Detailed Structures of the Subducted Philippine Sea Plate Beneath

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 inversion 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 excellent 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 otherin 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

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Plate 1. Map showinglocal seismicityin the southernmostRyukyu arc-Taiwanregion. Hypocenters(circles)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 1019

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23" 121" 122" 123" Figure 2. Map showingthe epicentersand focal mechanismsof large- and moderate-sizedearthquakes occurringin the region. Equal-areaprojections of the lowerhemispheres of the focalspheres for 28 earthquakes selectedfrom Kao et al. [1998a] are plotted showingthe orientationof the nodal planes. Darkened areas showquadrants with compressionalP wave first motions;black andgray shadingsrepresent events shallower and deeperthan 60 km, respectively.Each fault planesolution is labeledby the correspondingevent number and focal depth (accordingto Table 1). Four major seismogenicstructures associated with the subducted Philippine Sea plate are delineated,as indicatedby different symbolsfor epicenters:collision seismic zone (solid squares),interface seismic zone (solid circles), Wadati-Benioff Zone in downdipextension (open triangles)and compression(solid triangle), and lateralcompression seismic zone (open stars).

Kao et aL [1998a] recentlydetermined the sourceparame- were determined from inversion of teleseismic P and SH ters of large- and moderate-sizedearthquakes that occurredin waveforms[N•belek, 1984]. We adapttheir resultsand listed the regionto investigatethe characteristicsof transitionfrom 28 selectedearthquakes in Table 1. In Figure 2, theseevents the obliquesubduction in the southernmostRyukyu arc to the are separatedinto groups,as representedby differentsymbols, collision in Taiwan. Precisefocal depthsand mechanisms based on their distinct characteristics of focal mechanisms. 1020 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE

18 15 20

26 22 27 21 19 25

B • A • 0

17 •o

-100 - o - -100 o

o o

-200 - - -200

Figure 3. Cross sections(without vertical exaggeration)showing local seismicityand focal mechanisms alongprofiles A-A' andB-B' in Plate 1. Faultplane solutions of earthquakesin Table 1 andFigure 2 (within -50 km from the profiles)are shownin equal-areaprojection of the back hemisphereof the focal sphere. Layout of shadingsand smallsymbols are the sameas in Figure2. In crosssection A-A', a typical subduction zoneis observed,with the additionof earthquakesshowing lateral compressional strain (events 1, 2, 3, 5, 7, and 17). The seismogenicportion of plateinterface is clearlyshown by manylow-angle thrust earthquakes at shallowdepths. The Wadati-BenioffZone (WBZ) dipsat 400-50ø between60 and130 km, but steepens slightlyto -60 ø at greaterdepths. The seismicpatterns in crosssection B-B', on the otherhand, showtwo majorfeatures. Earthquakes at shallowdepths are mostly thrust events related to the regionalcollision. Only onelow-angle thrust event (21) is observedthat marks the positionof plateinterface to the eastof thisprofile. The intraplateseismicity within the subductedplate seemsto distributealong a two-layeredstructure that spansfrom-40 to-80 km.

