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AUTHOR Pakiser, Louis C.TITLE Earthquakes.INSTITUTION Geological Survey (Dept. of Interior), Reston, Va.PUB DATE 83NOTE 24p.; From the series "Popular Publications of the

U.S. Geological Survey." Photographs may notreproduce well.

PUB TYPE Reference Materials - General (130)

EDRS PRICE MF01/PC01 Plus Postage.DESCRIPTORS' *Earthquakes; Geographic Regions; *Geophysics;

Instructional Materials; *Natural Disasters; PhysicalEnvironment; Scientific Research; SecondaryEducation; *Seismology

IDENTIFIERS PF Project; Tsunami Warnings

ABSTRACTOne of a series of general interest publications on

science topics, the booklet provides those interested in earthquakeswith an introduction to the subject. Following a section presentingan historical look at the world's major earthquakes, the bookletdismisses earthquake-prone geographic areas, the nature and workingsof earthquakes, earthquake generated seawaves (tsunamis) andlandslides, and the measurement' and prediction of earthquakes. Thetext is accompanied by numerous photographs illustrating thedestructive effects of earthquakes, maps showing major earthquakezones, and diagrams clarifying internal processes involved inearthquake formation. (LU)

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Reproductions supplied by EDRS are the best that can be madefrom the original document.

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by Louis C. Pakiser

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One of the most frightening anddestructive phenomena of naturethat man experiences is a severeearthquake and its terrible after-effects. Since long before manexisted, probably for more than 4billion years, the forces within theEarth that produce earthquakeshave played an active role inshaping the Earth's surface. Theseunpredictable catastrophic move-ments of the Earth have caused agreat number of human casualtiesand extensive property damage.

Today man is challenging theassumption that earthquakes mustpresent an uncontrollable andunpredictable hazard to life andproperty. Scientists have begun toexplore the means of predictingearthquakes. Sites of greatesthazard are being identified, armdefinite progress is being madein designing structures thatwill withstand the effects ofearthquakes.

uSGS scientist uses Portable seismicrecording equipment near MOunt StHelens. Washington.

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Earthquakes in History

The scientific study of earth-quakes is comparatively new.Until the 18th century few factualdescriptions of earthquakes wererecorded, and the natural cause ofearthquakes was little understood.Many people believed that anearthquake was a massive punish-ment and a warning to the un-repentant. A 16th century scholar,for example, suggested thatstatues of Mercury and Saturn beplaced on each wall of buildingsto protect against earthquakes.Those who did look for naturalcauses often reached fanciful con-clusions; one popular theory wasthat earthquakes were caused byair rushing out of caverns deepin the Earth's interior.

An early earthquake for whichwe have detailed descriptive infor-mation occurred on November 1,1755, in the vicinity of Lisbon,Portugal. Shocks from the quakewere felt in many parts of the world.In some parts of the United Stateschandeliers rattied, and in Europebuildings trembled.

After the quake, Portuguesepriests were asked to documenttheir observations. Their records,still preserved, represent the firstsystematic attempt to investigatean earthquake and its effects.Since then, detailed records havebeen kept of almost every majorearthquake.

The most widely felt earth-quakes in the recorded history ofNorth America were a series thatoccurred in 1811-12 near New

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"The sea rose boiling In the harbour and broke up all the craft harboured there; the cityburst into flames, and ashes covered the streets and squares; the houses came crashingdown, roots piling up on foundations. and even the foundations were smashed to pieces,Thirty thousand inhabitants of both sexes and all ages were crushed to death underthe ruins."

Madrid, Mo. The shocks startedDecember 16, 1811, and continuedintermittently for 2 days. Largeshocks followed on January 23,1812, and again on February 7. Thelargest of these quakes was feltover an area of 2 million squaremilesfrom Canada to the Gulf ofMexico and from the Rocky Moun-tains to the Atlantic Ocean.Because the most intense affectswere in a sparsely populated

region, the destruction of humanlife and property was slight. If thisearthquake occurred in the samearea today, it probably would causesevere damage to many cities inthe central Mississippi Valley,

The San Francisco earthquakeof 1906 was one of the most de-structive in the recorded history ofNorth Americathe earthquakeand the tire that followed killednearly 700 people and left the city

in reins. The Alaska earthquake ofMarch 27, 1964, however, was ofgreater magnitude than the SanFrancisco earthquake, it releasedperhaps twice as much energy andwas felt over an area of almost500.000 square miles. The groundmotion near the epicenter was soviolent that the tops of some treeswere snapped off. One hundredand fourteen people (some as faraway as California) died as a resultof this earthquake. but loss of lifeand property would have been fargreater had Alaska been moredensely populated.

