Thenatureofseismicity:formsandcauses
3.1.5.4Hazards
Whatyouneedtoknow
Thenatureofseismicityanditsrelationtoplatetectonics.
Formsofseismichazardincludingearthquakes,shockwaves,tsunamis,liquefaction,
landslides.
Spatialdistribution,randomness,magnitude,frequency,regularity,predictabilityof
hazardevents.
Introduction
ThecrustoftheEarthismadeupofsevenmajorplatesandseveralminorones.They
areallonthemove.Attheplatemargins,whereplatesaretravellingindifferent
directions,stresscanbuildup.Whenthepressure-resultingfromabuild-upoffriction
- isreleased,aseriesoftremorsorearthquakescanbefelt.
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Spatialdistributionofearthquakes
Thelocationofearthquakesiscloselyassociatedwithplatemargins.
• Atdestructiveplatemargins,earthquakestendtooccuratdepth.Theyare
associatedwiththesubductionofonecrustunderanotherinanarrowarea
knownastheBenioffzone,wherecompressionalforcesaregreatest.
Earthquakesoccurringherecanbeverypowerfulandmaytakeplaceunderthe
seaclosetoheavilypopulatedcoastalzones;thismakesthethreatofdangerous
tsunamimuchmorelikely.
• Atconstructiveplatemargins,earthquakestendtobemuchshallowerandless
powerful.Theyareassociatedwithtensionalforcesinthecrustandoccuralong
mid-oceanridgesawayfromlandorlargepopulations.
• Atconservativeplatemargins,earthquakestendtobeshallowfocused.Here
thecontinentalplatesaredraggingpasteachotherandcompressionalforces
arehigh.Earthquakesoccurringherecanbeverypowerfulanddamagecanbe
severeiftheyoccurindenselypopulatedareas.
Magnitudeofearthquakes
Mostoftheearthquakesthatoccureachyeararetoosmalltobefelt.Withincreasing
numbersofseismometersavailabletomeasureearthquakesandpeoplelivinginmore
remotelocations,wearereportingearthquakesmoretodaythanwehaveinthepast.
EarthquakemagnitudecanbemeasuredontheRichterscale,alogarithmicscale1-10,
where7onthescaleistentimesmorepowerfulthana6and100timesmorepowerful
thana5.Around120earthquakesofmagnitude6andaboveoccureachyear,ofthese
20measure7oraboveontheRichterscale(amajorearthquake).Greatearthquakes
(measuring8orabove)tendtooccuronceortwiceadecadeandforthepeopleliving
attheepicentreitcancompletelydestroytheircommunity.AlthoughtheRichterscale
isusefulformeasuringsmall-scaleearthquakes,itsaccuracydecreasesforlarger
earthquakes.TheMomentMagnitudescaleisameasureofthetotaldistanceafault
hasmovedandtheforceneededtogenerateit,knownasthemomentreleaseofthe
earthquake,andisnowusedworldwidebyseismologists.TheModifiedMercalli
Intensity(MMI)scaleismoreameasureoftheeffectonpeopleandhuman
infrastructure.The12-pointscaleclassifiestheimpactoftheearthquakeandindicates
severityonhumansratherthanobjectivemeasurementofearthquakeforce.A
moderateearthquakeunderamajorcitycanregisterhigherontheMMIthanastrong
earthquakeinalargelyunpopulatedregion.
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Thenatureofseismicity:formsandcauses
3.1.5.4Hazards
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While85percentofearthquakesoccurnearplateboundaries,someoccurinintraplate
locations.Theseearthquakes,internaltoplates,canhighlightundiscoveredfaultlines
andareasofstressincrustalrocks,buttheycanalsorelatetoothercausessuchas
isostaticrecoil,whentheweightofglaciersisliftedastheymeltandthecrustlifts.
Iceland’ssurfacehasliftedby60msincethepeakofthelasticeage;withupliftcomes
stressesandstrainsincrustalrockandconsequentlyearthquakes.
