Analysis of a Dense Broadband Survey Across the Los Angeles Basin RobertW.ClaytonandYiranMa

CaltechJaschaPolet,RaymondNgandMichaelHerrman

CalPolyPomonaPaulM.Davis

UCLAThisstudyisbasedonadensebroadbandsurveydoneacrosstheLosAngelesBasinfromLongBeachtoPuenteHills.Thearrayconsistedof71stationsthatweredeployedbystudentsandfacultyatCalPolyPomona(JaschaPolet),UCLA(PaulDavis),USGS/Pasadena(ElizabethCochran),Caltech(RobertClayton),NodalSeismicInc(DanHollis),andOccidentalPetroleum–nowCaliforniaResources.ThenetworkwasdeployedforapproximatelysixweeksintheFallof2014.Formoredetailsseehttp://web.gps.caltech.edu/~clay/LASSIE/LASSIE.htmlThesurveyisnamedLASSIE(LosAngelesSeismicSynclineInterferometryExperiment).Thiscomponentofthestudyresultedintwopublishedpaperslistedbelow.ThearraycrossesthemainpartoftheLosAngelesBasinasshowninFigure1.Thegoalistofindtheshapeandshear-wavevelocityofthebasin.Thesearedoneusingsurfaceanalysisusingvirtualsourcescreatedbyambientnoisecorrelation,andbyreceiverfunctionsusingteleseismicrecordings.TheproblemofutilizingsurfacewavesinsedimentarybasinsisdiscussedinPaper1,andthechiefissueisthepresenceofhighermodes.Amethodbasedonthedifferencebetweenretrograde(fundamentalmode)andprograde(1stovertonemode)motionsisusedtoseparatethemodes.BothLoveandRayleighwavesareusedinconstructingthevelocitymodel.ThestructureandvelocityresultsarepresentedinPaper2.Theshearwavevelocityisdeterminedfromastandarddispersioncurveinversion.Thestructureisdonewithreceiverfunctions,andboththebasin-basementinterfaceandMohoarefound.UndertheLosAngelesBasin,theMohorisesbyapproximately10km,afeaturethat

Figure1.TheLASSIEarrayusedinthestudy.Theyellowdotsareaquasi-lineararrayof40broadbandstationsacrosstheLosAngelesBasin.

isnotpresentinanyoftheSCECmodels.Thisisexpectedifthebasinisinisostaticequilibrium.Thisstudyresultedintwopapers:Ma,Y,R.ClaytonandD.Li,(2016a),Higher-modeambient-noiseRayleighwavesin

sedimentarybasins,Geophys.J.Int,206,1634-1644.doi:10.1093/gji/ggw235Ma,Y,andR.Clayton,(2016b),StructureoftheLosAngelesBasinfromambient

noiseandreceiverfunctions,Geophys.J.Int.,206,1645-1651.doi:10.1093/gji/ggw236

Horizontal-to-Vertical(H/V)spectralratioanalysisisawell-establishedapproachtoestimatetheresonanceperiodsandseismicamplificationofasiteandthereforeisusefulforsitecharacterizationandmicro-zonation.Sinceresonanceperiodsareproducedbystrongimpedancecontrastsatdepth,spectralratioanalysismayalsobeusedtomapsubsurfacegeologicalstructuresandisparticularlyadvantageoustoinvestigatesedimentarybasinsandtheinterfacebetweenunconsolidatedsedimentsandthebasementrocks.SinceLASSIEwasashort-termdeployment,nolargeearthquakesoccurredduringthistimewindow,andthereforeweusedthemicro-tremorapproach.Thebasicmethod(e.g.Nakamura,1989;LermoandChavez-Garcia,1993)hasbeenconfirmedtorevealthedominantsoilresonancefrequenciesofasite,althoughtheinterpretationoftheamplificationfactorisnotaswellunderstood(Pilzetal.,2009).TheGeopsysoftware(DiGiulioetal.,2006;Watheletetal.,2008)wasusedtogenerateH/VspectralratiocurvesforallLASSIEstations,andtheSESAMEcriteria(SESAMEEuropeanresearchproject,2004)wereusedtoassessthereliabilityoftheresultingpeakfrequenciesandpeakamplitudes.BasedonthefindingsbyZhaoetal.(1997),H/Vspectraareinsensitivetotheoriginandthepropagationpath,butaresensitivetothesubsurfacestructurelocatedneartheobservationpoint.WefoundseveralcoherentsetsofspectralratiopeaksinourdatasetoftheLASSIEdeployment,withagroupofpeaksthatcouldmosteasilybetrackedalongtheentirenetworkinthefrequencyrangeof0.05Hz-0.17Hz(Figures2and3).ThepeakamplitudeandpeakfrequencybothshowsignificantvariationacrosstheLosAngelesbasin.Highestpeakamplitudes(above5.0)weremeasuredfortheLongBeacharea,withadecreaseinamplitudewithdistancetotheNorth-East.PeakfrequenciesshowadecreasewithdistancetowardstheNorth-East.ThesefrequenciesdonotappeartocorrespondtotheresonanceperiodthatwouldbepredictedfortheinterfacerepresentedbythebasinbasementintheSCECUCVMCVM-Hmodel,butsuggestamoreshallowcontrastinacousticimpedancemayberesponsible.Toinvestigatelateralheterogeneity,weexploreazimuthalvariationsinmicrotremorH/Vratiofortwostations,A134andN118,intheLASSIEtransect.MicrotremorH/Vazimuthalvariationshavebeenusedinpreviousbasinstudiestoqualitativelyinterpretthelocationandeffectofthebasinedge.StationN118is

