the nature of seismicity: forms and causes 3.1.5.4 …...these earthquakes, internal to plates, can...

Thenatureofseismicity:formsandcauses3.1.5.4Hazards

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WhatyouneedtoknowThenatureofseismicityanditsrelationtoplatetectonics.Formsofseismichazardincludingearthquakes,shockwaves,tsunamis,liquefaction,landslides.Spatialdistribution,randomness,magnitude,frequency,regularity,predictabilityofhazardevents.

IntroductionThecrustoftheEarthismadeupofsevenmajorplatesandseveralminorones.Theyareallonthemove.Attheplatemargins,whereplatesaretravellingindifferentdirections,stresscanbuildup.Whenthepressure-resultingfromabuild-upoffriction- isreleased,aseriesoftremorsorearthquakescanbefelt.

SpatialdistributionofearthquakesThelocationofearthquakesiscloselyassociatedwithplatemargins.

• Atdestructiveplatemargins,earthquakestendtooccuratdepth.TheyareassociatedwiththesubductionofonecrustunderanotherinanarrowareaknownastheBenioffzone,wherecompressionalforcesaregreatest.Earthquakesoccurringherecanbeverypowerfulandmaytakeplaceundertheseaclosetoheavilypopulatedcoastalzones;thismakesthethreatofdangeroustsunamimuchmorelikely.

• Atconstructiveplatemargins,earthquakestendtobemuchshallowerandlesspowerful.Theyareassociatedwithtensionalforcesinthecrustandoccuralongmid-oceanridgesawayfromlandorlargepopulations.

• Atconservativeplatemargins,earthquakestendtobeshallowfocused.Herethecontinentalplatesaredraggingpasteachotherandcompressionalforcesarehigh.Earthquakesoccurringherecanbeverypowerfulanddamagecanbesevereiftheyoccurindenselypopulatedareas.

MagnitudeofearthquakesMostoftheearthquakesthatoccureachyeararetoosmalltobefelt.Withincreasingnumbersofseismometersavailabletomeasureearthquakesandpeoplelivinginmoreremotelocations,wearereportingearthquakesmoretodaythanwehaveinthepast.EarthquakemagnitudecanbemeasuredontheRichterscale,alogarithmicscale1-10,where7onthescaleistentimesmorepowerfulthana6and100timesmorepowerfulthana5.Around120earthquakesofmagnitude6andaboveoccureachyear,ofthese20measure7oraboveontheRichterscale(amajorearthquake).Greatearthquakes(measuring8orabove)tendtooccuronceortwiceadecadeandforthepeoplelivingattheepicentreitcancompletelydestroytheircommunity.AlthoughtheRichterscaleisusefulformeasuringsmall-scaleearthquakes,itsaccuracydecreasesforlargerearthquakes.TheMomentMagnitudescaleisameasureofthetotaldistanceafaulthasmovedandtheforceneededtogenerateit,knownasthemomentreleaseoftheearthquake,andisnowusedworldwidebyseismologists.TheModifiedMercalliIntensity(MMI)scaleismoreameasureoftheeffectonpeopleandhumaninfrastructure.The12-pointscaleclassifiestheimpactoftheearthquakeandindicatesseverityonhumansratherthanobjectivemeasurementofearthquakeforce.AmoderateearthquakeunderamajorcitycanregisterhigherontheMMIthanastrongearthquakeinalargelyunpopulatedregion.

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Thenatureofseismicity:formsandcauses3.1.5.4Hazards

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While85percentofearthquakesoccurnearplateboundaries,someoccurinintraplatelocations.Theseearthquakes,internaltoplates,canhighlightundiscoveredfaultlinesandareasofstressincrustalrocks,buttheycanalsorelatetoothercausessuchasisostaticrecoil,whentheweightofglaciersisliftedastheymeltandthecrustlifts.Iceland’ssurfacehasliftedby60msincethepeakofthelasticeage;withupliftcomesstressesandstrainsincrustalrockandconsequentlyearthquakes.Earthquakescanalsobearesultofhumanactivity.Thesequasi-naturalearthquakesaremostcommonlyassociatedwiththecrustaldeformationcausedbytheweightofwaterstoredinreservoirs.Someresearchershaveattributedthe7.9-magnitudeSichuanearthquakeinMay2008,whichkilledanestimated80,000people,totheconstructionoftheZipingpuDaminChinaandtheresultingpressuresimposedontherocksunderlyingitsreservoironceatfullcapacity.Primaryseismichazards