Earthquakescanalsobearesultofhumanactivity.Thesequasi-naturalearthquakes
aremostcommonlyassociatedwiththecrustaldeformationcausedbytheweightof
waterstoredinreservoirs.Someresearchershaveattributedthe7.9-magnitude
SichuanearthquakeinMay2008,whichkilledanestimated80,000people,tothe
constructionoftheZipingpuDaminChinaandtheresultingpressuresimposedonthe
rocksunderlyingitsreservoironceatfullcapacity.
Primaryseismichazards
• Earthquakesarethereleaseofenergyintheformofseismicwavesfromthe
pointknownasthefocuswherethepressureisreleased.Manyearthquakes
resultfrommovementsalongfracturesinrockscalledfaults.Oneofthemost
studiedfaultsisthetransformplatemarginofCalifornia,USA.Yearsofresearch
havefoundthatthisisnotasinglefaultline,butacomplexzoneoffaults.Once
pressureisbuiltupinarockandreleased,seismicorshockwavesradiateoutin
alldirections.ThepointonthesurfaceoftheEarthdirectlyabovethefocusis
knownastheepicentreandcanbethelocationofgreatestdamage.Thereare
fourtypesofseismicwaves.PwavesandSwavesarebodywavesthattravel
throughrock;Pwavesarethefastestandcanalsotravelthroughliquids.When
bodywavesarriveattheepicentretheycausepeopleandpropertytoriseand
fall.Thiscanseverwater,gasandotherinfrastructure.Escapinggasfrequently
ignitesandisdifficulttoextinguish,asrupturedwatermainsareunableto
supplywatertotheemergencyservices.
Surfacewaves(RayleighandLoveWaves)traveloutfromtheepicentreand
theyrepresentthemostseverehazardtopeopleandproperty.Therocking
motionassociatedwiththesewavescanshakeandtopplebuildingslike
dominoes.Bedrocktypecanalsoaffecttheextentofdamagetobuildings.Soft
claysandunconsolidatedbedrocktendtowobblelikejellyandamplifyshaking.
Solidbedrocktendstolimitshaking.AtKobe,Japan,theportexperienced
severedamageasaresultoftheamplificationofgroundwavesthroughthesoft
sandsandmudsitwasbuiltupon.Similarly,theMarinaDistrictofSan
Francisco,builtonbayfillandmudswasdevastatedasaresultoftheground
wavesofthe1989LomaPrietaearthquake.Ingeneralbuildingsconstructedon
solidbedrockaremuchlesslikelytosufferdamageorcollapsethanthose
locatedonsoftsediments.
Secondaryseismichazards
• Soilliquefaction
Whenseismicwavestravelthroughsoftsediments,theycauseittobehaveasifit
werealiquid,duetoanincreaseinporewaterpressure.Itaffectsunconsolidated
sedimentsatdepthsoflessthantenmetres,whicharesaturatedwithwater.Asa
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Thenatureofseismicity:formsandcauses
3.1.5.4Hazards
resultofsoilliquefaction,thefoundationsofbuildingsbecomeunsupportedand
consequentlytheysinkortoppleover.DuringtheChristchurchearthquakesthat
struckNewZealandin2011,liquefactionwaslargelyresponsiblefortheextensive
damagetoresidentialproperties.Soilliquefactionismostdangerouswhenit
causesthecollapseofroadandrailsupportingstructures,ashappenedtotheSan
Francisco–OaklandBayBridgeandtheCypressStreetViaduct(NimitzFreeway)
duringtheLomaPrietaearthquakein1989.
• Tsunami
Whenasubmarineearthquakeoccursitcangenerateseismicwavesthat,iflarge
enough,cangenerateatsunamiintheocean.Therapiddeformationoftheseabed
canupliftacolumnofwaterastheoceaniccrustisthrustupwards.Theresulting
collapsingcolumnofwateractslikeripplesinapondandradiatesenergyoutwards
fromthefocus.
