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Thenatureofseismicity:formsandcauses3.1.5.4Hazards
© Tutor2u Limited 2016 www.tutor2u.net
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.
SAM
PLE
Thenatureofseismicity:formsandcauses3.1.5.4Hazards
© Tutor2u Limited 2016 www.tutor2u.net
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
SAM
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Thenatureofseismicity:formsandcauses3.1.5.4Hazards
© Tutor2u Limited 2016 www.tutor2u.net
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).
SAM
PLE
Thenatureofseismicity:formsandcauses3.1.5.4Hazards
© Tutor2u Limited 2016 www.tutor2u.net
• 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.
SAM
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TheGreatTōhokuearthquake,Japan
© Tutor2u Limited 2016 www.tutor2u.net
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.
SAM
PLE
TheGreatTōhokuearthquake,Japan
© Tutor2u Limited 2016 www.tutor2u.net
• 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)
SAM
PLE
TheGreatTōhokuearthquake,Japan
© Tutor2u Limited 2016 www.tutor2u.net
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.
SAM
PLE
TheGreatTōhokuearthquake,Japan
© Tutor2u Limited 2016 www.tutor2u.net
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.
SAM
PLE
Hazards:Thenatureofseismicity–formsandcauses3.1.5.4
© Tutor2u Limited 2016 www.tutor2u.net
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
SAM
PLE
Hazards:Thenatureofseismicity–formsandcauses3.1.5.4
© Tutor2u Limited 2016 www.tutor2u.net
Q4 HowwouldseismicprocessesbedifferentalongthePacificBasinifthesevariables
werechanged?Strengthofearthquakes Damageanddeathtollfromearthquakes
Iftheplatesweremovingapart…
IfallthecountriessurroundingthePacificwerewealthy…
Heightoftsunamiwaves ThePacificBasinlandscapeIftheareawasonaconservativeplatemargin…
Iftheplatesweremovingapart…
Q5 Compareandcontrastthefollowingcharacteristicsofseismicprocesseson
constructiveanddestructiveplatemargins.Magnitudeofearthquakes:Frequencyofseismicevents:Probabilityoftsunamis:
SAM
PLE
Hazards:Thenatureofseismicity–formsandcauses3.1.5.4ANSWERS
© Tutor2u Limited 2016 www.tutor2u.net
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
SAM
PLE
Hazards:Thenatureofseismicity–formsandcauses3.1.5.4ANSWERS
© Tutor2u Limited 2016 www.tutor2u.net
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.
SAM
PLE