• Earthquakesarethereleaseofenergyintheformofseismicwavesfromthepointknownasthefocuswherethepressureisreleased.Manyearthquakesresultfrommovementsalongfracturesinrockscalledfaults.OneofthemoststudiedfaultsisthetransformplatemarginofCalifornia,USA.Yearsofresearchhavefoundthatthisisnotasinglefaultline,butacomplexzoneoffaults.Oncepressureisbuiltupinarockandreleased,seismicorshockwavesradiateoutinalldirections.ThepointonthesurfaceoftheEarthdirectlyabovethefocusisknownastheepicentreandcanbethelocationofgreatestdamage.Therearefourtypesofseismicwaves.PwavesandSwavesarebodywavesthattravelthroughrock;Pwavesarethefastestandcanalsotravelthroughliquids.Whenbodywavesarriveattheepicentretheycausepeopleandpropertytoriseandfall.Thiscanseverwater,gasandotherinfrastructure.Escapinggasfrequentlyignitesandisdifficulttoextinguish,asrupturedwatermainsareunabletosupplywatertotheemergencyservices.Surfacewaves(RayleighandLoveWaves)traveloutfromtheepicentreandtheyrepresentthemostseverehazardtopeopleandproperty.Therockingmotionassociatedwiththesewavescanshakeandtopplebuildingslikedominoes.Bedrocktypecanalsoaffecttheextentofdamagetobuildings.Softclaysandunconsolidatedbedrocktendtowobblelikejellyandamplifyshaking.Solidbedrocktendstolimitshaking.AtKobe,Japan,theportexperiencedseveredamageasaresultoftheamplificationofgroundwavesthroughthesoftsandsandmudsitwasbuiltupon.Similarly,theMarinaDistrictofSanFrancisco,builtonbayfillandmudswasdevastatedasaresultofthegroundwavesofthe1989LomaPrietaearthquake.Ingeneralbuildingsconstructedonsolidbedrockaremuchlesslikelytosufferdamageorcollapsethanthoselocatedonsoftsediments.

Secondaryseismichazards

• SoilliquefactionWhenseismicwavestravelthroughsoftsediments,theycauseittobehaveasifitwerealiquid,duetoanincreaseinporewaterpressure.Itaffectsunconsolidatedsedimentsatdepthsoflessthantenmetres,whicharesaturatedwithwater.Asa

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Thenatureofseismicity:formsandcauses3.1.5.4Hazards

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resultofsoilliquefaction,thefoundationsofbuildingsbecomeunsupportedandconsequentlytheysinkortoppleover.DuringtheChristchurchearthquakesthatstruckNewZealandin2011,liquefactionwaslargelyresponsiblefortheextensivedamagetoresidentialproperties.Soilliquefactionismostdangerouswhenitcausesthecollapseofroadandrailsupportingstructures,ashappenedtotheSanFrancisco–OaklandBayBridgeandtheCypressStreetViaduct(NimitzFreeway)duringtheLomaPrietaearthquakein1989.• TsunamiWhenasubmarineearthquakeoccursitcangenerateseismicwavesthat,iflargeenough,cangenerateatsunamiintheocean.Therapiddeformationoftheseabedcanupliftacolumnofwaterastheoceaniccrustisthrustupwards.Theresultingcollapsingcolumnofwateractslikeripplesinapondandradiatesenergyoutwardsfromthefocus.