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Some90percentofdamagingtsunamioccurinthePacificBasinastheyare
generatedatsubduction-convergentplatemargins,particularlythosebordering
Japan,theAleutiansandSouthAmerica.Themagnitude9.0TōhokuEarthquake
whichhitJapanin2011,triggeredapowerfultsunamiwithwavesthatreachedup
to40metresinheight.Theeffectsofthetsunamiwerefeltaroundtheworld,but
weremostsevereinnorth-eastJapanwhere130000homescollapsed,4.4million
householdswereleftwithoutelectricityandover16,000peopledied.
Source
NASA
(public
domain).
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Thenatureofseismicity:formsandcauses
3.1.5.4Hazards
• Landslides
Suddengroundshakingcancauseslopefailureonevengentle-slopes.Inmany
earthquakes,particularlyinmountainouszones,landslidescanbemoredangerous
thattheprimarygroundshaking.Between200and300millioncubicmetresof
landslidesoccurredasaresultoftheGorkhaearthquakethatstruckNepalinApril
2015.TheyblockedsomeofthekeyroutestoKathmandualongtheLangtang
valleyandobstructedtheentirevalleyalongtheTrisuliRiver.Anavalanche
triggeredbythequakesweptthroughaMountEverestbasecamp,killing17and
injuring61,makingitthedeadliestdayinEveresthistory.
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Thepredictabilityofhazardevents
Weknowmuchaboutwhereearthquakesarelikelytooccur,duetotheirassociation
withplatemargins.However,predictingwhenearthquakeswilloccurisalmost
impossible.Seismologistsarespecialistscientiststhatstudyearthquakes.Hazard
mappingcanhelptopredictwherethenextearthquakesmightoccur.Eachtimean
earthquakestrikes,itismapped.Placesonplatemarginsthathavenotrecently
experiencedanearthquakehaveahigherstrainbuildingupinthecrustalrocksanda
higherprobabilityofexperiencinganearthquake.This‘gaptheory’canbeusedto
provideearthquakeprobabilitiesandhelpauthoritiesplanforpossiblehazardevents.
The2015GorkhaearthquakeinNepalwaspredictedbackin2013,asmeasurements
suggestedthattherewassufficientaccumulatedenergytoproduceagreatearthquake
(magnitude8).Theprecisemomentofoccurrencewasmoredifficulttodefine,
however.Ifinhabitantsdon’thavefaithinearthquakepredictionsmadebyscientists,
orforecastshaveprovedtobeinaccurateinthepast,theyarelesslikelytoheed
warningsofimpendingseismicactivity.
Attemptstopredictearthquakesinvolvemonitoringpre-existingfaultlines.Thiscan
beexpensiveastheprecisetimeandlocationofearthquakesisunknown.The
HaywardfaultalongtheeasternsideofSanFranciscoBayisarguablyoneofthemost
hazardousfaultsintheworldandalsothemoststudied.
Monitoringmethodsincludethefollowing:
• Tiltmetersandmagnetometersareusedtodetectchangesinthegroundheight
andlocalmagneticfield.
• Seismographscandetectforeshockspriortothemainseismicevent.
• Watercanbemeasuredforradongasandchangesintheheightofthewater
table.
• Strainmeterscanmonitortheincreaseinstressexperiencedincrustalrocks.
• Unusualanimalbehaviourcanindicateanimminentearthquake.Horses,dogs,
birdsandotheranimalsseemtosenseelectromagneticdisturbanceasrock
frictionincreases.ImmediatelypriortothedevastatingBoxingDayearthquake
intheIndianOceanandresultanttsunamiaffectingmuchofthecoastofsouth
eastAsia,workingelephantswereseentobecomeextremelyanxiousandbreak
theirlegchainsbeforerunninginlandintotheforest.
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TheGreatTōhokuearthquake,Japan
Specificationtopic:seismicityformsandcauses3.1.5.4
Casestudy:GreatTōhokuearthquake
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Japanislocatedinahazardousareaatthejunctionbetweenfourmajorplates.Inthe
northofthecountrythePacificplate,movingwestwardsatarateof83mmayear,is
beingforcedunderthecontinentalEurasianplateatasubductionzoneknownasthe
JapanTrench.Thefaultupliftedbyover30metresalonga300kmsectionoftheJapan
Trench.ItwasthemostpowerfulearthquakeeverrecordedinJapan.Thesubmarine
earthquakeoccurredatadepthof30km;thisisfairlyshallowandtheconsequent
groundshakingwasviolentandlastedoverthreeminutes.Japan’scoastalareaisflat,
low-lyingandtheunconsolidatedsoilamplifiedtheshockwaves.Coastalbaysaredeep
andamplifytsunamiwaves.The11Marchearthquakewasprecededbyaseriesof
largeforeshocksovertheprevioustwodays.TheJapanMeteorologicalAgencysentout
anearthquakewarningonnationaltelevisionandradio,givingpeopleminutesto
evacuate.