At shallow depths(_<35 km), the predominantfeature is the profile A-A', the configurationof seismogenicstructures is largenumber of eventsshowing low-angle thrust mechanisms basicallythe same as that in the southernRyukyu arc [e.g., locatedto the eastof Taiwan (events6, 10, 15, 18, 19, 20, 21, Kao and Chen, 1991]. While intenseseismicity was found 25; solid circles). These eventspresumably reflect the rela- along the plate interfaceand in the back arc region, lateral tive slip betweenthe subductedPhilippine Sea plate and the compressionalearthquakes were observedat some greater overridingEurasia plate along the interface. The collision- depths(_<50 km). The WBZ hasan averagedip of 400-50ø, as related compressivestrain is representedby two groups:one clearly defined by numerousintraplate earthquakes occurred alongthe easterncoastline at depthsless than 35 km (collision between 60 and 130 km. Below that, it seemsto steepen seismiczone; events9, 22, 23, 24, 27; solid squares)and an- slightly to-60 ø, althoughthe number of events is signifi- other extendingeastward from NE Taiwan to southernRyu- cantly less. kyu in a wider deeprange (lateral compressionseismic zone; On the other hand, seismicpatterns along profile B-B' events 1, 2, 3, 5, 7, 12, 17, 26, 28; stars). All these events show two major features. For depths>40 km, the subducted have maximumcompressive axes (P axes) in either E-W or PhilippineSea plate extendswestward to beneaththis profile NE-SW directions. Earthquakesin the shallowpart of WBZ and is clearly visible. The plate boundaryat shallowdepth, (_<120kin; events4, 8, 11, 13, 14; opentriangles) consistently however, is greatly distortedand locatedto the east of this showmaximum tensile axes (T axes)in the slab'sdowndip di- profile [Kao et aL, 1998a], as representedby event21 show- rection(i.e., downdipextension). However, the stateof strain ing typical low-angle thrustmechanism (Table 1 and Figure switchesto downdipcompression at a greaterdepth, as indi- 2). The denselydistributed earthquakes at depths<40 km catedby event 16 (solidtriangle; Figure 2). showpredominating thrust faulting with P-axes in E-W/NE- In Figure 3, local seismicity(Plate 1) and focal mecha- SW directions(Figures 2 and3). They presumablyreflect the nisms (Figure 2) are plotted together along two profiles ongoingprocess of collisionand may not be associatedwith trending N15øE-S15øW, a direction approximatelyperpen- the subductionof PhilippineSea plate at all. Settingaside the dicular to the strike of the subductedslab. Along the eastern collision-relatedevents at shallow depths, intraplate seis- KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE 1021 micity within the subductedlithosphere seems to distribute only prominentarrivals are admittedinto our dataset and only along a two-layeredstructure that spansfrom-40 to-80 km. horizontal componentsare used to time $ observations. In At a first glance,this could be an artifact from depthuncer- general,the readingerror is limited to only a few sampling tainty. However, as we shall illustratein the next section,the intervals(i.e., 0.02-0.05 s). The total numbersof P and $ ar- two-layereddistribution of seismicitybecomes even betterde- rivals used in the inversionare 42,806 and 22,256, respec- fined afterthe joint inversionof 3-D velocity tomographyand tively. earthquakerelocation. The averagedip of the WBZ seemsto We used a grid systemof 10 layerswith the orthogonaldi- increasewith depthfrom-35 ø at 40 km to-65 ø at >100 km. rectionsparallel (NI5øE, the Y direction) and perpendicular It is interestingto note that eventsshowing P axesparallel (S75øE,the X direction)to the local strikeof the slab(Plate 1 to the local strike of the arc are all confinedat depthsless than and Figure 4). The dimensionof our model is 100 km (X di- -100 km (Figure 3). Theselateral compressionalevents also rection) by 110 km (Y direction)by 150 km (depth). The show consistentT axes in the downdipdirection of the sub- distancebetween each layer dependson the averagenumber ducted slab if they occurredbetween 40 and 100 kin, pre- of passingrays, rangingfrom 5 km at the top to 40 km at the sumablywithin the WBZ (e.g., events 1, 2, 3, 5, 7, 13, 17 in bottom. Seismicvelocities and locationsof hypocenterswere profile A-A' and events8, 11, 28 in profile B-B'; Figure 3). inverted iteratively by a damped, linear, least squarestech- Such a pattern can be explainedas the result of interaction nique from the observedarrival times, as depictedby Thurber betweenthe compressivestress regime due to regional colli- [ 1983, 1993] and Evans et al. [ 1994]. sion (which is most significantwithin the lithospherein the The 3-D inversionconverged after six iterations,reducing lateral direction[Kao and Chen, 1991]) and the downdipex- the weighted root-mean-square(RMS) arrival time residual tensionalstress regime due to slab subduction[e.g., Spence, from its initial value of 0.34 to 0.21 s. The averageuncer- 1987]. Furthermore,there seemsto have no obviousdiffer- tainties of earthquakelocations are 3 km horizontallyand 5 ence between the fbcal mechanismsof events in the upper km vertically. For some deeper earthquakesand those oc- (events1 and 28) and lower (events2, 3, 5, 7, 8, 11, 13, and curredjust outsidethe network, uncertaintiesmay increaseto 17) layers. 10 km horizontallyand 15 km vertically. To quantify the goodnessof fit of the model parameters, 3. Three-Dimensional Tomography and two approacheswere employedin this study. First, the spread Relocation of Earthquake Hypocenters function of the model resolution was calculated based on the L2 norm of the difference between the resolution matrix and With the help of focal mechanismsof large and moderate- an identity matrix [Menke, 1989]. Velocity grid point with sized earthquakes,it is rather straightforwardto identify the smallerspread function represents higher resolution. Second, stateof strainof variousseismogenic structures. Nonetheless, a synthetictest named "restoringresolution test" [Zhao et al., the locationsof hypocentersreported in local bulletinsmight 1992] was conducted. The test takesthe tomographicimage lack the requiredresolution to constrainthe detailedgeometry obtainedby inverting the real data set as the referenceand of the subductedslab becausepossible mislocation resulted computesthe synthetic.travel times between the sourceand from assuminga 1-D layeredmodel could introducesome ar- receiver locations. After adding random noise, the same in- tifactsto the image. Suchexamples have been suggestedby versionprocedure was performedusing the syntheticdata set. many previousstudies to explainan apparentkink of WBZ at By comparingthe two inversionresults, we may know how -100 km beneaththe Alaska-Aleutian arc [e.g., Frohlich et well the tomographicfeatures can be recovered. al., 1982; Hauksson, 1985; McLaren and Frohlich, 1985]. Consequently,we performa joint inversionfor 3-D veloc- 3.2. Results ity tomographyand earthquakerelocation by taking advantage of the vast, high-qualitydigital datarecorded by the Tai- Plate 2a showsa crosssection along profile C-C' in Figure wan SeismicNetwork. Consideringthe distributionof seis- 4. The locationof this profile follows much of the profile B- mic stations(and thereforethe tomographicresolution of data; B' in Plate 1, only being slightly shorteras limited by the di- Plate 1), we focusour effortson the NE Taiwan regionthat is mension of the seismic network. Relocatedhypocenters to completelycovered by the network. the east of the profile within a distancerange of 25 km are projectedonto the crosssection to best show the detailedge- 3.1. Data and Method ometry of WBZ. The Taiwan Seismic Network has a total of 75 stations The most interestingfeature in Plate 2a is the significant distributingdensely on Taiwan and offshoreislets (Figure 1). diffbrencein the seismicpatterns within the subductedPhilip- Three-component,digital data (sampling at 100 Hz) are pine Sea plate at greater depthsand in the collisionzone at transmittedreal time to the SeismologicalObservation Center shallowdepths. In the collisionzone, earthquakestake place of the Central Weather Bureau where the routine determina- more or lessuniformly between0 and 50 kin, whereasin the tion of earthquakehypocenters is performed.From January 1, Philippine Sea plate earthquakestend to occur along a two- 1991, to March 31, 1997, a total of 69,024 earthquakesrang- layered structureand form an apparentdouble seismic zone. ing from 0.5 to 7.0 in ML were reported. Out of this dataset, The locationof the upper layer is immediatelybelow the in- 2480 events are selectedfor the inversion(Figure 4). The terplate thrust zone inferred from earthquakefocal mecha- event selectioncriteria are (1) the earthquakeshaving P arri- nisms(Figure 3). This layer extendsdown to 70-80 km at a vals recordedat more than eight stationsand (2) the largest more or lessconstant dip of 40ø-50ø . Below approximately azimuthalgap betweenstations being less than 180ø. Seis- 100 km, the dip increasesdramatically to 70ø-80ø. In other mogramsrecorded at 39 stationsin the northernpart of Tai- words, there seemsto have a significantgeometric variation wan are carefully examinedto determinearrival times for the in the shapeof the WBZ acrossthe depth range of 70-100 first P and S phases. To ensurethe highestaccuracy in time, km. 1022 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE

:25' 00'

24' 30'

o 0%0%o 0 ß o • 24' 00'

o o o o

121' 00' 121' 30' 122' 00'

. o o 0

ML= 2 3 4 5 6 Figure 4. Distributionof earthquakeepicenters (open circles) and stations (solid triangles) used in ourjoint inversionfor velocitystructure and hypocentral locations. The grid systemconsists of 10 layerswith the or- thogonaldirections parallel and perpendicularto the local strikeof the slab. Profile C-C' marksthe location of crosssection shown in Plate 2 and Figure5. The plusmarks the earthquakethat generatedclear S to P convertedphase recorded at stationTWA, as shownin Figure6.

The lower layer commencesat 45-50 km and maintains betweenthe two layersis not well definedbecause the reloca- approximatelyparallel to the upperlayer with a separationof tion errorsof earthquakesoccurred in the upperlayer seemto 15+5 km in betweendown to 70-80 km. Similarto the upper overlapwith that of the lowerlayer. This patternimplies that layer, the dip of the lower layer increasesat depths>90-100 the two layers are separatedby no more than-15 km at km, but with a value of about 10ø-15 ø less. Thus the distance greaterdepths. betweenthe two layers decreaseswith increasingdepth and The tomographicimage clearly shows a high-velocity they seemto mergeinto one WBZ at depth_>130 km. anomalydipping toward north with increasingdepth (see the The resultof our 3-D relocationmakes the gap betweenthe 8.0and 8.5 km s -] isopachsinPlate 2a), presumably indicat- two seismiclayers more prominent(compare Plate 2a to the ing the location of the subductedslab. The location of this profile B-B' in Figure 3). In Figure 5a, we plot the corre- high-velocityanomaly is consistentwith the distributionof spondinghypocentral uncertainties. From the distributionof seismicity.The 8.0 km s -] isopachfollows approximately the uncertainties,it seemsvery unlikely that the separationof the top boundaryof the upperseismic layer between depths of 50 two layers is merely due to mislocationof earthquakehypo- and90 km. In additionto the high-velocityanomaly observed centers,particularly for depthsbetween 40 and 80 km where at depth,a prominentlow-velocity region is mappeddirectly the resolutionof our 3-D tomographicinversion is good (a abovethe subductedslab in the backarc region, which proba- subjectwe shall addressnext). At 80 km andbelow, the gap bly correspondsto the mantle wedge (Plate 2a). Previous KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE 1023

