Geology Applied to Civil Engineering 土木工程地质 英文课件 第七章 Introduction to Earthquakes

7.1IntroductiontoEarthquakesGeologyAppliedtoCivilEngineering123contentsClassificationofearthquakesSeismicWavesBasicConceptsofEarthquakes1BasicConceptsofEarthquakesEarthquakeisacommongeologicalphenomenon.Onaverage,approximately5millionearthquakeeventsoccurredaroundworldwideperyear.Mostofearthquakeeventsaretooweaktobefeltbypeople.Amongthese5millionearthquakeevents,thereareabout50thousandeventsthatcanbefeltbypeople.Devastatingearthquakesaccountfor1,000events,ofwhichonly10eventsarethethelargerdevastatingones1.1TerminologyofAnEarthquakeEventFocaldepthofanearthquakeeventreferstothedistancebetweenseismicfocusandepicenter.Epicentraldistanceofanearthquakeeventreferstothehorizontaldistancebetweenseismostationandepicenter.Focaldistanceofanearthquakeeventreferstothedistancebetweenseismicfocusandseismostation.Isoseismiclineofanearthquakeeventreferstotheconnectedpointsofthesamedamagedegreewithsameintensity.Seismicfocus(orhypocenter)ofanearthquakeeventreferstothesourcearea,wheretheseismicenergyfromtheearthquakeeventreleases.Seismicfocusisalsoknownashypocenter.Thisisthecenteroftheearthquake,thepointofinitialbreakageandmovementonafault.Epicenterofanearthquakeeventreferstotheposition,whereistheverticalprojectionpointoftheseismicfocusislocatedontheEarth’ssurfaceground.1.2LocationsofearthquakesEarthquakescommonlyoccurinthestrongearthquakezone,whichcanbeclassifiedasCircum-PacificSeismicBelt(approximately80%events),MediterraneantoCentralAsiaSeismicBelt(approximately15%events),andMid-oceanRidgeandContinentalRiftSeismicBelt.Chinaisanearthquake-pronecountry,locatedbetweentheCircum-PacificseismicBeltandtheMediterraneantoCentralAsiaSeismicBelt.Thereare5seismicbeltsinChina:1SoutheasternCoastalSeismicBeltandTaiwanSeismicBelt2Tancheng-LujiangSeismicBelt3NorthChinaSeismicBelt4North-SouthSeismicBelt5Tibet-WestYunnanSeismicBelt2ClassificationofEarthquakesEarthquakescanbeclassifiedbytheirgenesisastectonicearthquake,volcanicearthquake,collapseearthquake,reservoirearthquakeandartificialearthquake.Tectonicearthquakeisalsoknownasfaultedearthquake,whichgeneratedfromthecrustalmovement.Theinclinedstrata,fold,fault,rupture,andvolcanicmagmaticactivitycanbeformedfromcrustalmovement,whichalsoresultsinthecrustalshakingthroughthegeo-stressreleaseandfaultdislocation.Atectonicearthquakeaccountsforapproximately90%ofthetotalnumberofearthquakeeventsaroundtheworld.Theelasticreboundtheoryandbondsliptheoryarethetwomostpopulartheories,whichcanexplainthemechanismoftectonicearthquake.2.1ClassificationBasedonGenesis2.1ClassificationBasedonGenesisIntheelasticreboundtheory,thestresswillbegatheredintherockmasswhentherockmassdeformationresultsfromcrustalmovement.Therockmasswillsuddenlybefracturedordislocatedwhentheaccumulatedstressislargerthantheultimaterockstrength.Atthesametime,therockmasswillreleasestrainenergytoformtheearthquake,whichalsoprompttherockreboundbacktotheoriginalstate.2.1ClassificationBasedonGenesisVolcanicearthquakeiscausedbyvolcaniceruptions.Thisearthquaketypewillresultinlargeearthquakeintensity,buttheearthquake-proneareaisrelativelysmall.Thevolcanicearthquakejustaccountsforabout7%ofthetotalearthquakeevents.2.1ClassificationBasedonGenesisCollapseearthquakeiscausedbythecollapseofthezawnandmine.Inaddition,thecollapseearthquakealsocangeneratefromtherockfallandgiantlandslide.Thestratasubsidencecommonlyoccursinthecarbonatiteregion,withkarstphenomena.Thekarsteffectwillresultinthecollapseofthetopmoststrata,whichgeneratethecollapseearthquake.Inaddition,thecollapseearthquakealsocommonlygeneratesfromthetopmoststratacollapseintheundergroundminingarea,especiallyinthecoalmineregion.Collapseearthquakesjustaccountforapproximately3%ofthetotalearthquakeevents.2.1ClassificationBasedonGenesisReservoirearthquakeisgeneratedfromthereservoirfillingorwaterinjection.Thisearthquaketypejustoccursatthesomespecialreservoirareaandoil-fieldarea.Forexample,theKoinnaearthquake,India,withM6.5occurredon10th,December,1967andresultedin177fatalities,2300injuries,andmostbrickhousescollapsedinKoinnacity.TheKoinnadamwithitsauxiliaryconstructionswereseverelybrokenandthenstrengthenedtreatmentafteremptyingthereservoir.2.1ClassificationBasedonGenesisArtificialearthquakeiscausedfromthehuman-madeactivities,suchasundergroundnuclearexplosionandexplosiveblasting.Withthedevelopmentofthehumanengineeringactivities,theartificialearthquakeleadstomoreandmorepopularattentionbyhumans.Inaddition,somescholarsputthereservoirearthquakeandartificialearthquaketogetherascalledartificialearthquake.2.2ClassificationBasedonSeismicFocusDepthBasedonthefocusdepthofanearthquakeevent,theearthquakecanbecategorizedasdeep-focusearthquake，intermediate-focusearthquakeandshallow-focusearthquake.Generally,wecallthefocaldepthlessthan70kmofthequakeasshallow-focusearthquake.Thefocaldepthofanearthquakeeventbetween70kmand300kmiscalledintermediate-focusearthquake.Ifthefocaldepthofanearthquakeeventismorethan300km,thisearthquakecanbecalledasdeep-focusearthquake.Mostofdestructiveearthquakesgenerallybelongtoshallow-focusearthquakes,whichaccountfor90%ofthetotalearthquakeeventswithfocusdepthof5~20km.2.2ClassificationBasedonEpicentralDistanceBasedontheepicentraldistance,theearthquakecanbeclassifiedaslocalearthquakewithepicentraldistancelessthan100km,nearearthquakewithepicentraldistanceof100~1,000km,anddistantearthquakewithepicentraldistancelargerthan1,000km.3SeismicWavesWhentheearthquakehappens,someearthquakeenergypropagatesaselasticvibrationwavesintheearthcalledseismicwaves.Basedonthepropagationmode,theseismicwavecanbeclassifiedasbodywavesandsurfacewaves.BodywavestravelthroughtheEarth’sinterior,spreadingoutwardfromthefocusinalldirections.SurfacewavestravelontheEarth’ssurfaceawayfromtheepicenter,likewaterwavesspreadingoutfromapebblethrownintoapond.3.1BodyWavesBodywavesincludeprimarywaves(P-wave)andsecondarywaves(S-wave).P-waveisalsoknownascompressionwaveorlongitudinalwaveinwhichrockvibratesbackandforthparalleltothedirectionofwavepropagation.Itbelongstothefastestseismicwave,travelingthroughnear-surfacerocksatspeedsof5~6km/s.P-waveisthefirst(orprimary)wavethatarrivesatarecordingstationfollowinganearthquake.Therefore,theEarth’ssurfacegroundfirstlymovesasup-downshaking,whichresultsinaweakbreakage.P-wavescaneasilypassthroughthesolidrockandfluid(gasorliquid).3.1BodyWavesS-waveisalsoknownasshearwaveortransversewaveinwhichrockvibratesperpendiculartothedirectionofwavepropagation,thatis,crosswisetothedirectionthewavesaremoving.S-waveisaslowerwavethattravelsthroughnear-surfacerockatspeedsof3~4km/s.AnS-waveispropagatedbyashearingmotionmuchlikethatinastretched,shakenrope.S-wavescanpassthroughthesolidrock,butcan’tpassthroughafluid(gasorliquid),

becausefluidisresistanttosheardeformation.WhenthePoisson’sratioofrockorsoilisμ=0.22,theP-wavevelocity(uP)isthe1.67foldsoftheS-wavevelocity(uS),thatis,uP=1.67uS.3.2SurfaceWavesSurfacewavesdonotcomefromaseismicfocus,butisproducedbytheencounterofP-waveandS-waveattheEarth’ssurfaceground.ItjustpropagatesalongtheEarth’ssurfaceorinterfacebetweendifferentmedia.Asurfacewaveistheslowestwavesetoffbyquakeandcausespropertydamagemorethanbodywavebecauseasurfacewavewithlongwavelengthandlargeamplitudeproducesmoregroundmovementandtravelsmoreslowly,resultinginlongertimetopassaregion.Twomostimportanttypesofsurfacewavesareso-calledLovewavesandRayleighwaves.3.2SurfaceWavesLovewaveisformedbytheinterferencesuperpositionfromtheS-waveneartheearth’ssurfaceground.LovewavesaremostlikeS-wavesthathavenoverticaldisplacement.Thegroundmovessidetosideasa“snake”shapeinahorizontalplanethatisperpendiculartothedirectionthethewavepropagatingdirection.Lovewavecannottravelthroughliquidsandwouldnotbefeltonthebodyofwater.3.2SurfaceWavesRayleighwavesbehavelikearollingoceanwave.ARayleighwaveisformedbytheintersectorsuperpositionfromtheplaneP-waveandplaneS-waveneartheEarth’ssurfaceground.ThevibrationmodelofaRayleighwavecombinesthe