Primaryeffects
• 16,000peoplewerekilledand6,000injured.
• TheearthquakeshiftedtheEarthonitsaxisby10cm.
• BuildingsinthecityofSendai(130kmfromtheepicentre)andTokyo(370km
fromtheepicentre)swayed.Damagewaslimitedduetoearthquake-resistant
design.
• MostcasualtiesanddamageoccurredinIwate,MiyagiandFukushimawhere
127000buildingswerecompletelydestroyedandamillionweredamaged
acrossnortheastJapan
• OneschoolinIshinomaki,Miyagilost84of121studentsandteachers.
• Around4.4millionhouseholdsinnortheastJapanwereleftwithoutelectricity
and1.5millionwithoutwater.
• 1100sectionsoftraintrackweredamaged.Onlyfourtrainswerederailed
becauseallhigh-speedtrainshadbeenautomaticallystoppedbyanearly
warningsystem.
Secondaryeffects
• Atleast1800housesweredestroyedinFukushimawhentheFujinumadam
failed.
• OilrefineriesinIchiharaandSendaisetonfire.
• Tsunamiwavesonlytook20minutestoarriveandreachedheightsofover30
metresatMiyako.Watertravelledupto10kminlandatSendai.56bridgesand
26railwayswerewashedawayalongtheeastcoastofHonshufromChibato
Aomoriand332400peoplelefthomeless
• TheearthquakeknockedoutthepowersupplytotheFukushimaDaiichiNuclear
PowerPlant.Thetsunamithenfloodedback-upgeneratorsandtriggered
nuclearmeltdowninthreereactors.
• Elsewhere,thetsunamicausedmassiveslabsoficetocalvefromtheSulzberger
IceShelf,AntarcticaanddamagepropertyinCalifornia,HawaiiandEcuador.
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TheGreatTōhokuearthquake,Japan
•
Thecostofthisdisasterwasinexcessof$300billion,themostexpensive
naturaldisasterinhistory.
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Responses
Japanwasoverwhelmedbythemagnitudeofthedisaster.Responsesfrom116
countriesand28internationalorganizationscamequickly.Themilitarywereforcedto
burythedeadinmassgravestopreventthespreadofdisease.Japandeclaredastate
ofemergencyfollowingproblemsatsixnuclearreactors.150000residentswere
evacuatedfromazone20kmaroundtheFukushimanuclearpowerplant.All55
nuclearreactorsacrossJapanweretakenofflineforsafetyreasons;rollingpower
blackoutswerecommonforweeks.ToyotaandSonystoppedproduction,whileother
companiesdecidedtorelocatetosafercountries.Someelderlypeoplecontinuedto
livenearFukushima;otherwiseitwasallbutabandoned.Japan’sfoodexportswere
restrictedduetoradiationfears.Recoveryandrebuildingtorestorenormalitywilltake
yearsduetothescaleofthedisaster.
Examstylequestions:
1.
2.
‘Theextenttowhichearthquakesrepresenthazardsdependsonwherethey
occur’Howfardoyouagreewiththisstatement?(20marks)
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Foranamedareathatexperiencesvolcanic/andorearthquakehazards,
explainhowpeopleperceiveandareimpactedbythehazard.(9marks)
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TheGreatTōhokuearthquake,Japan
1.
Foranamedareathatexperiencesvolcanic/andorearthquakehazards,explainhow
peopleperceiveandareimpactedbythehazard.(9marks)
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Studentsneedtojustifywhypeopleperceivehazardsinparticularways,thisanswerrefersto
theGreatTōhokuearthquake,butthefocuscouldalternativelybeavolcanosuchasMount
Ontake.