Some90percentofdamagingtsunamioccurinthePacificBasinastheyaregeneratedatsubduction-convergentplatemargins,particularlythoseborderingJapan,theAleutiansandSouthAmerica.Themagnitude9.0TōhokuEarthquakewhichhitJapanin2011,triggeredapowerfultsunamiwithwavesthatreachedupto40metresinheight.Theeffectsofthetsunamiwerefeltaroundtheworld,butweremostsevereinnorth-eastJapanwhere130000homescollapsed,4.4millionhouseholdswereleftwithoutelectricityandover16,000peopledied.

SourceNASA(publicdomain).

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Thenatureofseismicity:formsandcauses3.1.5.4Hazards

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• LandslidesSuddengroundshakingcancauseslopefailureonevengentle-slopes.Inmanyearthquakes,particularlyinmountainouszones,landslidescanbemoredangerousthattheprimarygroundshaking.Between200and300millioncubicmetresoflandslidesoccurredasaresultoftheGorkhaearthquakethatstruckNepalinApril2015.TheyblockedsomeofthekeyroutestoKathmandualongtheLangtangvalleyandobstructedtheentirevalleyalongtheTrisuliRiver.AnavalanchetriggeredbythequakesweptthroughaMountEverestbasecamp,killing17andinjuring61,makingitthedeadliestdayinEveresthistory.

ThepredictabilityofhazardeventsWeknowmuchaboutwhereearthquakesarelikelytooccur,duetotheirassociationwithplatemargins.However,predictingwhenearthquakeswilloccurisalmostimpossible.Seismologistsarespecialistscientiststhatstudyearthquakes.Hazardmappingcanhelptopredictwherethenextearthquakesmightoccur.Eachtimeanearthquakestrikes,itismapped.Placesonplatemarginsthathavenotrecentlyexperiencedanearthquakehaveahigherstrainbuildingupinthecrustalrocksandahigherprobabilityofexperiencinganearthquake.This‘gaptheory’canbeusedtoprovideearthquakeprobabilitiesandhelpauthoritiesplanforpossiblehazardevents.The2015GorkhaearthquakeinNepalwaspredictedbackin2013,asmeasurementssuggestedthattherewassufficientaccumulatedenergytoproduceagreatearthquake(magnitude8).Theprecisemomentofoccurrencewasmoredifficulttodefine,however.Ifinhabitantsdon’thavefaithinearthquakepredictionsmadebyscientists,orforecastshaveprovedtobeinaccurateinthepast,theyarelesslikelytoheedwarningsofimpendingseismicactivity.Attemptstopredictearthquakesinvolvemonitoringpre-existingfaultlines.Thiscanbeexpensiveastheprecisetimeandlocationofearthquakesisunknown.TheHaywardfaultalongtheeasternsideofSanFranciscoBayisarguablyoneofthemosthazardousfaultsintheworldandalsothemoststudied.Monitoringmethodsincludethefollowing:

• Tiltmetersandmagnetometersareusedtodetectchangesinthegroundheightandlocalmagneticfield.

• Seismographscandetectforeshockspriortothemainseismicevent.• Watercanbemeasuredforradongasandchangesintheheightofthewater

table.• Strainmeterscanmonitortheincreaseinstressexperiencedincrustalrocks.• Unusualanimalbehaviourcanindicateanimminentearthquake.Horses,dogs,

birdsandotheranimalsseemtosenseelectromagneticdisturbanceasrockfrictionincreases.ImmediatelypriortothedevastatingBoxingDayearthquakeintheIndianOceanandresultanttsunamiaffectingmuchofthecoastofsoutheastAsia,workingelephantswereseentobecomeextremelyanxiousandbreaktheirlegchainsbeforerunninginlandintotheforest.