(a) Spread Function

-40

•. -8o

-120

40 80 120 160 Distance (km)

(b) Hypocentral Location Uncertainties

-12o

' I ' I ' I ' 0 40 80 120 160 Distance (km)

Figure 5. (a) The associateddistribution of spreadfunctions. Smallernumber means higher resolution. Similarto Plate2b, mosttomographic images are well resolved(<3) exceptfor regionsnear the northernand bottomedges of ourmodel. (b) The hypocentraluncertainties of earthquakes.It seemsvery unlikelythat the separationof thetwo layersis merelyan artifactdue to earthquakemislocation 1024 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE seismicand geodeticstudies have suggestedthat the Ilan In our case, owing to the unique tectonic setting of arc- Plainis understrong influence of the Okinawatrough system continent collision, the station distribution is excellent such [e.g.,¾eh et at., 1989;Liu, 1995]. Our resultsindicate that that the local network has sufficient aperturein both along- the centerof the openingcan be traceddown to as deepas and normal-to-the-arcdirections (Figure 4). This advantage -30 km,as shown by the 6.0 km s -1 isopach. Seismic activi- providestight constraintson the earthquakehypocenters and tiesinside the trough concentrate within the very top 10 km, a thus significantlyreduces the possiblelocation error. As far patternconsistent with the inferencethat the low-velocity as the slab effect is concerned,we have performedjoint inver- anomalyis associatedwith thepartial melting of crustalmate- sionof 3-D velocity tomographyand earthquakerelocation, a rialsresponsible for the Okinawatrough opening. processthat automaticallytakes various velocity anomalies Plate2b andFigure 5a showthe resultof "restoredresolu- into account,including not only the high velocity subducted tion test"and the spreadfunction associated with our 3-D in- PhilippineSea slabbut alsothe low-velocityOkinawa trough version. For the synthetictest, randomnoise in normal at shallowerdepths. Hence we believethat the variouseffects (Gaussian)distribution with standard deviations of 0.05s (the ofhypocentralmislocation on our resultsshould be minimum. largestpossible reading error), 0.1 s, 0.2 s, 3 km, and5 km Finally, it is notedthat the mislocationresulted from either was addedto the error-freeP, $-P arrival times, origin times, 3-D velocity complexityor inadequatenetwork configuration horizontal,and vertical hypocentrallocations, respectively. is by no means random but systematicallyincreasing with Three inferences can be concluded from these results. First, depth [e.g., McLaren and Frohtich, 1985]. Thus even if the the tomographicimages obtained from the inversionof real slab effects cannot be completely eliminated, the most we data are, on the whole, correctlyreconstructed from the syn- would expect is a slight changein the dip angle of WBZ at thetic data. Second,the resolutionis generallygood for the depth. It seemsunlikely to radicallyoverthrow our inferences obtainedimages. All the importanttomographic features of the existenceof a type II doubleseismic zone between40 (e.g.,the subducted lithosphere and the Okinawa trough) have and 100 km and the significant geometricvariation of the spreadfunctions less than 3, indicatingthat they are not arti- subductedPhilippine Sea plate. factsdue to insufficientdata resolution. Finally, althoughthe resultanthypocenters from inversion of the syntheticdata are 4. Interpretations and Discussion more scatteredthan that from real data, main patterns of earthquakedistribution remain unchanged. Notice that the differencein calculatedtravel time using From the locationof low-anglethrust earthquakes and the betweenthe Cartesiancoordinate (Thurber's code, this study) high-velocityanomaly in the tomographicimage (Figures 3 and the sphericalcoordinate (which shouldbe used for the and 5 and Plate 2), it seemsthat the two-layeredWBZ is lo- Earth being a sphere)may exceed0.1 s if the epicentraldis- catedwithin the subductedPhilippine Sea plate with theupper tancesand focal depthsare greaterthan -120 and 100 km, re- layer immediatelybelow the top boundaryof the slab. To spectively [Sato, 1978]. Such a calculatedtravel time error furtherconfirm this inference,which is crucialto possiblein- (0.1 s) is slightly higher than the overall readingerror of our terpretations,we carefully examinea large quantityof seis- data set and may affect the resolutionof the resultedvelocity mogramslooking for convertedphases to pinpointthe top images and/or the relocatedhypocenters. Fortunately,less boundaryof the subductedlithosphere [e.g., James and $noke, than 0.5% of our data are in the ray path rangecorresponding 1994; Matsuzawaet at., 1986; Zhao et at., 1997]. Our efforts to greatertravel time errorsand all of them have focal depths had limited success,and one exampleis shownin Figure6. >100 km, thus having little effect on the focus (depths< 80 The eventtook placenear the top of the lower seismiczone km) of the presentstudy. at 65 km and the recordingstation (TWA) is in northernTai- wan (Figures4 and6b). The particlemotion of the prominent convertedphase (marked by an "X" in Figure 6a) clearly re- 3.3. PossibleHypocentral Mislocation semblesthat of the P ratherthan $ phases,suggesting that it is It is well documentedin the literaturethat a high-velocity probablyan $-to-P conversionat the top boundaryof the sub- subductedslab might introducesignificant hypocentral mislo- ductedplate. The time differencesfrom P to X and from X to cationthat leadsto artifactsin the seismicpattern. One of the $ are 1.69 and 6.25 s, respectively. Given the 3-D velocity most famous examplesis the sharp kink in WBZ along the model (Plate 2 and Figure 5), they correspondto a boundary centraland easternAleutian at a depthof-110 km [e.g.,Eng- 17-20 km above the hypocenter(Figure 6b), a pattem re- daht, 1977; House and dacob, 1982; Reyners and Cotes, markably consistentwith our previousconclusion. Unfortu- 1982]. While seismicityshows the WBZ in two segments nately, we were unableto find sufficientnumber of clear con- with the deeperdips 20ø more than the shallowerone, many vertedphases to map out the boundaryof the subductedplate studiesargued that the sharpdip increaseis merely an artifact in further detail. Future deploymentof a denselydistributed due to systematicmislocation of hypocentersusing local data high-resolutionbroadband array in the regionwould probably with conventionallayered model [e.g., Frohtich et at., 1982; be necessaryto resolvethe issue. Hauksson,1985; McLaren and Frohtich, 1985]. Another sourceof mislocationis due to the poor azimuthal 4.1. Double SeismicZones: Type II versusType I coverageand/or limited apertureof local networks. Accord- Double seismic zones within the subducted slab were ing to the control study of McLaren and Frohtich [1985], documentedfor the Japanand Kuril arcsmore than two dec- earthquakehypocenters located by a local networkwith most ades ago [e.g., Umino and Hasegawa, 1975; Veith, 1974]. stationsdistributed along the strike of arc (i.e., on smallvol- Subsequently,double seismiczones have been reportedfor canic islands)could be shiftedby as much as 100 km. Fur- severalother regions, includingvarious segmentsalong the thermore,there would be an apparentincrease in the dip of Japanarc [e.g., Hasegawa et al., 1978; Kawakatsuand $eno, WBZ beneatha certaindepth. 1983; Matsuzawa et at., 1986], the Kuril arc [e.g., Kao and KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE 1025