locatedabovethedeepestportionoftheLAbasinandstationA134islocatednearthebasinedgeoftheCVMmodel.ResultsindicatethattheorthogonalazimuthsthatproduceamaximumdifferenceinamplitudesfordirectionalH/VatA134

correspondwiththemajorandminorhorizontalaxisoftheLAbasin,whereasnosignificantvariationisseenforN118.ThisstudyresultedintwoCPPMSctheses:Ng, R. (Fall 2016), Evaluation of Site Response in the Los Angeles Basin from

SpectralRatioAnalysisofMicrotremorDatafromaHighDensityTemporaryBroadbandDeployment

Herrman,M.(Winter2017),SeismicWaveformModelingofBroadbandDataFromaTemporaryHigh-DensityDeploymentintheLosAngelesBasin

ReferencesDi Giulio, G., C. Cornou, M. Ohrnberger, M. Wathelet, and A. Rovellii (2006). Deriving

WavefieldCharacteristicsandShear-VelocityProfilesfromTwo-DimensionalSmall-Aperture Arrays Analysis of Ambient Vibrations in a Small-Size Alluvial Basin,Colfiorito,Italy.BulletinoftheSeismologicalSocietyofAmerica,96,1915-1933.

Lermo,J.,&Chávez-García,F.J.(1993).Siteeffectevaluationusingspectralratioswithonlyonestation.BulletinoftheseismologicalsocietyofAmerica,83(5),1574-1594.

Nakamura,Y.(1989).Amethodfordynamiccharacteristicsestimationofsubsurfaceusingmicrotremoronthegroundsurface.RailwayTechnicalResearchInstitute,QuarterlyReports,30(1).

Figure2.TypicalspectralratiocurvefortypicalLAbasinstation.Apeakisfoundatafrequencyof.15Hz,withanamplitudeof4.7.

Pilz,M.,Parolai,S.,Leyton,F.,Campos,J.,&Zschau,J.(2009).Acomparisonofsiteresponsetechniques using earthquake data and ambient seismic noise analysis in the largeurban areas of Santiago de Chile. Geophysical Journal International, 178(2), 713-728.

BasinEdgeFocusingBecausefocusingatdamagingseismicfrequenciesrequireshighlyconvexbasinstructuresthemostlikelygeometriesarebasinsedgedbyreversefaultssuchastheLAbasin.Thereversemotioncanunderthrustlow-velocitysedimentsbeneathhighervelocitybedrock.MaandClayton(2015)haveusednoisecorrelationtoobtainphasevelocitiesalongtheLASSIEline.TheyinvertedthatdataforSwavestructureandfindalowvelocityconvexzoneunderthePuenteHills(nexttostation134)thatisacandidatestructureforgeneratingfocusing.Wefindthatlocaleventsexhibitanomalouslylargeamplitudesindicatingfocusinginthisregion.Theseismograms(fromevent657)wereband-passedfiltered,andtheabsoluteamplitudeoftheHilberttransformisplottedtoviewtheenergyintheenvelopes.AnS-wavewindowofabout2seclongwaschosen,andtheRMSamplitudecalculated.TheamplitudeaswellasthenoiseareshowninFigure(4).ApeakoccursatstationsA132-A133-A134-A135(Figure4).Asmoothmodelofspatialdecayandattenuationwasfittothedata.Figure4illustrateshowtheinner

4stationsjustmentionedrobtheenergyofthestationseithersidetoformafocusingpeak.

Figure3.Mapofpeakfrequencyandamplitudeforthelowerfrequencypeaksacrossthenetwork.Colorofthecircleindicatesthefrequencyofthespectralratiopeak,theradiusofthecircletheamplitude.

-118.3˚ -118.25˚ -118.2˚ -118.15˚ -118.1˚ -118.05˚ -118˚ -117.95˚ -117.9˚33.7˚

33.75˚

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33.85˚

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33.95˚

34˚

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34.1˚

0.05 0.07 0.09 0.11 0.13 0.15 0.17

Frequency of low-frequency peak in Hz

0 10

Figure4.Exampleoffocusing.RedcurveisRMSenergyinwindowsofS-waveseismograms.Bluecurveissimplesmoothspatialdecaymodel.BlackcurveisRMSnoiseofseismogramsbeforearrivalofPwaves.FocusingapparentlyoccursatstationsA133-135thatarewithin1kmofthedamagezoneoftheWhittierearthquake.