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TheGreatTōhokuearthquake,Japan

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Specificationtopic:seismicityformsandcauses3.1.5.4Casestudy:GreatTōhokuearthquakeJapanislocatedinahazardousareaatthejunctionbetweenfourmajorplates.InthenorthofthecountrythePacificplate,movingwestwardsatarateof83mmayear,isbeingforcedunderthecontinentalEurasianplateatasubductionzoneknownastheJapanTrench.Thefaultupliftedbyover30metresalonga300kmsectionoftheJapanTrench.ItwasthemostpowerfulearthquakeeverrecordedinJapan.Thesubmarineearthquakeoccurredatadepthof30km;thisisfairlyshallowandtheconsequentgroundshakingwasviolentandlastedoverthreeminutes.Japan’scoastalareaisflat,low-lyingandtheunconsolidatedsoilamplifiedtheshockwaves.Coastalbaysaredeepandamplifytsunamiwaves.The11Marchearthquakewasprecededbyaseriesoflargeforeshocksovertheprevioustwodays.TheJapanMeteorologicalAgencysentoutanearthquakewarningonnationaltelevisionandradio,givingpeopleminutestoevacuate.Primaryeffects

• 16,000peoplewerekilledand6,000injured.• TheearthquakeshiftedtheEarthonitsaxisby10cm.• BuildingsinthecityofSendai(130kmfromtheepicentre)andTokyo(370km

fromtheepicentre)swayed.Damagewaslimitedduetoearthquake-resistantdesign.

• MostcasualtiesanddamageoccurredinIwate,MiyagiandFukushimawhere127000buildingswerecompletelydestroyedandamillionweredamagedacrossnortheastJapan

• OneschoolinIshinomaki,Miyagilost84of121studentsandteachers.• Around4.4millionhouseholdsinnortheastJapanwereleftwithoutelectricity

and1.5millionwithoutwater.• 1100sectionsoftraintrackweredamaged.Onlyfourtrainswerederailed

becauseallhigh-speedtrainshadbeenautomaticallystoppedbyanearlywarningsystem.

Secondaryeffects

• Atleast1800housesweredestroyedinFukushimawhentheFujinumadamfailed.

• OilrefineriesinIchiharaandSendaisetonfire.• Tsunamiwavesonlytook20minutestoarriveandreachedheightsofover30

metresatMiyako.Watertravelledupto10kminlandatSendai.56bridgesand26railwayswerewashedawayalongtheeastcoastofHonshufromChibatoAomoriand332400peoplelefthomeless

• TheearthquakeknockedoutthepowersupplytotheFukushimaDaiichiNuclearPowerPlant.Thetsunamithenfloodedback-upgeneratorsandtriggerednuclearmeltdowninthreereactors.

• Elsewhere,thetsunamicausedmassiveslabsoficetocalvefromtheSulzbergerIceShelf,AntarcticaanddamagepropertyinCalifornia,HawaiiandEcuador.

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TheGreatTōhokuearthquake,Japan

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• Thecostofthisdisasterwasinexcessof$300billion,themostexpensivenaturaldisasterinhistory.

ResponsesJapanwasoverwhelmedbythemagnitudeofthedisaster.Responsesfrom116countriesand28internationalorganizationscamequickly.Themilitarywereforcedtoburythedeadinmassgravestopreventthespreadofdisease.Japandeclaredastateofemergencyfollowingproblemsatsixnuclearreactors.150000residentswereevacuatedfromazone20kmaroundtheFukushimanuclearpowerplant.All55nuclearreactorsacrossJapanweretakenofflineforsafetyreasons;rollingpowerblackoutswerecommonforweeks.ToyotaandSonystoppedproduction,whileothercompaniesdecidedtorelocatetosafercountries.SomeelderlypeoplecontinuedtolivenearFukushima;otherwiseitwasallbutabandoned.Japan’sfoodexportswererestrictedduetoradiationfears.Recoveryandrebuildingtorestorenormalitywilltakeyearsduetothescaleofthedisaster.Examstylequestions:1. Foranamedareathatexperiencesvolcanic/andorearthquakehazards,

explainhowpeopleperceiveandareimpactedbythehazard.(9marks)2. ‘Theextenttowhichearthquakesrepresenthazardsdependsonwherethey

occur’Howfardoyouagreewiththisstatement?(20marks)

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TheGreatTōhokuearthquake,Japan

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1. Foranamedareathatexperiencesvolcanic/andorearthquakehazards,explainhowpeopleperceiveandareimpactedbythehazard.(9marks)

Studentsneedtojustifywhypeopleperceivehazardsinparticularways,thisanswerreferstotheGreatTōhokuearthquake,butthefocuscouldalternativelybeavolcanosuchasMountOntake.

• InJapan,peopleperceivehazardsinavarietyofways.• Japanisamulti-hazardousenvironmentandisaffectedbytyphoons,volcanoes,

earthquakes,tsunamisandgeomorphologicalhazardssuchaslandslides.Theriskprobabilityforeachofthesevariesaccordingtowherepeopleactuallylive.

• Coastalzonesarelesslikelytosuffertheeffectsofvolcanoesorlandslidesandtheinlandmountainousregionhasalowprobabilityofbeingaffectedbytsunamisorflooding.

• Hazardsalienceistherelativeimportanceoftheearthquakeriskcomparedwithotherconcerns.TheJapaneseeconomycouldcauseindividualsgreaterconcernsthantheriskfromnaturalhazards.

• Perceptionisoftenrelatedtoexperienceinlivingmemory.Astyphoonsoccurfairlyregularlytheyareperceivedtoberisky,thisiswhymosthomespriorto1995werebuiltwithheavytiledroofstoprotectpeoplefromheavyrainfall,butaftertheKobeearthquakein1995,newhomeswerebuilttoprotectinhabitantsfromearthquakes.

PeoplewereimpactedbytheGreatTōhokuearthquakeinanumberofways:

• 16,000peoplewerekilled.Thiscausedpsychologicalproblemsformany,especiallywhenyouconsiderdeathtollsinsomecoastalcommunitieswasmassive.

• Oilrefineryfiresaswellasrupturedwaterpipesandgaslinesmeantpeoplewerecutofffromvitalinfrastructure(around4.4millionhouseholdswithoutelectricityand1.5millionwithoutwater),thismaderecoverydifficult.

• 127,000buildingsweredestroyedandamillionweredamaged,thisleftahugeamountofrubbletoclearbeforereconstructioncouldoccur.

• Peoplehadfeltsafebehind6-8metreharbourwalls,butthesewerebreachedduringthetsunami.

• Nuclearmeltdownwasapossibility.Someworkersriskedtheirlivestostopmeltdownandthepossibilityofradiationreachingthousandsofothers.

2. ‘Theextenttowhichearthquakesrepresenthazardsdependsonwheretheyoccur’

Howfardoyouagreewiththisstatement? (20marks)Studentsarerequiredtousetheirknowledgeandunderstandinginordertoassessarangeoffactorsthatmakeearthquakeshazardoustohumansandconcludeastowhetherlocationisthemostsignificantfactor.

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TheGreatTōhokuearthquake,Japan

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Locationalfactors Non-locationalfactorsLocationinrelationtothenearestplatemarginandtypeofmargin

Timeofday–moredeathscanoccurifpeopleareasleep/inside,especiallywhenbuildingsarepoorlybuilt

Locationofepicentre–iftheepicentreisnearurbanareasmorebuildingsandpeoplearegoingtobeaffected

Levelofdevelopment–richcountriescanaffordtomitigatetheimpactofearthquakesthroughprediction,protectionandpreparation

Depthoffocus–shallow-focusedearthquakescausemoreshaking

Planning-buildingcontrolandaseismicbuildingdesignscanpreventbuildingcollapseandsavelives

Geology-unconsolidatedrockssuchasclayandsandcanamplifyshockwavesandcausethecollapseofbuildings

Prediction–forecastingwhenandwhereanearthquakemightstrikecanprovideplanningtimeforpeople

Argumentforlocationasasignificantfactor

• PlacesondestructiveplatemarginssuchasJapancansufferfromearthquakesandtsunami,makingthemveryhazardous.Japanhasdenselypopulatedcoastalareasmeaningthepopulationisveryvulnerablee.g.Kobe1995,GreatTōhoku2011.

• Placesonconservativeplatemarginscansufferstrongearthquakeswithnowarning.ThesearehazardousandcanaffectdenselypopulatedareassuchasCaliforniae.g.LomaPrieta1989.

• Placesonconstructiveplatemarginscansufferweakshallow-focusedearthquakes.Thesearelesshazardous.

• Thefurtherawayfromaplatemargin,thelesshazardousearthquakesare;theBritishIsleshasearthquakes,buttheyhavealowmagnitudeandcauselittlemorethanlightshaking,similartoalorrypassingby.

• ThefocusoftheGreatTōhokuearthquakewasshallow.Shakingwasstrongandlastedseveralminutes.Thisdestroyedthousandsofbuildings,cutpowersupplies,rupturedgasmainsandcausedfires.

• Geologyvarieswithlocation.Unconsolidatedsoftsedimentamplifiesshockwavesmakingearthquakesthatstrikecoastalbays,oldlakebedsandreclaimedlandmorehazardous.

• Topographyvarieswithlocation.Theveryflatcoastalareainthenorth-eastofJapanallowedtsunamiwavestotravelfarinlandwithoutimpediment.Italsofloodedtheback-upgeneratorsforthenuclearpowerplantresultinginthisbeingacomplexandveryhazardousdisaster.

Argumentagainstlocationasasignificantfactor

• ThethreePs(planning,predictionandpreparation)canbeimplementedanywhereandcanreducethehazardassociatedwithearthquakes.Deathtollsinmodernearthquakesaremuchlowerthaninthepastduetoriskmanagement.

• AlthoughearthquakesarecommoninCalifornia,thepopulationhasahighcapacitytocopeandthereforedeathtollislow.

• Thetimeofdaycanhaveahugeeffectonthedeathtoll,ifpeopleareinopenspaces(usuallyduringtheday)orinbedinhazard-resistantbuildings,theirchanceofsurvivinganearthquakearemuchhigher.

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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4

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Q1 MatchthetermswiththeirprocessdescriptionA Whentheweightofglaciersisliftedandlandsubsequentlyuplifts

B Whensoftsedimentsbehavelikealiquidduetogroundtremors

C Whenpressurebuiltupbetweentwoplatesisreleased

D Whenadensertectonicplateisforcedunderalessdenseone

E Theresultantgiantwavescausedbysubmarineearthquakes

F Slopefailureandmassmovementduetogroundtremors

Selectfrom:landslidessubductionearthquakeliquefactionisostaticrecoiltsunami

Q2 Tickwhethertheseareprimaryorsecondaryseismichazards Primary SecondaryA Theresultantgiantwavescausedbysubmarineearthquakes

B Slopefailureandlandslidesduetogroundtremors

C Whenpressurebuiltupbetweentwoplatesisreleased

D Whenshakingcausesloosesnowtoavalanchedownslope

E Whensoftsedimentsbehavelikealiquidduetogroundtremors

Q3 Tickthe2factorsoutofeachtriothatwillbemostinfluentialinthefollowingprocessesA Avalanches Depthofsnow Strengthof

earthquakeRocktype

B Landslides Angleofslope Altitudeofmountain

Strengthofearthquake

C Liquefaction Watercontent Distanceinland Depthofsoftsediments

D Earthquakeintensity Depthoffocus Populationdensity

Distancetoepicentre

E Tsunami Proximitytocoast

Depthoffocus Strengthofearthquake

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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4

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Q4 HowwouldseismicprocessesbedifferentalongthePacificBasinifthesevariables

werechanged?Strengthofearthquakes Damageanddeathtollfromearthquakes

Iftheplatesweremovingapart…

IfallthecountriessurroundingthePacificwerewealthy…

Heightoftsunamiwaves ThePacificBasinlandscapeIftheareawasonaconservativeplatemargin…

Iftheplatesweremovingapart…

Q5 Compareandcontrastthefollowingcharacteristicsofseismicprocesseson

constructiveanddestructiveplatemargins.Magnitudeofearthquakes:Frequencyofseismicevents:Probabilityoftsunamis:

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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4ANSWERS

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Q1 MatchthetermswiththeirprocessdescriptionA Whentheweightofglaciersisliftedandlandsubsequentlyuplifts

isostaticrecoil

B Whensoftsedimentsbehavelikealiquidduetogroundtremors

liquefaction

C Whenpressurebuiltupbetweentwoplatesisreleased

earthquake

D Whenadensertectonicplateisforcedunderalessdenseone

subduction

E Theresultantgiantwavescausedbysubmarineearthquakes

tsunami

F Slopefailureandmassmovementduetogroundtremors

landslides

Selectfrom:earthquakelandslidessubductionliquefactionisostaticrecoiltsunami

Q2 Tickwhethertheseareprimaryorsecondaryseismichazards Primary SecondaryA Theresultantgiantwavescausedbysubmarineearthquakes

ü

B Slopefailureandlandslidesduetogroundtremors

ü

C Whenpressurebuiltupbetweentwoplatesisreleased

ü

D Whenshakingcausesloosesnowtoavalanchedownslope

ü

E Whensoftsedimentsbehavelikealiquidduetogroundtremors

ü

Q3 Tickthe2factorsoutofeachtriothatwillbemostinfluentialinthefollowingprocessesA Avalanches Depthofsnow Strengthof

earthquakeRocktype

ü

ü

B Landslides Angleofslope Altitudeofmountain

Strengthofearthquake

ü

ü

C Liquefaction Watercontent Distanceinland Depthofsoftsediments

ü

ü

D Earthquakeintensity Depthoffocus Populationdensity

Distancetoepicentre

ü

ü

E Tsunami Proximitytocoast

Depthoffocus Strengthofearthquake

ü

ü

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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4ANSWERS

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Q4 HowwouldseismicprocessesbedifferentalongthePacificBasinifthesevariables

werechanged?Strengthofearthquakes Damageanddeathtollfromearthquakes

Iftheplatesweremovingapart…Earthquakeswouldhavealowermagnitudeastheplateswouldhavetensionalratherthancompressionalforcesactingonthem.

IfallthecountriessurroundingthePacificwerewealthy…Thepeoplelivinginthecoastalzonewouldbebetterpreparedandprotectedintheeventofanearthquake.Lesspeoplewoulddie.

Heightoftsunamiwaves ThePacificBasinlandscapeIftheareawasonaconservativeplatemargin…Whenplatesmovepasteachothertheyarelesslikelytogeneratetsunamiwavesastheearthquakewillnotoccurdirectlyundertheocean.

Iftheplatesweremovingapart…Iftheplatesweremovingapart,magmawouldbebasalticandcompressionalforceswouldnotbeatworkthereforelargefoldmountainswouldnotform.Volcanoeswouldbeshield-shapedratherthancompositecones,sothelandheightwouldbemuchlower.

Q5 Compareandcontrastthefollowingcharacteristicsofseismicprocesseson

constructiveanddestructiveplatemargins.Magnitudeofearthquakes:Themagnitudeofearthquakestendstobeloweronconstructiveplatemarginsduetotensionalforcesandtheytendtobehigherondestructiveplatemarginswheretheplatesexperiencestrongercompressionalforces.Frequencyofseismicevents:Thefrequencyofseismiceventsisprimarilyrelatedtothespeedofplatetravelratherthanthetypeofplatemarginwheretheyoccur.Greatermovementisexperiencedwhereplatesaremovinginoppositedirections.Constructiveplatemarginstendtoexperiencemoresmallerfrequencyearthquakesthanatdestructiveplatemarginswheregreaterpressurebuildsuptocauselessfrequent,butstrongerearthquakes.Probabilityoftsunamis:Tsunamisaremorelikelytooccuratdestructiveplatemarginswheredenseroceanicplatesaresubductedunderotherplatesandpowerfulearthquakesoccur.Atconstructiveplatemargins,earthquakesarelesspowerfulandrarelycausetheupliftofwaterrequiredtotriggeratsunami.

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