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Abilityof1-HzGPSdatatoinferthesourceprocessofamedium-sizedearthquake:Thecaseofthe2008Iwate-MiyagiNairiku,Japan,earthquakeYusukeYokota,1KazukiKoketsu,1KazuhitoHikima,1,2andShin’ichiMiyazaki3Received18February2009;revised23April2009;accepted8May2009;published16June2009.[1]Wedemonstratetheabilityof1-HzGlobalPositioningSystem(GPS)datatoinferamedium-sized(M6class)earthquakesourceprocessthroughaninversionstudyofthe2008Iwate-MiyagiNairiku,Japan,earthquake(M
6.9).1-HzGPSwaveformsforthisearthquakearecompared,andshowgoodagreementwithstrongmotionwaveforms.GroundmotionsarewellrecordedatasufficientnumberofGPSstationsinandaroundsourceregion,allowingustoperformwaveforminversionusing1-HzGPSdataonly.The1-HzGPSinversionresultshowsfairlygoodagreementwithajointinversionofgeodeticandstrongmotiondata,suchastheseismicmoment(Mw
6.9),cumulativeslipdistribution,andrupturepropagation.Suchagreementsdemonstratethat1-HzGPSisabletoinferthedynamicfeaturesoftheruptureprocessaswellaspermanent(orstatic)slipevenforaM6classmedium-sizedearthquake.Citation:Yokota,Y.,K.Koketsu,K.Hikima,andS.Miyazaki(2009),Abilityof1-HzGPSdatatoinferthesourceprocessofamedium-sizedearthquake:Thecaseofthe2008Iwate-MiyagiNairiku,Japan,earthquake,Geophys.Res.Lett.,36,L12301,doi:10.1029/2009GL037799.1.Introduction[2]High-rate(1Hzorhigher)samplingGPSdatacanbeusedasagrounddisplacementseismometer.Overthepastseveralyears,high-rateGPShasbeenusedforsuchpurposesasmonitoringvolcanodeformation[Mattiaetal.,2004]andtsunami[Katoetal.,2005].Thecapabilityofhigh-rateGPStorecordseismicwavefieldsforlargemagnitude(M
8class)earthquakeshasbeendemonstratedbycomparing1-HzGPSwaveformswithintegratedstrongmotionrecords,e.g.,forthe2002Denalifaultearthquake(Mw
7.9)[Larsonetal.,2003],andthe2003Tokachi-okiearthquake(Mw
8.1)[Koyamaetal.,2004;Irwanetal.,2004;Miyazakietal.,2004].Ruptureprocesswasinferredforthe2003Tokachi-okiearthquakesolelyfrom1-HzGPSdata[e.g.,Miyazakietal.,2004].Oneadvantageofusing1-HzGPSdataisthattheyinvolvepermanent(orstatic)slipcomponentaswellasdynamicsourceprocesses.Strongmotionseismometersare,ontheotherhand,affectedbytheproblemofbaselineshiftsduringseismicshaking,whichmakestherecoveryofthestaticdisplacementbydoubleintegrationdifficult.Thereareafewexampleswhere1-HzGPSdatawereusedsupplementarilyinwaveforminver-sionswithstrongmotiondataformedium-sizedearth-quakes,e.g.,theSanSimeonearthquake(Mw
6.5)[Jietal.,2004]andthe2004Fukuoka-okiearthquake(Mw
6.6)[Kobayashietal.,2006].However,noneofthepreviousstudieshavesucceededininferringthesourceprocesssolelyfrom1-HzGPSdata.Becausejointinversionsmaysufferfrominadequatedataweightingamongdifferentdatatypes,itwouldbebettertouseasingledatatype.[3]Inthisstudy,wefirstcompare1-HzGPSwithintegratedstrongmotionwaveformsforthe2008Iwate-MiyagiNairiku,Japan,earthquake.Wethenperformawaveforminversionforthesourceprocessofthiseventusing1-HzGPSdataonly.Inparticular,weshowthatboththestaticanddynamicpartsofthesourceprocessareinferredsolelyfrom1-HzGPSdata,withasufficientnumberofGPSstationsinandaroundthesourceregion.WethencomparetheresultwiththejointinversionresultfromstrongmotionandstaticGPSdisplacements.2.DataandModels[4]The2008Iwate-MiyagiNairiku,Japan,earthquakeoccurredat8:43a.m.onJune14,2008(JST).GlobalCMTProject()estimateditsmagnitude(Mw)tobe6.9.Thisearthquakecaused13fatalities,451injuries,30collapsedhouses,andseveralseverelandslides.Tenpersonswerestillmissingand143houseswereseverelydamaged,asreportedbytheFireandDisasterManagementAgency[2009].Below,weexaminethesourceprocessofthisearthquake,whichisbelievedtohavecausedtheabovedamage.[5]Staticanddynamicgrounddisplacementsforthisearthquakewereobservedatone-secondintervalsbyadenseGPSnetwork,calledGEONET,operatedbytheGeographicalSurveyInstituteofJapanbyprocessingGPSphasedatausingthemethoddescribedbyLarsonetal.[2003](Figure1).Asanexample,the1-HzGPSdisplace-mentsatStation0928areshowninFigure2.TheycomparefavorablywiththedisplacementseismogramsobtainedbydoubleintegrationofstrongmotionrecordsatK-NET[Kinoshita,1998]andKiK-net[Aoietal.,2000]stations.Becauseofthestrongmotionseismometers,itshouldbenotedthataseriouslyclippedseismicrecord[e.g.,Bilichetal.,2008]wasnotfound.Thesestationsarelocatedsoclosetothe0928GPSstationthatwecancomparewaveforms.Goodagreementsofthesewaveformsimplythat1-HzGPSdisplacementsmaybeinvertedtoinfertheruptureprocess.Temporallycorrelatednoiseisnotreducedthroughmodi-fiedsiderealfiltering(MSF)describedbyChoietal.[2004].Hence,thisnoisemaynotbecausedbyrepeatableerrorsGEOPHYSICALRESEARCHLETTERS,VOL.36,L12301,doi:10.1029/2009GL037799,20091EarthquakeResearchInstitute,UniversityofTokyo,Tokyo,Japan.2NowatTokyoElectricPowerCompany,Yokohama,Japan.3GraduateSchoolofScience,KyotoUniversity,Kyoto,Japan.Copyright2009bytheAmericanGeophysicalUnion.0094-8276/09/2009GL037799L123011of4suchasmultipath.Weusedthe1-HzGPSdataof12stations,whichwereselectedfromstationswithinapprox-imately50kmofthehypocentersoastocoveralldirections.TheGPSwaveformdatawerewindowedfor60sinordertoadequatelyincludetheP-andS-