by the geographic position of its epicenter and by its focal depth4 ,earthquakes beneath the ocean floor sometimes generate immense sea waves or tsunamis 7,japan's dread 8huge wave074 ,these waves travel across the ocean at speeds as great as #ifj kilometers per hour 7#eig miles per hour7 and may be #ae meters 7#di feet7 high or higher by the time they reach the shore4 ,during the #aifd ,alaska earthquake1 tsunamis engulfing coastal areas caused most of the destruction at ,kodiak1 ,cordova1 and ,seward and caused severe damage along the west coast of ,north ,america1 particularly at ,crescent ,city1 ,calif4 ,some waves raced across the ocean to the coasts of ,japan4 ,illustration3 ,tsunami destruction on ,kamehameha ,avenue on ,hilo's waterfront1 #aidf4 7,photograph provided by the ,u4,s4 ,army ,corps of ,engineers47 ,liquefaction1 which happens when loosely packed1 water-logged sediments lose their strength in response to strong shaking1 causes major damage during earthquakes4 ,during the #aihi ,loma ,prieta earthquake1 liquefaction of the soils and debris used to fill in a lagoon caused major subsidence1 fracturing1 and horizontal sliding of the ground surface in the ,marina district in ,san ,francisco4 ,illustration3 ,liquefaction of sands and debris caused major damage throughout the ,marina district in ,san ,francisco during the ,loma ,prieta earthquake4 ,landslides triggered by earthquakes often cause more destruction than the earthquakes themselves4 ,during the #aifd ,alaska quake1 shock-induced landslides devastated the ,turnagain ,heights residential development and many downtown areas in ,anchorage4 ,an observer gave a vivid report of the breakup of the unstable earth materials in the ,turnagain ,heights region3 ,i got out of my car1 ran northward toward my driveway1 and then saw that the bluff had broken back approximately #cjj feet southward from its original edge4 ,additional slumping of the bluff caused me to return to my car and back southward approximately #ahj feet to the corner of ,mc,collie and ,turnagain ,parkway4 ,the bluff slowly broke until the corner of ,turnagain ,parkway and ,mc,collie had slumped northward4 ,illustration3 ,many homes were damaged by landslides triggered by the #aifd ,alaska earthquake 7above7 and the #aihi ,loma ,prieta shock 7below74 ,illustration3 ,home damaged by landslide triggered by the #aihi ,loma ,prieta shock4 ,illustration3 ,scientist examining seismographic equipment4 ,measuring ,earthquakes ,the vibrations produced by earthquakes are detected1 recorded1 and measured by instruments called seismographs4 ,the zig-zag line made by a seismograph1 called a 8seismogram10 reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument4 ,from the data expressed in seismograms1 scientists can determine the time1 the epicenter1 the focal depth1 and the type of faulting of an earthquake and can estimate how much energy was released4 ,the two general types of vibrations produced by earthquakes are surface waves1 which travel along the ,earth's surface1 and body waves1 which travel through the ,earth4 ,surface waves usually have the strongest vibrations and probably cause most of the damage done by earthquakes4 ,body waves are of two types1 compressional and shear4 ,both types pass through the ,earth's interior from the focus of an earthquake to distant points on the surface1 but only compressional waves travel through the ,earth's molten core4 ,because compressional waves travel at great speeds and ordinarily reach the surface first1 they are often called 8primary waves0 or simply 8,p0 waves4 ,p waves push tiny particles of ,earth material directly ahead of them or displace the particles directly behind their line of travel4 ,shear waves do not travel as rapidly through the ,earth's crust and mantle as do compressional waves1 and because they ordinarily reach the surface later they are called 8secondary0 or 8,s0 waves4 ,instead of affecting material directly behind or ahead of their line of travel1 shear waves displace material at right angles to their path and are therefore sometimes called 8transverse0 waves4 ,illustration3 ,diagram of propagation of seismic waves4 ,the first indication of an earthquake is often a sharp thud1 signaling the arrival of compressional waves4 ,this is followed by the