(a) C RealData Inversion C' E • • CentralRange

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Plate2. (a) Crosssection showing the inversionresults of tomographyand earthquake relocation. Surface topographyis at thetop showing the locations of WesternFoothills (WF), HsuehshanRange (HR), andIlan Plain(IP, whichis thewestern extension of theOkinawa trough). The subductedPhilippine Sea plate and Okinawatrough are clearly shown by high-and low-velocity anomalies at deepand shallow depths, respectively. Relocatedhypocenters clearly distribute along a two-layeredstructure (a doubleseismic zone). The separationbetween the two layersis 15+5km downto a depthof 70-80 km. Belowthat, the two layersseem to graduallymerge into one Wadati-Benioff Zone. (b) Resultsof "restoringresolution test." Mostof theto- mographicimages and seismic patterns on Plate 2a canbe successfully reconstructed from the synthetic data. 1026 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE

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Figure 6a. (top) Seismograms(horizontal and vertical axesin secondand digital count,respectively) and (bottom)associated particle motions showing a clearexample of phaseconversion. The convertedphase (markedas "X") is 1.69 s afterthe P and 6.25 s beforethe S. Becausethe particlemotion of the converted phaseresembles that of P ratherthan S, we concludethat it is probablyan S to P conversionat the top boundary of the subductedplate. The relativetiming suggests that the boundaryis located17-20 km abovethe hypocenter. KAO AND RAU' NEW TYPE OF DOUBLE SEISMIC ZONE 1027

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0 4o 8o 120 160 Distance (km) Figure6b. A schematicdiagram showing the configuration of a typeII doubleseismic zone in NE Taiwan. Backgroundseismicity is adaptedfrom Plate 2a. Dashedlines mark the inferred upper boundaries of thesub- ductedcrust and mantle, whereas a thick solidline showsthe portionconfirmed by the convertedphase (Fig- ure 6a). The two layersof seismicityrepresent earthquakes occurring in the subductedupper crust and uppermostmantle, respectively. With increasingdepth, the separationbetween the two layersdecreases and earthquakesbegin to migratetoward the interiorof the subductedplate.