• InJapan,peopleperceivehazardsinavarietyofways.
• Japanisamulti-hazardousenvironmentandisaffectedbytyphoons,volcanoes,
earthquakes,tsunamisandgeomorphologicalhazardssuchaslandslides.Therisk
probabilityforeachofthesevariesaccordingtowherepeopleactuallylive.
• Coastalzonesarelesslikelytosuffertheeffectsofvolcanoesorlandslidesandthe
inlandmountainousregionhasalowprobabilityofbeingaffectedbytsunamisor
flooding.
• Hazardsalienceistherelativeimportanceoftheearthquakeriskcomparedwithother
concerns.TheJapaneseeconomycouldcauseindividualsgreaterconcernsthantherisk
fromnaturalhazards.
• Perceptionisoftenrelatedtoexperienceinlivingmemory.Astyphoonsoccurfairly
regularlytheyareperceivedtoberisky,thisiswhymosthomespriorto1995were
builtwithheavytiledroofstoprotectpeoplefromheavyrainfall,butaftertheKobe
earthquakein1995,newhomeswerebuilttoprotectinhabitantsfromearthquakes.
PeoplewereimpactedbytheGreatTōhokuearthquakeinanumberofways:
• 16,000peoplewerekilled.Thiscausedpsychologicalproblemsformany,especially
whenyouconsiderdeathtollsinsomecoastalcommunitieswasmassive.
• Oilrefineryfiresaswellasrupturedwaterpipesandgaslinesmeantpeoplewerecut
offfromvitalinfrastructure(around4.4millionhouseholdswithoutelectricityand1.5
millionwithoutwater),thismaderecoverydifficult.
• 127,000buildingsweredestroyedandamillionweredamaged,thisleftahugeamount
ofrubbletoclearbeforereconstructioncouldoccur.
• Peoplehadfeltsafebehind6-8metreharbourwalls,butthesewerebreachedduring
thetsunami.
• Nuclearmeltdownwasapossibility.Someworkersriskedtheirlivestostopmeltdown
andthepossibilityofradiationreachingthousandsofothers.
2.
‘Theextenttowhichearthquakesrepresenthazardsdependsonwheretheyoccur’
Howfardoyouagreewiththisstatement? (20marks)
Studentsarerequiredtousetheirknowledgeandunderstandinginordertoassessarangeof
factorsthatmakeearthquakeshazardoustohumansandconcludeastowhetherlocationis
themostsignificantfactor.
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TheGreatTōhokuearthquake,Japan
Locationalfactors
Locationinrelationtothenearestplate
marginandtypeofmargin
Locationofepicentre–iftheepicentre
isnearurbanareasmorebuildingsand
peoplearegoingtobeaffected
Depthoffocus–shallow-focused
earthquakescausemoreshaking
Geology-unconsolidatedrockssuchas
clayandsandcanamplifyshockwaves
andcausethecollapseofbuildings
Non-locationalfactors
Timeofday–moredeathscanoccurif
peopleareasleep/inside,especiallywhen
buildingsarepoorlybuilt
Levelofdevelopment–richcountriescan
affordtomitigatetheimpactof
earthquakesthroughprediction,
protectionandpreparation
Planning-buildingcontrolandaseismic
buildingdesignscanpreventbuilding
collapseandsavelives
Prediction–forecastingwhenandwhere
anearthquakemightstrikecanprovide
planningtimeforpeople
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Argumentforlocationasasignificantfactor
• PlacesondestructiveplatemarginssuchasJapancansufferfromearthquakesand
tsunami,makingthemveryhazardous.Japanhasdenselypopulatedcoastalareas
meaningthepopulationisveryvulnerablee.g.Kobe1995,GreatTōhoku2011.
• Placesonconservativeplatemarginscansufferstrongearthquakeswithnowarning.
ThesearehazardousandcanaffectdenselypopulatedareassuchasCaliforniae.g.
LomaPrieta1989.
