JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B1, PAGES 1015-1033,JANUARY 10, 1999 Detailed structuresof the subductedPhilippine Sea plate beneath northeast Taiwan' A new type of double seismiczone Honn Kao and Ruey-JuinRau Instituteof Earth Sciences,Academia Sinica, Taipei, Taiwan Abstract. We studiedthe detailedstructure of the subductedPhilippine Sea plate beneath northeastTaiwan whereoblique subduction, regional collision, and back arc openingare all activelyoccurring. Simultaneousinversion for velocitystructure and earthquakehypocenters areperformed using the vast,high-quality data recorded by the TaiwanSeismic Network. We furthersupplement the inversionresults with earthquakesource parameters determined from inversionof teleseismicP and $H waveforms,a criticalstep to definethe positionof plateinterface and the stateof strainwithin the subductedslab. The mostinteresting feature is that relocatedhypocenters tend to occuralong a two-layeredstructure. The upperlayer is locatedimmediately below the plate interfaceand extendsdown to 70-80 krn at a dip of 40ø- 50ø. Below approximately100 km, the dip increasesdramatically to 700-80ø. The lower layer commencesat 45-50 km andstays approximately parallel to the upperlayer with a separationof 15+5 km in betweendown to 70-80 km. Below thatthe separationdecreases andthe two layersseem to graduallymerge into one Wadati-BenioffZone. We proposeto term the classicdouble seismic zones observed beneath Japan and Kuril as "type I" andthat we observedas "typeII," respectively.A globalsurvey indicates that type II doubleseismic zonesare alsoobserved in New Zealandnear the southernmostNorth Island,Cascadia, just northof the Mendocinotriple junction, and the Cook Inlet areaof Alaska. All of them are locatednear the terrniniof subductedslabs in a tectonicsetting of obliquesubduction. We interpretthe seismogenesisof type II doubleseismic zones as reflecting the lateralcompres- sivestress between the subductedplate and the adjacentlithosphere (originming from oblique subduction)and the downdipextension (from slabpulling force). The upperseismic layer representsseismicity occurring in the uppercrust of a subductedplate and/or along the plate interface,whereas the lower layer is associatedwith eventsin the uppermostmantle. 1. Introduction ward beneathNE Taiwan to a depth of at least 150 km (Plate Northeast Taiwan is a tectonically complicated region 1) [e.g., Kao et at., 1998a]. (Figure 1). The Philippine Sea plate subductsbeneath the The regionaltectonic setting is further complicatedby the Eurasiaplate along the Ryukyu arc at a rateof 7+4cm yr -1 existence of numerous extensional features in the back arc re- [Senoet at., 1993]. Most of the Ryukyu trench strikesNE- gion, namely,the Okinawa trough(Figure 1). Evidencesfrom SW except to the west of 125øE where the orientation seismicity [e.g., Ouchi and Kawakami, 1989; Sato et at., becomesnearly E-W. Bathymetricsignature of the trench 1994], earthquakefocal mechanisms[e.g., Kao and Chen, disappearsto the west of 123øE. The westernterminus of the 1991; Shiono et at., 1980], and seismic reflection profiles arc is boundedby the active collisionbetween the Luzon arc [e.g.,Sibuet et at., 1987] all indicatethat the troughis actively and Eurasia continent in the Taiwan region (Figure 1), an deforming,presumably reflecting the present-dayprocess of ongoingevent that began-4 Ma [e.g., Lee and Lawyer, 1994; back arc opening. Teng, 1990; Wu, 1978b]. Given the complextectonic setting in the region, the pur- Previous studies have shown that seismic patterns along pose of this study is thereforeto addressthe fundamentalis- the Ryukyu arc vary systematicallyfrom north to south[e.g., sue as how the subductedlithosphere responds to various Kao and Chen, 1991; Shiono et al., 1980]. In additionto the tectonic processes,both compressionaland extensional,at typical subduction,the subductedPhilippine Sea plate exhib- shallower depths. In particular, does the regional collision, its increasingamounts of lateralcompression as it approaches which results in the complex orogeny in Taiwan, also have Taiwan. Such a patternis interpretedas a resultof the trans- profound effects at depth? If indeed they do, what is the mitted strain originated from the collision [e.g., Kao and depth range and how are they reflected in terms of the seis- Chen, 1991; Kao et at., 1998a]. Local seismicityclearly indi- mogenicbehavior within the subductedlithosphere? cates that the subductedPhilippine Sea plate extendswest- To answerthe above questions,we investigatethe detailed configurationof the subductedPhilippine Sea plate beneath NE Taiwan where subduction,collision, and back arc opening Copyright1999 by theAmerican Geophysical Union. are all actively occurring. Our