SeismicAttenuation地震波衰减

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1、SeismicAttenuation---fromseismicwavetraveltimestoamplitudesHaijiangZhangCourtesyofmanydifferentsourcesHaberlandetal.,GRL2003EARTHQUAKEMAGNITUDEEarliestmeasureofearthquakesizeDimensionlessnumbermeasuredvariousways,includingMLlocalmagnitudembbodywavemagnitudeMssurfacewavemagnitudeMwm

2、omentmagnitudeEasytomeasureNodirecttietophysicsoffaulting(4)intrinsicattenuation:duetoanelasticitytherealearthmaterialsarealways“lossy”,leadingtoreducedwaveamplitudes,orintrinsicattenuation.Mechanismstoloseenergy:(1)Movementsalongmineraldislocations(2)ShearheatingatgrainboundariesTh

3、esearecalled“internalfriction”.……(3)Scattering:Awaytopartitionenergyofsupposedlymainarrivalsintoboundaryorcornerdiffracted,scatteredenergy.AttnuationMechanisms:(1)Geometricalspreading:wavefrontspreadingoutwhileenergyperunitareabecomesless.(2)Multipathing:wavesseekalternativepathstot

4、hereceiver.Somearedispersedandsomearebundled,therebyaffectingamplitudes.Key:verywavelengthdependent.4Geometricspreading:lightmovesoutwardfromlampinexpandingsphericalwavefronts.Byconservationofenergy,theenergyinaunitareaofthegrowingwavefrontdecreasesasr-2,whereristheradiusofthesphere

5、ordistancefromthelamp.Analogoustolightbehavoirs…Expectminimumat=90º,andmaximaat0ºand180ºGEOMETRICSPREADING:SURFACEWAVESFromgeometricspreadingalone,expectminimumat=90º,andmaximaat0ºand180ºAlsohaveeffectsofanelasticityForbodywaves,considerasphericalwavefrontmovingawayfromadeepearthq

6、uake.Energyisconservedontheexpandingsphericalwavefrontwhoseareais4πr2,whereristheradiusofthewavefront.Thustheenergyperunitwavefrontdecaysas1/r2,andtheamplitudedecreasesas1/rGEOMETRICSPREADING:BODYWAVESMULTIPATHINGSeismicwavesarefocusedanddefocusedbyvariationsinvelocity.IdeaofRayTube

7、sinbodywavesRaytubesizeaffectsamplitudes,smallerareameanslargeramplitudeAngledependent:raybundleexpandsorcontractsduetovelocitystructure.Also:amplitudevarieswithtakeoffangleEffectofMultipathing10Scattering:Scatteringoccurswhentherearevelocityheterogeneitiesinthemediumwithwavelengths

8、ontheorderofλofthew