Chen, 1994, 1995; Gorbatov et al., 1994], the Aleutian- much shallowerdepth, around20-40 insteadof 70 km. Sec- Alaskaarc [e.g:,Engdahl and $cholz, 1977; House and Jacob, ond, the separationbetween the two layers is at most 15-½5 1982; Reynersand Coles, 1982], the Mariana arc [e.g., $a- km, which is approximatelyhalf of that observedfor a type I rnowitzand Forsyth,1981], the Tongaarc [e.g.,Kawakatsu, doubleseismic zone. Furthermore,the mergeof the two lay- 1985],and the New Britainregion [e.g., McGuire and Wiens, erstakes place at a muchshallower depth (<150 km). 1995]. Notice that a type II doubleseismic zone is hardto identify A classicdouble seismic zone is characterizedby an upper becausethe smaller separationbetween the two layers re- layerof seismicitywith downdipcompressional focal mecha- quiresa higherresolution in locatingearthquake hypocenters, nismsand a lower layer in downdipextension. The two lay- a conditionnot easily met by most local networkson island ersinitiate at a depthof approximately70 km with a distance arcs. Fortunately,the situationhas improved significantly of 35-40 km in between. The separationdecreases with in- over the pastdecade, mainly due to the rapid advancein the creasingdepth and the two layersmerge into a singlelayer at technicalaspect of seismologicalobservation and a betterun- depthsof approximately200 km. derstandingof velocity structuresin subductionzones. It is To preventfurther confusion between the doubleseismic rather encouragingto note that NE Taiwan is not the only zone mentioned above and the one we observed beneath NE place on Earth where a type II doubleseismic zone is ob- Taiwan,we proposeto termthe two as "typeI" and"type II," served. Similar exampleswere recentlydocumented for the respectively.The geometryof a type II doubleseismic zone subduction zones in New Zealand near the southernmost differsfrom that of a type I in severalaspects, as summarized North Island [e.g., Eberhart-Phillipsand Reyners, 1997; in Table 2. First, the lower layer of seismicityinitiates at a Reynerset al., 1997],in Cascadiajust northof the Mendocino 1028 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE

Table 2. GeometricDifferences Between Type I andType II the Gorda plate from the Pacific plate, and the so-called Double Seismic Zones "Gorda Deformation Zone" to the north of the Mendocino transformfault (Figure 7b). Plate kinematicsand numerical Features Type I Type II modelingboth indicatethat the regionis understrong N-S (lateral)compression, most likely dueto the northwardpush Commenceof the lower layer, km -70 20-40 of the Pacific plate [e.g., Wang et aL, 1997; Wilson,1989, 1993]. Earthquakefocal mechanismsshow that most events Separationbetween the two layers,km 35-40 15+5 which occurred in the double seismic zone have P axes in N- S directionparallel to the strike of subductedslab (lateral Merging depthof the two layers,km -200 <150 compression),whereas events farther to the east are in downdipextension (slab pull) [e.g., Schwartz,1995; Smith and Knapp, 1993]. triple junction [e.g., Smith and Knapp, 1993; Wang and In Alaskanear the Cook Inlet area (Figure7c), a type II Rogers, 1994], and in the Cook Inlet area, Alaska [e.g., double seismic zone is delineated between 50 and 100 km Ratchkovskyet al., 1997]. [e.g.,Ratchkovsky et al., 1997]. This regionis practicallythe easternend of the Alaska-Aleutianarc system. Globalplate 4.2. Characteristic Tectonic Setting: Termini of Oblique motionmodels predict an obliquesubduction setting with a Subduction and Lateral Compression 300-40 ø difference between the trench normal and the relative Figure 7 showsthe overall tectonicsettings and crosssec- plate convergencedirections [e.g., DeMets et al., 1994; Lar- tions for the New Zealand, Cascadia,and Alaska regions son et al., 1997]. Recentstress inversion using earthquake where the type II double seismiczones were reported. De- focalmechanisms indicates that the predominatingstress field spite that NE Taiwan seemsto differ from theseregions in withinthe subductedslab is a combinationof downdipexten- many aspects,it is rather interestingto find a commontec- sion and along-arc(i.e., lateral) compression[e.g., Lu et al., tonic characteristicamong them. 1997]. Beneath NE Taiwan, the double seismic zone is observed In conclusion,all regionswhere type II double seismic along the westernmostsegment of the subductedplate, less zonesare found seemto sharea commontectonic setting, than 50 km from the terminus along the 121.5øE meridian namely,oblique subduction with significantlateral compres- (Plate 1 and Figure 4). The PhilippineSea plate is moving sionbetween the subductedplate and the adjacentlithosphere. along310 ø at 7 cmyr -1 (Figure 1) [Senoet al., 1993], result- Furthermore, all of them are within a short distancefrom the ing in a significantoblique subduction such that the westward terminiof the subductedslabs where the lateralcompression componentis approximatelyparallel to the strike of the slab is expectedto be the greatest. [e.g., Fitch, 1972; Shiono et al., 1980; Kao et al., 1998a]. This lateral compressionis clearly shown by the consistent 4.3. SeismogenicOrigin orientationof P axesin earthquakefocal mechanisms (Figures As demonstratedthat a type II double seismic zone is 2 and 3) [Kao and Chen, 1991; Kao et al., 1998a]. unique in its geometry and tectonic characteristic,does it Similarly, the double seismiczone observedin New Zea- meanthat the seismogenicorigin of a type II doubleseismic land is located near the southernmost North Island where the zone is fundamentallydifferent from that of a type I? Given subductionsystem is changinginto a transformfault (Figure the knowledgewe have,how muchcan we addressthis issue? 7a). The relative motion betweenthe Pacific plate and the Generallyspeaking, the occurrenceof earthquakesrequires Australianplate is highly obliqueto the trenchnormal direc- two conditions.The first is the presenceof sufficientdeviato- tion [e.g., DeMets et al., 1994], a settingbasically similar to ric stressthat generatesthe sheardeformation and the second that in the southernmostRyukyu. Reynerset al. [1997] stud- is the adequatemechanism(s) that can storeand releasethe ied earthquakefocal mechanismsoccurring in the regionand strain in a seismogenicway. The occurrenceof a double reportedlow-angle thrust events just abovethe upperseismic seismiczone is gettingmore complicatedbecause we needto layer between12 and 25 km. The majorityof earthquakes simultaneouslyexplain seismogenesis along both layers. As withinthe lowerseismic layer (8 outof 12) showmechanisms far as the sourceof stressis concerned,proposed interpreta- with P and T axes parallel to the strike and dip of the sub- tionsinclude unbending of the subductedplate [e.g., Engdahl ductedslab, respectively. It is particularlytrue for the region and Scholz, 1977; Kawakatsu, 1986], thermoelasticstresses south of Cook Strait, within 150 km of the subductedslab's [e.g., Fujita and Kanamori, 1981; Hamaguchi et al., 1983; terminus. This patternis analogousto the observationsin NE House and dacob, 1982], and saggingof the subductedslab Taiwan (Plate 1 and Figure 3). [e.g., Sleep, 1979]. In the southernmost Cascadia subduction zone near the On the otherhand, possible seismogenic mechanisms for a Mendocino triple junction, a double seismic zone is docu- double seismic zone were addressed much less in the litera- mented at depthsas shallow as 15-20 km [e.g., Smith and ture. At shallow depths,brittle failure is the predominant Knapp, 1993; Wang and Rogers, 1994]. Despitethat the de- seismogenicmechanism for earthquakes[e.g., Scholz,1990]. tailedtectonic configuration of this regionis very complicated At intermediatedepths, brittle failure due to increasingpore and still under vigorousdebate [e.g., Schwartz,1995; Smith pressure,which probablyresulted from dehydrationof hy- and Knapp, 1993; Wangand Rogers,1994; Wanget al., 1997; drateminerals in the crust,is believedto be responsiblefor Wilson, 1989, 1993], the overall tectonic framework is well mostseismogenesis [e.g., Raleigh and Paterson,1965; Meade understood. There are three major components:an oblique and deanloz, 1989]. An alternativeis to attributeto the me- subductionsystem between the Gordaplate andNorth Amer- tastablephase transition from basaltto eclogite[e.g., Fukao et ica plate, an E-W Mendocinotransform fault that separates al., 1983; Pennington,1983; Ringwoodand Green, 1966]. KAO AND RAU' NEW TYPE OF DOUBLE SEISMIC ZONE 1029