resultis basedon the joint in- version of three-dimensional(3-D) velocity tomographyand Papernumber 1998JB900010. relocation of earthquakehypocenters using the vast, high- 0148-0227/99/1998JB900010509.00 quality datarecorded by the Taiwan SeismicNetwork (Figure 1015 1016 KAO AND RAU' NEW TYPE OF DOUBLE SEISMIC ZONE 28 ø E. China Sea 26 ø 2.00 24 ø • RyukyuTrenCh Philippine Sea 22 ø Plate 7+4 cm/yr 118 ø 120 ø 122 ø 124 ø 126 ø 128 ø Figure 1. Map showingbathymetry in the Ryukyuarc-Taiwan region. Morphologyof the Ryukyutrench disappearsto the west of 123øEwhere it is interceptedby the GaguaRidge. The PhilippineSea plate is movingat 7+4 cm yr -1 toward NW relative to the Eurasia plate, resulting inthe Taiwan Collision Zone. The regionis furthercomplicated by theopening process of theOkinawa trough to thenorth. Solidtriangles mark the stationlocations of the Taiwan SeismicNetwork. Its apertureand high-quality digital data provide excel- lent resolutionto delineatedetailed velocity structures associated with the subductedPhilippine Sea plate un- derneath NE Taiwan. 1 and Plate 1) [Shin, 1993]. We furthersupplement the result Zealand near the southernmostNorth Island [e.g., Reynerset with earthquakefocal mechanismsand depths determined al., 1997], Cascadiajust north of the Mendocinotriple junc- from inversionof teleseismicP and$H waveforms.This step tion [e.g., Smith and Knapp, 1993], and Alaska in the Cook providescritical informationon the natureof distinctseismo- Inlet area [e.g., Ratchkovskyet al., 1997]. All four regions genic structuresand henceavoids the possibilityof mistaking have a commontectonic setting, that is, locatednear the ter- the interplatethrust zone or structuresin the overridingplate minus of an oblique subductionsystem with a significantlat- as the Wadati-Benioff Zone (WBZ) [e.g., Kao and Chen, eral compressivestress field. We interpretthe observedtwo 1991, 1995; $eno and Kroeger, 1983]. layersof a type II doubleseismic zone as earthquakesin the In the following text we first examinethe local seismicity subducted(and probably thickened) crust and uppermost along two profiles,one directly beneathNE Taiwan and an- mantle, respectively. The regional collision and subduction other in the near offshore,to identify the overall configuration providethe necessarydeviatoric stresses for the seismogene- of the subductedPhilippine Sea plate. The effectsof regional sis in the correspondinglayers. Such an interpretationcan collision are identified from the changingpatterns of earth- alsoexplain the largegeometric variation of WBZ at depth. quake focal mechanisms.Then we show the resultsof joint inversionof 3-D velocity tomographyand earthquakereloca- 2. Local Seismicityand Focal Mechanisms tion. The most interestingfeature of our resultsis that relo- cated hypocenterstend to form a two-layered structure(a doubleseismic zone). It differs from the classicdouble seis- Plate 1 showsthe distributionof seismicitybetween 1991 mic zones(e.g., thoseobserved beneath the Japanand Kuril and 1996 reportedby the SeismologicalObservation Center arcs)in both geometryand depthrange. To avoid any confu- of the CentralWeather Bureau, Taiwan, along with the loca- sion, we proposeto term the classicdouble seismic zones as tions of local seismographicstations. While shallowseis- "type I" and that we observedas "type II," respectively. We micityis very intensealong the easterncoastline and offshore, alsofind that the dip of WBZ steepensat depthsgreater than most earthquakesdeeper than -60 km occurredwithin the -90 km. WBZ striking-E-W that extendswestward beneath NE Tai- Finally, we compareour resultsto similar studiesin other wan. The westernboundary of the subductedPhilippine Sea major subductionzones and discussthe correspondingtec- slabroughly follows the 121.6øEmeridian between depths of tonic implications. It is noted that type II double seismic 60 and 200 km. Earthquakesdeeper than 200 km occurred zoneswere documentedpreviously along three regions: New mostlyto the eastof 122øEin a significantlyless number. KAO AND RAU' NEW TYPE OF DOUBLE SEISMIC ZONE 1017 26 ø i ] • ßee ß • ß I 25 ø •' "' ,( C)o 0 o t . o 24 ø • >200 100-200 ,o o A o o 60-100 0 0 0-60km o 123 ø ML = 3 4 5 6 Plate 1. Map showinglocal seismicityin the southernmostRyukyu arc-Taiwanregion. Hypocenters(cir- cles)were reported by the SeismologicalObservation Center of the CentralWeather Bureau, Taiwan. Differ- ent colorsand sizesrepresent different depths and magnitudes (Mr), respectively.Triangles show the station locations of the Taiwan Seismic Network. Profiles A-A' and B-B' mark the locations of cross sections in Figure3 showingthe distributionof projectedhypocenters with depth. 1018 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE