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Strike-slipintraplateearthquakesintheWesternPhilippineSeaPlateJing-YiLina,,Yen-FuChen ,Chao-ShingLee ,Shu-KunHsu ,Chin-WeiLiangabaa,b,Yi-ChinLin ,Hsin-SungHsiehaaaDepartmentofEarthSciencesandInstituteofGeophysics,NationalCentralUniversity,300JhongdaRoad,JhongliCity,TaoyuanCounty32001,TaiwanbInstituteofAppliedGeophysics,NationalTaiwanOceanUniversity,2Pei-NingRoad,Keelung202,Taiwanabstractarticle infoArticlehistory:Received27February2013Receivedinrevisedform12July2013Accepted29August2013Availableonline7September2013 Keywords:Strike-slipfaultCollisionConjugatefaultWestPhilippineSeaPlateFracturezoneRyukyusubductionsystemOn26April2010,astrike-slipearthquake(Mw6.5)occurredintheWesternPhilippineSeaPlate.Wedeployed14ocean-bottomseismometerstorecordthecorrespondingaftershockstoacquireinformationregardingtheseintraplateevents.Ourresultsshowthattheaftershockswerelocatedalongtwolinearfeaturesthatintersectwithanangleofapproximately120andareconsideredaconjugatefaultset.ThePaxisofthemainshockfocalmech-anism is consistent with the compressive stress direction induced by the arccontinent collision occurring ineastern Taiwan. The pre-existing oceanic fracture zones and tectonic fabrics do not appear to be reactivatedbased on thedistinct rupturedirectionsdetermined fromtherelocatedaftershocks.However,the abrupthaltoftheaftershocksattheborderofthefracturezonesuggeststhatpre-existingweakzonescouldactasabarriertorupturepropagation.Moreover,mostlargeearthquakeshaveoccurrednearfracturezones,indicatingthatthepre-existingweaknessmay favor the generation ofearthquakescomparedtotheotherportionofthe oceanicplateduetotherelativelylowrockstrengthofthiszone.2013ElsevierB.V.Allrightsreserved.1.IntroductionThe majority of large earthquakes that occur in oceans occur atsubductionzones,whereonetectonicplateisthrustingbeneathanoth-er.However,the11April2012,Mw8.6andMw8.2earthquakesoffthewestcoastofnorthernSumatra,Indonesia,occurredasaresultofstrike-slip faulting within theIndo-Australia plate. Is such a great strike-slipevent within the oceanic lithosphere a special case, or could it occurelsewhereintheworld?Overthelastfewdecades,severalstrike-slip-type earthquakes have been observed within theWest PhilippineSeaPlate (WPSP) (Fig. 1). Nearly all of these earthquakes possessed asimilar focal mechanism pattern with one fault plane sub-parallel toapproximately N35WN45W (Fig . 1b). Based on bathymetric andmagneticanomalydata,previousstudieshaveidentiedseveraldistincttectonic structures intheWPSP,suchastheGagua Ridge,whichisanancient spreading center, and oceanic fracture zones (Deschampse t a l .,2002 ; Hi l deandLee , 1984).However,lessinformationregardingthepresentactivityofthesestructureshasbeenacquired.On26April2010, an Mw 6.5 strike-slip-type earthquake occurred on the easternsideofGaguaRidge(Fig.1b),wherethedetectableseismicactivityisgen-erally low. To understand the earthquake mechanism, we conducteda passive ocean-bottom seismometer (OBS) experiment approximately1weekaftertheoccurrenceoftheearthquake.2.GeologicalbackgroundThe Philippine Sea Plate (PHS) is moving northwestward (Yuet a l., 1997) (~N306312) relative to the Eurasian Plate (EU),with an 89cm/yr PHS/EU plate convergence vector near Taiwan(Fig . 1). The northwestern corner of the PHS is colliding westwardwith the EU margin and is creating the Taiwan orogen; however,the PHS is also subducting northward beneath the Ryukyu Arc(Fig . 