Locations of Earthquakes'

Most earthquakes occur in areasbordering the Pacific Ocean. ThiscarcumPacific belt, called the "ringof fire," includes the Pacific coastsof North and South America, theAleutians, Japan, Southeast Asia,and Australasia. Half a millionpeople within his area have losttheir lives because of earthquakes,and property ,,alued in billions ofdollars has been severely damagedor destroyed.

Earthquakes occa most frequentlythose zones shOion in yellow below.

Earthquake movement Juring the SanFrancisco earthquake caused the tracksto buckle.

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The United States has experi-enced less destruction than othercountries located in this ring offire, but millions of Americans livein potential quake areas. Largeparts of the Western United Statesare known to be particularly vul-nerable. Nuclear reactors, greatdams, schools, and high-rise apart-ments and other housing develop-ments are being planned and builtin places where the danger ofmajor earthquakes is e ter present.This has created an urgent need for

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more information on the nature,causes, and effects of earthquakes.

Scientists, including those of theU.S. Geological Survey's NationalCenter for Earthquake Research inMenlo Park, Calif., are studying thecauses of earthquakes in hopes offinding methods of prediction andof developing practices that willreduce the destructive effects ofearthquakes.

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Nature of Earthquakes

An earthquake is the oscillatory,sometimes violent movement of theEarth's surface that follows arelease of energy in the Earth'scrust. This energy can be gener-ated by a sudden dislocation ofsegments of the crust, by a vol-canic eruption, or even by man-made explosions. Most of thedestructive quakes, however, arecaused by dislocations of thecrust. When subjected to deep-seated forces (whose origins andnatures are largely unknown) thecrust may first bend and then,when the stress exceeds thestrength of the rocks, break and"snap" to a nevosition. In theprocess of breaking, vibrationscalled "seismic waves" are gen-erated. These waves travel fromthe source of the earthquake tomore distant places along thesurface and through the Earth atvarying speeds depending on themedium through which they Move.Some of the vibrations are of highenough frequency to be audible,

while others are of very low fre-quencyactually many secondsor minutes between swings. Thesevibrations cause the entire planetto quiver or ring like a bell or atuning fork.

A fault is a fracture in the Earth'scrust along which two blocks ofthe crust have slipped with respectto each other. One crustal blockmay move horizontally in one direc-tion while the block facing it movesin the opposite direction, or oneblock may move upward whim theother moves downward. Faults aredistinguished by the kinds ofmovements that characterize them.Movement along California's fa-mous San Andreas Fault is pre-dominantly horizontal, and the faultis called a strike-slip fault. A faultin which the movement is verticalis called a dip-slip fault. AlongMany faults, movement is bothhorizontal and vertical.

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Movement along stnkeslip fault (left).Movement along 1:10slip fault (right).

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Geologists have found thatearthquakes tend to reoccur alongfaults, which reflect zones ofweakness in the Earth's crust. Thefact that A fault zone has recentlyexperienced an earthquake offersno assurance that enough stresshas been relieved to preventanother quake.

The local depth of an earthquakeis the depth from the Earth's sur-face to the region (locus) where anearthquake's energy originates.Earthquakes with focal depthsfrom the surface to about SOkilometers (38 miles) are classifiedas shallow. Earthquakes with focaldepths from 60 to 300 kilometers(38 to 188 miles) are classified asintermediate. The focus of deepearthquakes may reach depths of700 kilometers (440 moles).

The focuses of most earth-quakes are concentrated in thecrust and upper mantle. Comparedto a depth of about 4,000 miles tothe center of the Earth's core,earthquakes can be considered tooriginate in relatively shallow partsof the Earth's interior. Earthquakes

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in California along the San Andreasand associated faults have shallowfocal depths; for most the depthis less than 10 miles. During thepast 100 years, Earth movementshave occurred along more thanhalf the entire length of the SanAndreas Fault and the ruptureitself is visible at the land surfacein many places.

Very shallow earthquakes areprobably caused by fracturing ofthe brittle rock in the crust or byinternal stresses that overcome thefrictional resistance locking oppo-site sides of a fault. The immediatecause of intermediate and deepearthquake is not yet fully under-stood.

The epicenter of an earthquakeis the point on the Earth's surfacedirectly above the focus. The loca-tion of an earthquake is commonlydescribed by the geographic posi-tion of its epicenter and by itsfocal depth.

Earthquakes beneath the oceanfloor sometimes generate immensesea waves or "tsunamis" (Japan'sdread "huge wave"). These wavestravel across the ocean at speedsas great as 960 kilometers per hour(600 miles per hour) and may be 15meters (50 feet) high or higher bythe time they reach the shore.During the 1964 Alaska earthquake,tsunamis engulfing coastal areascaused most of the destruction at

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Kodiak, Cordova, and Sewardand caused severe damage alongthe west coast of North America,particularly at Crescent City, Calif.Some waves raced across theocean to the coasts of Japan.

Water levels in artesian wellsfluctuate as seismic waves travelthrough the rock layers that holJthe water. During passage ofseismic waves from a large earth-quake, water levels in some wellsfluctuate wildly, not only in theimmediate vicinity of the earth-quake but also at great distancesfrom it. The water level changemay be long lasting or even per-manent. The Alaska quake appearstc have caused changes in wells inmany areas, both local and remote.Water levels in wells in NewOrleans, La., for example, rose andfell as a result of the quake.

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Landslides triggered by earth-quakes often cause more destruc-tion than the earthquake shocksthemselves. During the 1964 Alaskaquake, shockinduced landslidesdevastated the Turnagasn Heightsresidential development and manydowntown areas .n Anchorage. Anobserver ga,,e a vivid report of triebreakup of the unstable Earthmaterials in the Turnagain Heightsregion:

I got out of my car, rannorthward toward my drive-way, and then saw that thebluff had broken back ap-proximately 300 feet south-ward from ifs original edge.Additional slumping of thebluff caused me to return tomy car and back southwardapproximately 180 feet tothe corner of McCollie andTurnagain Parkway, The

- bluff slowly broke until thecorner of T.urnagain Park-way and McCollie hadslumped northward.

Background: Fracture and pressure ridgesdeveloped in,.ice on a lake in northernKenai Lowlands.

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Measurement ofEarthquakes

The vibrations 1:i:educed byevthquakes are detected, re-corded, and measured by instru-ments called seismographs. Thezig-zag line made by a seismo-graph, called a "seismogram."reflects the varying amplitude ofthe vibrations by responding to themotion of the ground surface be-neath the instrument. From thedata expressed in seismograms,the time, the epicenter, and thefocal depth of an earthquake canbe determined, and estimates canbe made of the amount of energythat was released.

The two general types of vibra-tions produced by earthquakes aresurface waves which travel atom:,the Earth's surface and body waveswhich travel through the Earth.Surface waves usually have thestrongest vibrations and probablycause most of the damage done byearthquakes.

Body waves are of two types,compressional and shear. Bothtypes of body waves pass throughthe Earth's interior from the focusof an earthquake to distant pointson the surface, but only compres-sional waves travel through theEarth's molten core. Becausecompressional waves travel atgreat speeds and ordinarily reachthe surface first, they are oftencalled "primary waves" or simply"P" waves. P waves push tinyparticles of Earth material directlyahead of them or displace theparticles directly behind their lineof travel.

Shear waves do not travel asrapidly through the Earth's crustand mantle as do compressionalwaves, and because they ordinar-ily reach the surface later they arecalled "secondary" or "5" waves.Ins'ead of affecting materialdirectly behind or ahead of theirline of travel, shear waves displace

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material at right ang'es to 'heirpath and are therefore sometimescalled "transverse" waves.