• Placesonconstructiveplatemarginscansufferweakshallow-focusedearthquakes.
Thesearelesshazardous.
• Thefurtherawayfromaplatemargin,thelesshazardousearthquakesare;theBritish
Isleshasearthquakes,buttheyhavealowmagnitudeandcauselittlemorethanlight
shaking,similartoalorrypassingby.
• ThefocusoftheGreatTōhokuearthquakewasshallow.Shakingwasstrongandlasted
severalminutes.Thisdestroyedthousandsofbuildings,cutpowersupplies,ruptured
gasmainsandcausedfires.
• Geologyvarieswithlocation.Unconsolidatedsoftsedimentamplifiesshockwaves
makingearthquakesthatstrikecoastalbays,oldlakebedsandreclaimedlandmore
hazardous.
• Topographyvarieswithlocation.Theveryflatcoastalareainthenorth-eastofJapan
allowedtsunamiwavestotravelfarinlandwithoutimpediment.Italsofloodedthe
back-upgeneratorsforthenuclearpowerplantresultinginthisbeingacomplexand
veryhazardousdisaster.
Argumentagainstlocationasasignificantfactor
• ThethreePs(planning,predictionandpreparation)canbeimplementedanywhereand
canreducethehazardassociatedwithearthquakes.Deathtollsinmodernearthquakes
aremuchlowerthaninthepastduetoriskmanagement.
• AlthoughearthquakesarecommoninCalifornia,thepopulationhasahighcapacityto
copeandthereforedeathtollislow.
• Thetimeofdaycanhaveahugeeffectonthedeathtoll,ifpeopleareinopenspaces
(usuallyduringtheday)orinbedinhazard-resistantbuildings,theirchanceofsurviving
anearthquakearemuchhigher.
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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4
Q1
A
B
C
D
E
F
Matchthetermswiththeirprocessdescription
Whentheweightofglaciersisliftedandlandsubsequentlyuplifts
Whensoftsedimentsbehavelikealiquidduetogroundtremors
Whenpressurebuiltupbetweentwoplatesisreleased
Whenadensertectonicplateisforcedunderalessdenseone
Theresultantgiantwavescausedbysubmarineearthquakes
Slopefailureandmassmovementduetogroundtremors
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Selectfrom:landslidessubductionearthquakeliquefactionisostaticrecoiltsunami
Q2
Tickwhethertheseareprimaryorsecondaryseismichazards
Primary
Secondary
A
Theresultantgiantwavescausedbysubmarineearthquakes
B
Slopefailureandlandslidesduetogroundtremors
C
Whenpressurebuiltupbetweentwoplatesisreleased
D
Whenshakingcausesloosesnowtoavalanchedownslope
E
Whensoftsedimentsbehavelikealiquidduetogroundtremors Q3
Tickthe2factorsoutofeachtriothatwillbemostinfluentialinthefollowingprocesses
A
Avalanches
Depthofsnow
Strengthof
Rocktype
earthquake
B
Landslides
Angleofslope
Altitudeof
Strengthof
mountain
earthquake
C
Liquefaction
Watercontent
Distanceinland
Depthofsoft
sediments
D
Earthquakeintensity
Depthoffocus
Population
Distanceto
density
epicentre
E
Tsunami
Proximityto
Depthoffocus
Strengthof
coast
earthquake
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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4
Q4
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HowwouldseismicprocessesbedifferentalongthePacificBasinifthesevariables
werechanged?
Strengthofearthquakes
Damageanddeathtollfromearthquakes
Iftheplatesweremovingapart…
IfallthecountriessurroundingthePacific
werewealthy…
Heightoftsunamiwaves
ThePacificBasinlandscape
Iftheareawasonaconservativeplate
Iftheplatesweremovingapart…
margin…
Q5
Compareandcontrastthefollowingcharacteristicsofseismicprocesseson
constructiveanddestructiveplatemargins.