1a). Thus,mostearthquakesinandaroundTaiwanare relatedto the convergence of these two plates (Kao and Jian, 2001; Kaoe t al. , 1998, 2000; Kubo and Fukuyama, 2003). The Gagua Ridge,located off the eastern shore of Taiwan, is a major narrow linearhigh that enters the Ryukyu Arc and isolates the Huatung Basinto the west from the main West Philippine Basin (WPB) (Fig . 1b)(Deschamps and Lallemand, 2002 ; Dom i nguez et al ., 1998; Hsueta l., 1996).S i bue t etal . (2002)proposedthattheGaguaRidgeisazone of weakness and could have been a plate boundary betweenthePhilippineSeaand Huatungplates. Lin e t a l. (2004a , 2004b)showedthattheGaguaRidgewasaformerplateboundaryandwassheared beneath the Ryukyu subduction zone. East of the GaguaRidge,severalmajorNESW-orientedfracturezoneshavebeendenedusingbathymetricdata(Hsue t al.,2013).TheLuzonOkinawaFractureZone (LOFZ) is the largest of these structures based on the apparentgeomorphology (Fig . 1b). West of the LOFZ, the spreading fabric isillustratedbytheN120E-trendingabyssalhills,whichlieperpendicularto the fracture zones (Deschamps and La l lemand , 2002 ; Deschampse t al.,2002). Tec t onophysics608(2013)499504 Correspondingauthor.Tel.:+88634227151x65613.E-mailaddresses:jylin.gep@gmail.com,jylin@ncu.edu. t w(J.-Y.Lin). 0040-1951/$seefrontmatter2013ElsevierB.V.Allrightsreserved.ht t p: // dx.doi.org/10.1016 / j.tecto.2013.08.038 ContentslistsavailableatScienceDirectTectonophysicsjournalhomepage:www.elsevier.com/locate/tecto

3.DataprocessingWe deployed 16 OBSs from May 2 to 25, 2010, within a 250 200km area covering parts of the West Philippine Sea Basin, Gagua2RidgeandtheHuatungBasin(Fig. 1c). Unfortunately, 2 OBSs were notsuccessfully recovered (open triangles in Fig. 1c) and only 14 OBSswere used for the data processing. The spacing between OBSs variedfrom18to60km.Earthquakeeventswereselectedmanuallyfromcon-tinuousseismic records,and weightswere assignedtoP-andS-wavearrivals based on the quality of the signal. Events possessing morethansix arrivals wereinitiallyidentied usinganIASP911-Dvelocitymodel (Kennett and Engdahl , 1991). In total, 1476 earthquakes wereidentiedandlocatedduringanapproximately22-dayrecord(Fig. 2aand b). However, the use of only six wave arrivals and a global 1-Dvelocitymodelmayintroducemanualselectionandvelocityerrorsintoour localization. To improve the accuracy of the hypocenter locations,we applied the double-difference (hypoDD) method (Waldhauser andEllsworth, 2000) to the 527 earthquakes possessing at least 10 arrivals(Fig.2c).Forthisprocess,anappropriate1-Dvelocitymodeloftheareawas derived using the seismic wave arrivals and the VELEST program(Kisslingetal.,1994).TheinitialvelocitymodelinputfortheVELESTpro-gram was extracted from the results of a wide-angle seismic reectionsurvey conducted in the eastern part of the Gagua Ridge (Chen, 2009).To avoid the inuence of arrivals from outside of our target area, onlythose events that occurred around the epicenter area (~123.6124E;22.122.5N) were used for the inversion. During the inversion, theRMSresidualdecreasedwitheachiterationandbecamestableafterthefourthiteration (Fig. 3a).Theinverted velocitymodelwasthenused asthe input for the hypoDD relocation. To ensure the stability of thismodel, we repeated the inversion by altering the initial velocity valueby asmuchas13%.Theresultshows that evenwhendifferentinitialmodelsareused,asimilarvelocityvaluecanbeobtained(Fig.3b).Finally,326eventswererelocatedusingthehypoDDsoftwareandtheinverted1-Dvelocity model (Fig. 2d). The magnitude ranged between 0.6 and4.3,andthedepthrangedfrom5.4to34.4 km.Thespatialerrorsoftherelocation reported by hypoDD program are 13 to 558 m for the eastwest, 15 to 247 m for the northsouth direction and 16 to 605 m forthedepth.4.Results4.1.EpicenterdistributionOurresultsshowthatthemajorityofidentiedearthquakesoccurredinthevicinityoftheApril26mainshockarea,andonlyasmallnumberofeventswerelocatedintheeasternTaiwanandRyukyusubductionzones Fig.1.