shear waves and then the 8ground roll0 caused by the surface waves4 ,a geologist who was at ,valdez1 ,alaska1 during the #aifd earthquake described this sequence3 ,the first tremors were hard enough to stop a moving person1 and shock waves were immediately noticeable on the surface of the ground4 ,these shock waves continued with a rather long frequency1 which gave the observer an impression of a rolling feeling rather than abrupt hard jolts4 ,after about #a minute the amplitude or strength of the shock waves increased in intensity and failures in buildings as well as the frozen ground surface began to occur4444 ,after about #c minutes the severe shock waves ended and people began to react as could be expected4 ,illustration3 ,large earthquakes cause more damage east of the ,rocky ,mountains2 this map shows areas that suffered major architectural damage 7striped areas7 and minor damage 7dotted areas7 during the magnitude-#h earthquakes in ,new ,madrid and ,san ,francisco and the smaller but still damaging quakes in ,charleston and ,san ,fernando4 ,the severity of an earthquake can be expressed in several ways4 ,the magnitude of an earthquake1 usually expressed by the ,richter ,scale1 is a measure of the amplitude of the seismic waves4 ,the moment magnitude of an earthquake is a measure of the amount of energy released--an amount that can be estimated from seismograph recordings4 ,the intensity1 as expressed by the ,modified ,mercalli ,scale1 is a subjective measure that describes how strong a shock was felt at a particular location4 ,the ,richter ,scale1 named after ,dr4 ,charles ,f4 ,richter of the ,california ,institute of ,technology1 is the best known scale for measuring the magnitude of earthquakes4 ,the scale is logarithmic so that a recording of #g1 for example1 indicates a disturbance with ground motion #aj times as large as a recording of #f4 ,a quake of magnitude #b is the smallest quake normally felt by people4 ,earthquakes with a ,richter value of #f or more are commonly considered major2 great earthquakes have magnitudes of #h or more on the ,richter scale4 ,the ,modified ,mercalli ,scale expresses the intensity of an earthquake's effects in a given locality in values ranging from ,i to ,,xii4 ,the most commonly used adaptation covers the range of intensity from the condition of 8,i--,not felt except by a very few under especially favorable conditions10 to 8,,xii--,damage total4 ,lines of sight and level are distorted4 ,objects thrown upward into the air40 ,evaluation of earthquake intensity can be made only after eyewitness reports and results of field investigations are studied and interpreted4 ,the maximum intensity experienced in the ,alaska earthquake of #aifd was ,x2 damage from the ,san ,francisco and ,new ,madrid earthquakes reached a maximum intensity of ,,xi4 ,illustration3 ,the ,january #ag1 #aiid1 earthquake at ,northridge1 ,california1 caused this collapse of a major highway interchange4 7,photograph by ,james ,w4 ,dewey1 ,,usgs47 ,earthquakes of large magnitude do not necessarily cause the most intense surface effects4 ,the effect in a given region depends to a large degree on local surface and subsurface geologic conditions4 ,an area underlain by unstable ground 7sand1 clay1 or other unconsolidated materials71 for example1 is likely to experience much more noticeable effects than an area equally distant from an earthquake's epicenter but underlain by firm ground such as granite4 ,in general1 earthquakes east of the ,rocky ,mountains affect a much larger area than earthquakes west of the ,rockies4 ,an earthquake's destructiveness depends on many factors4 ,in addition to magnitude and the local geologic conditions1 these factors include the focal depth1 the distance from the epicenter1 and the design of buildings and other structures4 ,the extent of damage also depends on the density of population and construction in the area shaken by the quake4 ,the ,loma ,prieta earthquake of #aihi demonstrated a wide range of effects4 ,the ,santa ,cruz mountains suffered little damage from the seismic waves1 even though they were close to the epicenter4 ,the central core of the city of ,santa ,cruz1 about #bd kilometers 7#ae miles7 away from the epicenter1 was almost completely destroyed4 ,more than #hj kilometers 7#ej miles7 