Magnitudeofearthquakes:
Frequencyofseismicevents:
Probabilityoftsunamis:
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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4ANSWERS
Q1
A
B
C
D
E
F
Matchthetermswiththeirprocessdescription
Whentheweightofglaciersisliftedandlandsubsequentlyuplifts
Whensoftsedimentsbehavelikealiquidduetogroundtremors
Whenpressurebuiltupbetweentwoplatesisreleased
Whenadensertectonicplateisforcedunderalessdenseone
Theresultantgiantwavescausedbysubmarineearthquakes
Slopefailureandmassmovementduetogroundtremors
isostaticrecoil
liquefaction
earthquake
subduction
tsunami
landslides
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Selectfrom:earthquakelandslidessubductionliquefactionisostaticrecoiltsunami
Q2
Tickwhethertheseareprimaryorsecondaryseismichazards
Primary
Secondary
ü
A
Theresultantgiantwavescausedbysubmarineearthquakes
ü
B
Slopefailureandlandslidesduetogroundtremors
ü
C
Whenpressurebuiltupbetweentwoplatesisreleased
ü
D
Whenshakingcausesloosesnowtoavalanchedownslope
ü
E
Whensoftsedimentsbehavelikealiquidduetogroundtremors
Q3
Tickthe2factorsoutofeachtriothatwillbemostinfluentialinthefollowingprocesses
A
Avalanches
Depthofsnow
Strengthof
Rocktype
earthquake
ü
ü
B
Landslides
Angleofslope
Altitudeof
Strengthof
mountain
earthquake
ü
ü
C
Liquefaction
Watercontent
Distanceinland
Depthofsoft
sediments
ü
ü
D
Earthquakeintensity
Depthoffocus
Population
Distanceto
density
epicentre
ü
ü
E
Tsunami
Proximityto
Depthoffocus
Strengthof
coast
earthquake
ü
ü
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Hazards:Thenatureofseismicity–formsandcauses3.1.5.4ANSWERS
Q4
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HowwouldseismicprocessesbedifferentalongthePacificBasinifthesevariables
werechanged?
Strengthofearthquakes
Damageanddeathtollfromearthquakes
Iftheplatesweremovingapart…
IfallthecountriessurroundingthePacific
werewealthy…
Earthquakeswouldhavealowermagnitude
Thepeoplelivinginthecoastalzonewould
astheplateswouldhavetensionalrather
bebetterpreparedandprotectedinthe
thancompressionalforcesactingonthem.
eventofanearthquake.Lesspeoplewould
die.
Heightoftsunamiwaves
ThePacificBasinlandscape
Iftheareawasonaconservativeplate
Iftheplatesweremovingapart…
margin…
Iftheplatesweremovingapart,magma
Whenplatesmovepasteachothertheyare
wouldbebasalticandcompressionalforces
lesslikelytogeneratetsunamiwavesasthe
wouldnotbeatworkthereforelargefold
earthquakewillnotoccurdirectlyunderthe
mountainswouldnotform.Volcanoeswould
ocean.
beshield-shapedratherthancomposite
cones,sothelandheightwouldbemuch
lower.
Q5
Compareandcontrastthefollowingcharacteristicsofseismicprocesseson
constructiveanddestructiveplatemargins.
Magnitudeofearthquakes:
Themagnitudeofearthquakestendstobeloweronconstructiveplatemarginsdueto
tensionalforcesandtheytendtobehigherondestructiveplatemarginswheretheplates
experiencestrongercompressionalforces.
Frequencyofseismicevents:
Thefrequencyofseismiceventsisprimarilyrelatedtothespeedofplatetravelratherthan
thetypeofplatemarginwheretheyoccur.Greatermovementisexperiencedwhereplates
aremovinginoppositedirections.Constructiveplatemarginstendtoexperiencemore
smallerfrequencyearthquakesthanatdestructiveplatemarginswheregreaterpressure
buildsuptocauselessfrequent,butstrongerearthquakes.
Probabilityoftsunamis:
Tsunamisaremorelikelytooccuratdestructiveplatemarginswheredenseroceanicplates
aresubductedunderotherplatesandpowerfulearthquakesoccur.Atconstructiveplate
margins,earthquakesarelesspowerfulandrarelycausetheupliftofwaterrequiredtotrigger
atsunami.
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