(a)GeneralmapofthetectonicenvironmentintheWesternPhilippineBasin.Thedashedgrayrectangleshowsthepositionof(b).(b)FocalmechanismsfromtheGlobalCMTcat-alog(ht t p: / /www.globalcmt.org /)fortheperiodbetweenJanuaryof1976andSeptemberof2012areplottedonthebathymetricmapoftheRyukyu-Taiwansubductioncollisionzone.Thered,blueandyellowbeachballsymbolsshowextensional,compressiveandstrike-slipfocalmechanisms,respectively.Blacklinesindicatetheoceanicfracturezonesdeterminedfromdetailedbathymetricdata(Hsue t a l., 2013).Thedetailedbathymetryofthearealocatedinthewhiterectanglein(b)andthetargetearthquake(c)areshown.Whitetrianglesin-dicatethepositionsofthedeployedOBSs.ThetwoopentrianglesaretheOBSsthatwerenotsuccessfullyrecovered.Thewhitearrowshowstherelativeplatemotion(Yue t al ., 1997).TheyellowfocalmechanismrepresentstheMw6.526April2010mainshockwithafocaldepthof24km.EU,Eurasia;GR,GaguaRidge;HB,HuatungBasin;LU,Luzon;LOFZ,LuzonOkinawaFractureZone;BMFZ,Batan-Miyakofracturezone;OT,OkinawaTrough;PHS,PhilippineSeaPlate;WPB,WestPhilippineBasin.500J.-Y.Linetal./Tectonophysics608(2013)499504

(Fig.2a).Thisresultcouldbeduetotherelativelygreatdistancebetweenournetworkandthetwotectonicallyactiveareas.NoeventwasreportedbytheglobalCMTcatalog(http://www.globalcmt.org /)andonly8earth-quakes have been recorded by the International Seismological Centercatalog (ISC, http://colossus.iris.washington.edu/) in our study areaduring the recording period, indicating the relatively small magnitudeofearthquakescollectedbyournetwork.Aftertherelocation,thedistri-bution of the aftershocks appeared to be more concentrated (Fig. 2d).ThemostobviousearthquakeclusterisaN45Wtrending,approximately19-km long feature (F1 in Fig. 2e) crossing the latitude range between Fig.2.Aftershockdistribution.(a)Epicentersof1476earthquakeswithatleastsixwavearrivalsrecordedbytheOBSnetworkandrelocatedbythe1-DIASP91globalvelocitymodel(Kennet t andEngdahl , 1991).(b)Epicentersof1409earthquakeslocatedinthebluerectangleareain(a).(c)Positionof527earthquakespossessingatleast10arrivalsandrelocatedbytheIASP91velocitymodel.(d)Positionsof326earthquakesrelocatedusingthehypoDDprogram(Waldhau s erandE l lsworth,2000)andthe1-DvelocitymodelinvertedusingtheVELESTprogram(Kis s lingetal. , 1994).AAandBBarethepositionsofthetwocrosssectionsshownin(f)and(g),respectively.(e)Possiblefaultgeometrydeterminedbythedistributionofaftershockclusters.F1,F2andF3arethethreemainclusters.Thebrownbroaddashedlinesshowtheancientfracturezoneandspreadingfabricsdeterminedfromdetailedbathymetrydata(Hsue t a l., 2013).ThefocalmechanismshowninyellowrepresentstheMw6.526April2010mainshock.(f)Earthquakeslocatedinthe10-km-widebandwidthsoneachsideoftheAAproleareplotted.(g)Earthquakeslocatedinthe3-km-widebandwidthsoneachsideoftheBBproleareplotted.Thegurelocatedintheupperportionof(f)and(g)showsthebathymetryofthetwocrosssections. Fig.3.(a)RMSdistributionasafunctionofthenumberofiterationsduringthe1-DvelocityinversionusingtheVELESTprogram.(b)Initialinputvelocitymodel(dashedlines)andtheircorrespondinginvertedmodel(solidlines).Theinversionwasperformedbyvaryingtheinitialvelocityvalueby13%.501J.-Y.Linetal./Tectonophysics608(2013)499504

approximately22.25Nand22.35N.Southwestofthecluster,inthevi-cinity of 22.24N, is another cluster characterized by a relatively smalllength of approximately 6 km (F2) that extends along a direction sub-parallel to that of F1. The third cluster (F3), occurring along a 17-km-longpath,intersectsF1atanangleofapproximately120.Regardlessofthe earthquake cluster, the western prolongation of these earthquakesappearstoberestrictedbyaNNESSW-trendingfracturezone(Fig.2d).4.2.CrosssectionTwoverticalcrosssections,oneorientedinaNESWdirectionandtheotherinaNWSEdirection,areshowninFig.2fandg(AAandBB).Thevertical distribution of earthquakes along Prole AAshowsthat the F1andF2earthquakeclustersbelongtotwo3-km-widestrips(Fig.2f).F1ischaracterizedbyalargerdepthrangeof835kmcomparedwiththerangeof1525kmfortheF2feature.Allotherearthquakesoccurredatdepths of 10 to 25 km. The depth distribution pattern showsthat F1 islikely the most important seismic structure, having the largest ruptureextent, and can pass through the oceanic crust to the lithosphere. TheNWSEProleBBshowsthattheearthquakesofF1becomedeeperto-ward the southeast (Fig. 2g). The lower limit is from 17 km for themost northwestern area, near the oceanic fracture zone, to 30km inthe southeastern portion. In addition, both proles show relatively lowseismicityinthedepthrangeof1114km.5.Discussion5.1.FaultplanedeterminationbasedontheaftershockdistributionBasedontheconcentrationandsizeoftheaftershockdistribution,weobservedthatthemajorityofthepost-seismicactivityoccurredalongF1andF2,twoN45Wtrendinglinearfeatures(Fig.2e).ThepresenceofasmallbathymetricscrapetrendingalongF1isalsoindicativeofactivede-formation along this structure (Figs. 2dand4c).Therefore,weproposethatthemainruptureshouldpropagateinaNWSEdirection,whichisconsistent with the NWSE fault plane solution of the Global CentroidMoment Tensor (CMT) focal mechanism (http://www.globalcmt.org /)forthemainshock.Inthiscase,asinistralrupturealonganapproximatelyvertical fault plane is determined for the 26 April 2010 Mw 6.5 earth-quake. Moreover, the strike-slip events, observed in the vicinity of theLOFZareaapproximately200 kmeastofourstudyarea,arecharacterizedbysimilarfocalmechanisms(Fig . 4b)(Linetal.,2013).Thebackgroundseismicity and swath-bathymetric features of this area also indicate aNWSEfaultplane(Matsumotoetal.,2001).Consequently,thepredom-inance of NWSE tectonic structures in the Western Philippine Basinisrevealed.Meanwhile,someNWWSEEbathymetriclineaments,representing the former tectonic fabrics, are observed on each side ofthefracturezone(thebrownlinesinFig.2e).Wenoticedthattheorien-tationofthesetectonicfabricsisnotinparallelwithF1andF2(Fig.2e),suggestingthattheoccurrenceofaftershockscouldnotbecorrelatedtothereactivationoftheancientpre-existingfeatures.ItisworthnotingthatF1 appearstobethemostwidelydistributedseismically active feature, along with the reported epicenter of themainshocklocated in F2, and is arelatively small cluster (Fig. 2d). Thisphenomenon can be discussed from two different perspectives: (1) Inthemarinearea,possibleearthquakelocationerrorscouldbegeneratedbyaninaccuratevelocitymodelandpoorraycoverage.Thus,ahypocen-tralmislocationmayhaveoccurred,andthehypocenterofthemainshockshouldbelocatedfarthernorthinthepositionofF1.(2)Themainshockdid occur in F2 and triggered the activity along F1. As determined bytheaftershockdistribution,thephysicaldimensionofF2isestimatedtobe about 5km by 8km, whereas the F1 is about 15km by 20km(Fig. 4c). Given a normal value of rigidity at the source location and anormal stress drop, the fault area of a Mw 6.5 event is expected to be Fig.4.(a)Principalcompressivestressdistribution(redlines)invertedbyWuetal.(2010)intheRyukyuTaiwancollisionzone.Thelengthoftheredlinesisproportionaltothehorizontalcomponent.ThefocalmechanismshowninyellowcorrespondstotheMw6.526April2010mainshock.TheblackandwhitedotsshowthedirectionofthePandTaxesofthemainshock.(b)FocalmechanismsoftheearthquakeslocatedinthePhilippineSeaPlatewithmagnitudeslargerthansixareplottedusingdifferentcolors.Aftershocksoccurringwithinonemonthaftertheoccurrenceofthemainshockareplottedusingthesamecolor.Thewhitearrowshowstherelativeplatemotion(Yue t al ., 1997).(c)Relocatedaftershockdistributionplottedasafunctionofitsmagnitude.TrianglesindicatetheOBSpositions.502J.-Y.Linetal./Tectonophysics608(2013)499504

on the order of 100200km , which is closer to the dimension of F1.2Moreover, the fault area inferred from aftershock distribution is some-what larger than that inferred from the theoretical fault model formostcase.Therefore,itseemsthatF1isabettercandidateforthepre-ferred rupture plane. Furthermore, it is widely accepted that most ofthe accumulated stress on the rupture area has been released duringthe mainshock. Thus, it is expected that aftershocks inside the mainrupture zone are relatively smaller than those on the peripheral oftherupturezone.Consequently,weidentifytheF1nodalplaneasthepreferredruptureplaneratherthanF2.5.2.ConjugatefaultandtectonicstressregimeAsdescribedpreviously,therelocatedepicentersaremainlydistrib-utedalongtwolinearfeaturesthatintersectatanglesofapproximately60 and 120 (Fig . 2e). This cross-cutting set of fault planes can beunderstoodasaconjugatefaultset:thedominantset,knownasF1andF2,formsintheNWdirectionandisthenlinkedbyasecondset,theF3feature, located along a NE alignment. This type of conjugate fault hasbeenreportedinseveraloceanicplates.Forexamples,thelargeJune18,2000, Mw 7.9 (13.87S, 97.3E) earthquake in the Wharton basin andthe 11 April 2012, Mw 8.6 and Mw 8.2 earthquakes off the west coastofnorthernSumatraappeartohaveinvolvedpredominantlyleft-lateralstrike-slipfaultingalongtheexpectedNNESSWorientation,althougha second fault orientation was also activated (Po l litz e t al., 2012;Robinsonetal ., 2001 ; Satr i anoe t al ., 2012 ; Yueetal ., 2012).Moreover,inNovemberof1987,anMw7.2earthquakeonafaulttrendingapprox-imatelyEWintheGulfofAlaskawasfollowedlessthan2 weekslaterby an Mw 7.8 earthquake on the conjugate NS fault (Pegler andDas , 1996). Generally, if strike-slip faults are formed by the Coulombfracture mechanism, which predicts the formation of X-shaped shearfractures 30 from the1direction, the intersection angles betweenconjugate strike-slip faults should be approximately 60 and 120(Donath , 1961 ; S i bsone t al ., 1988 ; Y i nandRana lli, 1992).Thismecha-nismisincompleteagreementwiththeresultobtainedfromourstudy.Todeterminetheoriginofthepresent-daystressregimeofourstudyarea,wecomparedthePaxisorientationobtainedfromthefocalmech-anismofthe2010mainshock(blackdotinFig . 4a)withtheprinciplecompressive stress directions inverted by Wu e t al . (2010) for thewesternmostportionoftheWPSParea(redbarsinFig . 4a).Asshownin Fig . 4a, the1direction is maintained within 5 from the relativeplate motion for the majority of central Taiwan. However, due to thetectonic compression induced by the collision between theLuzon ArcandtheEurasiaPlateinthesoutheasternregionofTaiwan,the1direc-tion deviated counterclockwise from an NWSE to an almost EWdirection(Wue t al.,2010),whichisconsistentwiththePaxisorienta-tion of the mainshock and other strike-slip events occurring in theWPSP. Based on the local seismicity and fault plane solutions, thisapproximatelyEW-trendingcompressionaldirectionwasalsoidenti-ed by Kao et al . (1998) for eastern Taiwan and offshore areas andwasconsideredaneffectofcollision.TheevidenceofthisEWregionalstressregimecanalsobeobservedinthefaultinggeometriesshownbythethreemainearthquakeclustersdeterminedinourstudy.Thethreeclusters show steeply dipping strike-slip orientations having either aleft-lateral slip on NWSE faults (Faults F1 and F2) or a right-lateralsliponENEWSWfaults(FaultF3),bothareconsistentwiththeperva-siveEWcompressionalstressorientationthroughouttheregion.Con-sequently, the arccontinent collision process between the Luzon ArcandtheEurasiaPlatemaybetheoriginofthestressregimefortheintra-plateeventslocatedwithintheWPSP.Hence,asimilarfocalmechanismpossessedbythestrike-slipevents,observedbetweenlongitudesofap-proximately123and126,maysuggestthattheresistingforceofthecollisionistransmittedoveradistanceofapproximately 500 kmfromeastern Taiwan to the LOFZ area (Fig. 4b). This type of stress transferwaspreviouslyreportedfortheWhartonBasinandoffshorenorthernSumatra,wherethecompressionalaxesofearthquakesareconsistentlyorientedinaNWSEdirection.Thisorientationindicatesthattheintra-platestressesintheregionareprimarilyinheritedfromtheIndia-Asiacollisionoveradistanceofapproximatelyseveralthousandkilometers(Dep l usetal ., 1998;Rob i nsone t a l., 2001).Toextendourdiscussion,weputforwardseveralquestions.First,inotheroceanicplatesaroundtheworld,suchasintheGulfofAlaska and the Wharton Basin, most intraplate earthquakes haveoccurredalongareactivatedrelictransformsandrelicridges.However,based on our results, no obvious seismic activity occurred along theformertectonicstructuresintheWPSParea(Fig.2a).AlloftheancienttectonicstructuresintheWPSPappeartobeseismicallyinactiveinthepresentday.Forexample,aportionoftheGaguaRidgeislocatedinournetworkandissurroundedbythesixwestern-mostOBSstations;how-ever,onlyafaintearthquakewasrecordedalongthisstructureduringour recorded period. Moreover, the orientation of the earthquakeclusters determined from the OBS data does not appear to extendalong the oceanic fracture zones or fabrics but has an intersectingangle(Fig.4b).Thus,therstquestionraisediswhytheoceanicfracturezones in the WPSP, which represent the tectonically weak area, havenotbeenreactivatedbytheearthquakesthathaveoccurredwithintheoceanic plate. In the WPSP as well as in the other tectonic areas, wenotethatthePaxisofthefocalmechanismsisconsistentwiththedirec-tionofcollisionregardlessoftheorientationofpre-existingweakzones.Therefore,weproposethattheregionalprinciplestressdirectionisthemainfactorcontrollingtheactivefaultplaneorientationinthepresentday, and the inuence of the pre-existing fracture zones should beminor.However, thepresenceofthepre-existingweaknesscontinuesto have certain effects on the distribution of large strike-slip events.Forexample,asshownbyourresults,almostalloftherecordedafter-shocksstopsuddenlyontheeasternsideofafracturezone,indicatingthatthepre-existingfeaturescouldactasabarriertorupturepropaga-tion. In addition, we note that the majority of relatively large earth-quakes occurred within the vicinity of the oceanic fracture zones(F i g.4b),suggestingthatthepre-existingweakzonemayfavorthegen-erationofearthquakesduetoitsrelativelylowrockstrengthcomparedwiththecompact,un-rupturedoceanicplate.Inaddition,basedonthepreviousdiscussion,wefoundthatthetec-tonic context within the WPSP area is similar to that in the WhartonBasin:shorteningbythrustearthquakesoccursonthewesternside,andthe oceanic lithosphere subducts along the trench system on the otherside.Thistectonicenvironmentcouldbethecauseofthesimilarcharac-teristics of the intraplate earthquakes located within the two oceanicareas.Consequently,analogousinteractionsbetweentheunderthrustingevents at the subduction interface and intraplate deformation offshoremaybeexpectedforthesetwoareas.Delescluseetal.(2012)demonstrat-ed that the 11 April 2012 twin strike-slip earthquakes were part of acontinuing enhancement of the intraplate deformation between Indiaand Australia that followed the Ache 2004 and Nias 2005 megathrustearthquakes.WillasimilartectonicprocessoccuralongtheRyukyusub-duction system? What do these earthquakes reveal about earthquakephysics,andhowmighttheychangeearthquakehazardassessment?Allofthesequestionsarecrucialandrequirefurtherinvestigation.6.ConclusionsOn26April2010,astrike-slip-typeearthquake(Mw6.5)occurredontheeasternportionoftheGaguaRidge.Wedeployed14OBSstorecordtheaftershockstoacquireinformationregardingthesestrike-slipearth-quakes in the oceanic plate. A total of 326 aftershocks were relocatedbased on the hypoDD analysis, and an appropriate 1-D velocity modelof the area was derived using the VELEST program. Our results showthat the majority of identied earthquakes occurred in the vicinity ofthemainshock area. Three clusters ofearthquakes were identied: twoof them trend along the N45W direction, and the other intersects therst two clusters at an angle of approximately 120. The origin of thestress regime is evidenced by the consistency of the P axis of the focal503J.-Y.Linetal./Tectonophysics608(2013)499504

mechanism for the mainshock and the principle compressive stressresultingfromthecollisionoccurringinsoutheasternTaiwan.Thus, thediffuse intraplate seismicity in this WPSP region should result from thecollision between the Luzon Arc and the Eurasia continental margin. Inaddition,theinuence ofthiscollisionprocessappearstohavereachedtheLOFZarea,approximately500 kmfromtheeasterncoastofTaiwan,over avery long distance. Because the strike of the earthquake clustersisdistinctfromthatoftheancienttectonicfeatures,theseismicitydoesnot appear to be linked to their reactivation. However, the abrupt haltoftheaftershockdistributionattheborderofthefracturezonesuggeststhatthepre-existingfeaturescouldactasabarriertotherupturepropa-gation.Inaddition,most large earthquakes occurred withinthevicinityof the oceanic fracture zones, suggesting that the pre-existing weakzonemayfavor thegenerationofearthquakes duetotherelativelylowrockstrengthcomparedwiththecompact,un-rupturedoceanicplate.AcknowledgementWethankDr.HonnKaoandananonymousreviewerfortheircarefulreviewswhichhelpalottoimprovethismanuscript.Pertinentdiscus-sionswithDr.Wen-NanWu,Chung-LiangLoandWen-BinDooareac-knowledged.WearegratefultoDr.Wen-TzongLiangforprovidingusassistancewithdataprocessing.FigureswerepreparedwiththeGenericMappingTool(GMT)software(Wesse l andSm i th , 1998).ThisresearchwassupportedbytheTaiwanEarthquakeResearchCenter(TEC)fundedthroughtheNationalScienceCouncil(NSC)withgrantnumbersNSC-102-3113-P-008-006 and NSC-102-2116-M-008-024. 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