away1 the cities of ,san ,francisco and ,oakland suffered selective but severe damage1 including the loss of more than #dj lives4 ,the greatest destruction occurred in areas where roads and elevated structures were built on unstable ground underlain by loose1 unconsolidated soils4 ,the ,northridge1 ,california1 earthquake of #aiid also produced a wide variety of effects1 even over distances of just a few hundred meters4 ,some buildings collapsed1 while adjacent buildings of similar age and construction remained standing4 ,similarly1 some highway spans collapsed1 while others nearby did not4 ,illustration3 ,a sudden increase in earthquake tremors signaled the beginning of a series of eruptions at ,redoubt ,volcano in #aihi-ij4 ,volcanoes and ,earthquakes ,earthquakes are associated with volcanic eruptions4 ,abrupt increases in earthquake activity heralded eruptions at ,mount ,st4 ,helens1 ,washington2 ,mount ,spurr and ,redoubt ,volcano1 ,alaska2 and ,kilauea and ,mauna ,loa1 ,hawaii4 ,the location and movement of swarms of tremors indicate the movement of magma through the volcano4 ,continuous records of seismic and tiltmeter 7a device that measures ground tilting7 data are maintained at ,u4,s4 ,geological ,survey volcano observatories in ,hawaii1 ,alaska1 ,california1 and the ,cascades1 where study of these records enables specialists to make short-range predictions of volcanic eruptions4 ,these warnings have been especially effective in ,alaska1 where the imminent eruption of a volcano requires the rerouting of international air traffic to enable airplanes to avoid volcanic clouds4 ,since #aihb1 at least seven jumbo jets1 carrying more than #a1ejj passengers1 have lost power in the air after flying into clouds of volcanic ash4 ,though all flights were able to restart their engines eventually and no lives were lost1 the aircraft suffered damages of tens of millions of dollars4 ,as a result of these close calls1 an international team of volcanologists1 meteorologists1 dispatchers1 pilots1 and controllers have begun to work together to alert each other to imminent volcanic eruptions and to detect and track volcanic ash clouds4 ,predicting ,earthquakes ,the goal of earthquake prediction is to give warning of potentially damaging earthquakes early enough to allow appropriate response to the disaster1 enabling people to minimize loss of life and property4 ,the ,u4,s4 ,geological ,survey conducts and supports research on the likelihood of future earthquakes4 ,this research includes field1 laboratory1 and theoretical investigations of earthquake mechanisms and fault zones4 ,a primary goal of earthquake research is to increase the reliability of earthquake probability estimates4 ,ultimately1 scientists would like to be able to specify a high probability for a specific earthquake on a particular fault within a particular year4 ,scientists estimate earthquake probabilities in two ways3 by studying the history of large earthquakes in a specific area and the rate at which strain accumulates in the rock4 ,scientists study the past frequency of large earthquakes in order to determine the future likelihood of similar large shocks4 ,for example1 if a region has experienced four magnitude #g or larger earthquakes during #bjj years of recorded history1 and if these shocks occurred randomly in time1 then scientists would assign a #ej percent probability 7that is1 just as likely to happen as not to happen7 to the occurrence of another magnitude #g or larger quake in the region during the next #ej years4 ,but in many places1 the assumption of random occurrence with time may not be true1 because when the strain is released along one part of the fault system1 it may actually increase on another part4 ,four magnitude #f4h or larger earthquakes and many magnitude #f-f4e shocks occurred in the ,san ,francisco ,bay region during the #ge years between #ahcf and #aiaa4 ,for the next #fh years 7until #aigi71 no earthquakes of magnitude #f or larger occurred in the region4 ,beginning with a magnitude #f4j shock in #aigi1 the earthquake activity in the region increased dramatically2 between #aigi and #aihi1 there were four magnitude #f or greater earthquakes1 including the magnitude #g4a ,loma ,prieta earthquake4 ,this clustering of earthquakes leads scientists to estimate that the probability of a magnitude #f4h or larger earthquake occurring during the next #cj years in the ,san ,francisco ,bay region is about #fg percent 7twice as likely as not74 ,another way to estimate the likelihood of future earthquakes is to study how fast strain accumulates4 ,when plate movements build the strain in rocks to a critical level1 like pulling a rubber band too tight1 the rocks will suddenly break and slip to a new position4 ,scientists measure how much strain accumulates along a fault segment each year1 how much time has passed since the last earthquake along the segment1 and how much strain was released in the last earthquake4 ,this information is then used to calculate the time required for the accumulating strain to build to the level that results in an earthquake4 ,this simple model is complicated by the fact that such detailed information about faults is rare4 ,in the ,united ,states1 only the ,san ,andreas fault system has adequate records for using this prediction method4 ,illustration3 ,using a two-color laser to detect movement along a fault near ,parkfield1 ,california4 ,both of these methods1 and a wide array of monitoring techniques1 are being tested along part of the ,san ,andreas fault4 ,for the past #aej years1 earthquakes of about magnitude #f have occurred an average of every #bb years on the ,san ,andreas fault near ,parkfield1 ,california4 ,the last shock was in #aiff4 ,because of the consistency and similarity of these earthquakes1 scientists have started an experiment to 8capture0 the next ,parkfield earthquake4 ,a dense web of monitoring instruments was deployed in the region during the late #aihjs4 ,the main goals of the ongoing ,parkfield ,earthquake ,prediction ,experiment are to record the geophysical signals before and after the expected earthquake2 to issue a short-term prediction2 and to develop effective methods of communication between earthquake scientists and community officials responsible for disaster response and mitigation4 ,this project has already made important contributions to both earth science and public policy4 ,illustration3 ,san ,andreas fault in the ,carrizo ,plain1 central ,california4 ,scientific understanding of earthquakes is of vital importance to the ,nation4 ,as the population increases1 expanding urban development and construction works encroach upon areas susceptible to earthquakes4 ,with a greater understanding of the causes and effects of earthquakes1 we may be able to reduce damage and loss of life from this destructive phenomenon4 ,illustration3 ,a statue of ,christ at ,cemetery ,hill overlooks the ruined town of ,yungay1 ,peru4 ,it and a few palm trees were all that remained standing after the ,may #ca1 #aigj1 earthquake4 ,u4,s4 ,geological ,survey ,information ,services ,p4,o4 ,box #bebhf ,denver1 ,,co #hjbbe ,u4,s4 ,,government ,,printing ,,office3 #aiif--dba-bje ,illustration3 ,,us ,,department ,,of ,,the ,,interior2 ,,march #c1 #ahdi ,as the ,nation's principal conservation agency1 the ,department of the ,interior has responsibility for most of our nationally owned public lands and natural and cultural resources4 ,this includes fostering sound use of our land and water resources2 protecting our fish1 wildlife1 and biological diversity2 preserving the environmental and cultural values of our national parks and historical places2 and providing for the enjoyment of life through outdoor recreation4 ,the ,department assesses our energy and mineral resources and works to ensure that their development is in the best interests of all our people by encouraging stewardship and citizen participation in their care4 ,the ,department also has a major responsibility for ,american ,indian reservation communities and for people who live in island territories under ,u4,s4 administration4 ,illustration3 7,cover photographs1 clockwise from top left73 ,mexico ,city1 ,mexico1 #aihe2 ,coalinga1 ,calif41 #aihc2 ,northridge1 ,calif41 #aiid2 ,anchorage1 ,alaska1 #aifd2 ,san ,francisco1 ,calif41 #aijf2 ,loma ,prieta1 ,calif41 #aihi4 ,transcriber's ,notes --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text versions only1 text in italics is delimited by underscores4 --,in the ,,ascii version only1 subscripted numbers are preceded by underscore and delimited by brackets4