The first indication of an earth-quake will often be a sharp thudsignaling the arrival of compres-sional waves. This will be followedby the shear waves and then tne"ground roll" caused by the sur-face waves. A geologist who was atValdez, Alaska, during the 1964earthquake described this phe-nomenon:

The first tremors were hardenough to stop a movingperson and shock waveswere immediately notice-able on The surlace of theground. These shock wavescontinued with a rather longfrequency which gave theobse;ver an impression of arolling feeling rather thanabrupt hard jolts. After about1 minute the amplitude orstrength of the shock wavesincreased in intensity andfailures in buildings as wellas the frozen ground sur-face began to occur . . .

After about 3 1 '2 minutesthe severe shock wavesended and people began toreact as could be expected.

The times of arrival of compres-sional and shear waves at selectedseismograph stations throughoutthe world indicate where and whenthe earthquake occurred and,sometimes, its focal depth. The re.corded amplitudes of seismicwaves indicate the amount of14

energy released by the quake.The severity of an earthquake

can be expressed in several ways.The magnitude of an earthquake,as expressed by the Richter Scale,is a measure of the amplitude ofthe seismic waves and is related tothe amount of energy releasedanamount that can be estimated fromseismograph recordings. The inten-sity, as expressed by the ModifiedMercalli Scale, is a subjectivemeasure which describes howsevere a shock was felt at a par-ticular location. Damage or loss oflife and property is another, andultimately the most important,measure of an earthquake'sseverity.

The Richter Scale, named afterDr. Chartes F. Richter of the Cali-fornia Institute of Technology, isthe best known scale for measuringthe magnitude of earthquakes. Thescale is logarithmic so that a re-cording of 7, for example, indicatesa disturbance with ground motion10 times as large as a recording of6. A quake of magnitude 2 is thesmallest quake normally felt byhumans. Earthquakes with a Rich-ter value of 6 or more are common-ly considered major inmagnitude.

The Modified Mercalli Scale ex-presses the intensity of an earth-quake's effects in a given localityin values ranging from I to XII. Themost commonly used adaptationcovers the range of intensity fromthe condition of "INot felt exceptby a very few under especially

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favorable conditions," to "XIIDamage total. Lines of sight andlevel are distorted. Objects thrownupward into the air." Evaluation ofearthquake intensity can be madeonly after eyewitness reports andresults of field investigations arestudied and interpreted. The maxi-mum intensity experienced in theAlaska earthquake of 1964 was X;the San Francisco earthquake of1906 reached a maximum intensityof Xl.

Earthquakes of large magnitudedo not necessarily cause the mostintense surface effects. The effectin a given region depends to alarge degree on local surface andsubsurface geologic conditions. Anarea underlain by unstable ground(sand, ,ay, or other unconsoli-dated materials), for example, islikely to experience much morenoticeable effects than an areaequally distant from an earthquake'sepicenter but underlain by firmground such as granite.

An earthquake's destructivenessdepends on many factors. In

addition to magnitude, these in-clude the focal depth, the distancefrom the epicenter, local geologicconditions, and the design ofbuildings and other works of man.The extent of damage alsc dependson the density of population andconstruction in the area shakenby the quake.

The Alaska earthquake of 1964demonstrated wide variations in itseffects. The town of Whittier, builton firm granite, suffered littledamage from the seismic wavesdespite the fact that it leas close tothe earthquake's epicenter.Whittier did experience greatdestruction and the loss of 13 livesfrom the enormous seawavesoroduced by submarine landslides,however. In Anchorage, muchfarther from the earthquake'sepicenter, damage was selective;the greatest destruction occurredin areas where homes and otherbuildings were constructed nearunstable slopes underlain byclay and other unconsolidatedmaterials.

Government Hill Elementary Scnool in Ancnorage. Alaska. destroyed in the 1964 earthquake.

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Prediction of Earthquakes

The prediction of earthquakes isone of the objectives of currentstudies by the U.S. GeologicalSurvey and other scientific groups.At present the ability to predictthe time, place, and size ofearthquakes is very limited, butconsiderable progress has beenmade in formulating the statisticalprobability that an earthquake ofgiven magnitude will occur in aregion within a specific span oftime.

In 1966 the National Center forEarthquake Research at theGeological Survey's Western Re-gion in Menlo Park, Calif., began aprogram to study the causes ofearthquakes, their influence onman and his property, and tech-niques for predicting the occur-rence of earthquakes. State andFederal agencies, as well as aca-demic and industrial organizations,are cooperating in this researcheffort. In all, the National Centerfor Earthquake Research has thelargest group of earth scientistsstudying earthquakes in the UnitedStates, and it has extensivefacilities for geophysical, geo-chemical, and geological research.

The greatest emphasis in theNational Center's investigations ofeiathquake-prone areas will befault zones in California andNevada. The faults will be mappedand identified by type and move-ments. Earthquake shocks alongthe faults will be continuouslyrecorded. Fault zones in otherWestern States and in Hawaii and

Alaska will also be studied. Scien-tists are hopeful that from thisresearch, patterns will emerge thatwill assist in earthquake prediction.

The physical and cttemicalnature of rocks, their behaviorunder stress, and their failureduring earthquakes are also beingstudied in the field and in labora-tories of the National Center forEarthquake Research. Thesestudies will aid engineers indesigning and building structuresin earthquake-prone areas.

For the present, the best protec-tion against earthquakes is toavoid construction in high-riskareas and to employ earthquakeresistant construction techniques.For example, the port of Valdez,Alaska, destroyed by the 1964earthquake, was found to havebeen built on incipient landslideblocks. These landslide blocks didnot provide stable budding sites.Because little property of valueremained standing after the earth-quake and because the town wasnot heavily populated, the peopleof Valdez chose to rebuild theirtown at a new siteone recom-mended by the results of geologicstudy. In the San Francisco area,on the other hand, the emphasiswill have to be on improved con-struction methods and zoning ofdamage-prone undeveloped landfor nonresidential use.

Earthquakes on the island ofHawaii, the site of active volcanoesKilauea and Mauna Loa, appearto be associated with volcanic

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activity. Abrupt increase in quakeactivity at times herald an eruption,and the location cf swarms oftremors can indicate where lavamay break out. Also, prior to aneruption, the volcano swells meas-urably in response to the upwardmovement of molten rock. Continu-ous seismic and liltmeter (a device

The photograph on the right shows howMauna Loa Volcano, 30 miles away.appears from a window at the HawaiianVolcano Observatory The observatory isshown below.

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that measures ground tilting)records are maintained at theGeological Survey's HawaiianVolcano Observatory, located onthe rim of <ilauea Volcano, wherestudy of these records enablesspecialist! to make short-rangepredictions of volcanic eruptions.

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Recent earth movementsalong fault have dis-placed stream channelrom A to B.

Repeated movements over thousandsof years have displaced channelfrom A to C.

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Scientific understanding ofearthquakes is of vital importanceto the Nation. As the populationincreases, expanding urban devel-opment and construction worksencroach upon areas susceptibleto earthquakes. With a greaterunderstanding of the cause andeffects of earthquakes, man may beable to reduce damage and lossof life from this destructivephen-Jmenon.

This publication is one of a series ofgenet& interest publications prepared bythe U.S. Geological Survey to provideinformation about the earth sciences.natural resources. and the environment. Toobtain a catalog of addiiimal titles in theseries "Popular Publications of the U.S.Geological Survey." write:

Branch of Distribution-U.S. Geological Survey604 South Pickett StreetAlexandria, VA 22304

OfBranch of DistributionU.S. Geological SurveyBox 25286. Federal CanterDenver, CO 80225

A statue 01 Christ at Cemetery Hilloverlooks the ruined town of YungaY.Peru It and a few palm trees were all thatremained standing after the May 31. 1970earthquake.

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As the Nation's principal conservation agency, the Departmentof the Interior has responsibility for most of our nationally ownedpublic lands and natural resources, Thia Includes lostering the 2

wisest use of our land and water resources, protecting our fish and: .

wildlife, preserving the environmental and cultural values of our.national parks and historical places, and providing for the enjoy-ment of life through outdoor recreation. The Department assesses :4$our energy and mineral resources and works to assure that theirdevelopment is in the best interests of all our people. The n

Department also has a major responsibility for American Indianreservation communities and for people who live in IslandTerritories under U.S. administration

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