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204 ø 206 ø 208 ø 210 ø 212 ø 214 ø Figure7. (left)Maps and (right) cross sections showing tectonic settings and distributions of seismicity for regionswhere type II doubleseismic zones were documented. Arrows show the directions of relative plate motions.Thick gray lines mark the locations ofcross sections. (a) New Zealand near the southernmost North Island.(b) Cascadiajust north of theMendocino triple junction where the Mendocino Transform Fault (MTF),San Andreas Fault (SAF), and Cascadia subduction zone meet. (c) Cook Inlet area, Alaska. Notice thatall regions are located within a shortdistance from the termini of subducted slabs and share a characteristictectonic setting of oblique subduction with significant lateral compression between the subducted plate and the adjacentlithosphere. 1030 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE

Both argumentsseem to apply only to the upper layer of a dition, becausethe CoastalRange, which is part of the former double seismic zone where both hydrate minerals and basalt Luzon arc, is located directly updip of the observedtype II are available. The occurrenceof earthquakesin the lower double seismic zone, the thickened crust might also corre- layer, however, remainsa puzzle. spondto the subductedarc materials. Kao and Liu [1995] study seismicpatterns and the tem- perature-pressureconditions associated with a classicdouble 4.4. A Testable Case: the Indo-Burma Subduction Zone seismiczone. They proposethe metastablephase transition If our proposedmodel is correct,a type II doubleseismic from Al-rich enstatite(pyroxene) to Al-poor enstatiteplus zone shouldbe observedin a region if it satisfiesthree condi- garnet to be a possibleseismogenic mechanism for earth- tions: significantoblique subduction,near the terminusof a quakesoccurring in the lowerlayer and part of the upperlayer subductionzone, and existence of considerablelateral com- (> -110 km). Alternatively,Seno and Yamanaka[1996] re- pressionbetween the subductedslab and the adjacentlitho- port a correlationbetween the existenceof doubleseismic sphere. In additionto the four casesshown in Plate 2 and zones and deep outer rise compressionalearthquakes. His Figures5 and 7, here we proposethat the subductionsystem interpretationcalls for dehydrationof solidifiedmagma en- beneaththe Indo-Burma rangesto be anotherplace where a trappedwithin the plate when it passedthe headof a super- type II doubleseismic zone might exist. plumesomewhere on its way from mid-oceanridges to sub- The N-S striking Indo-Burma subduction system is ductionzones. Unfortunately,both modelsare inadequateto boundedby the Himalaya CollisionZone to the north. Along explain a type II doubleseismic zone becausethe observed it, the Indian plate obliquely subductsbeneath the Eurasia lower layerbegins at a depthwhere the temperatureand pres- [e.g., Curray et al., 1979]. Chenand Molnar [1990] reported sure conditionsare not ready for metastablephase transition that the majority of large and moderate-sizedearthquakes in of Al-rich enstatite, and there are no deeper compressional the region shows a consistentpattem of N-S (lateral) com- earthquakesin the correspondingouter-rise regions. pression,despite that the N-S trendingIndo-Burma ranges We proposethat the compressivestress between the endof seemsto imply an E-W compression.They interpretthat the a subductedplate and the adjacentlithosphere (i.e., the lateral deformationin the Indo-Burm'aranges is decoupledfrom that componentof the oblique subduction)to be the sourceof in the underlyingIndian plate suchthat the observedpattern stressresponsible for type II double seismiczones. This of lateral compressionreflects the state of strain within the stressregime, together with the downdipextension from slab subductedslab. Ni et al. [1989] determinedthe geometryof pulling force, can explain the consistentpatterns of lateral the subductedplate from 184 hypocentersreported by the In- compressionand downdip extensionshown by most focal ternational SeismologicalCentre (ISC) and concludedthat mechanismsfound in type II doubleseismic zones. lateral compressiveearthquakes indeed occurred beneath the From our observationsalone, it is somewhat difficult to inferredplate interface. determine the correspondingseismogenic mechanism(s). Unfortunately,distribution of earthquakesreported in the Nonetheless,because the 15+5 km separationbetween the ISC bulletinsis too scatteredto well define the expectedtwo- two layers is comparableto the thicknessof the crust,it is layered structure. Furthermore,to our best knowledge,no possibleto regardthe observedupper seismic layer as seis- detailed study of 3-D tomographyand/or microearthquake micity occurredin the uppercrust of a subductedplate and/or relocationhas been done for the regionat the sameresolution alongthe plateinterface, whereas the lowerlayer is associated level as for thosepresented in Plate 2 and Figures5 and 7. with eventsin the uppermostmantle (Figure 6b). If so, then Hence, the existenceof a type II doubleseismic zone cannot interpretationfor the seismogenesisof type II doubleseismic be confirmed at this moment. Nonetheless,our model can be zonesbecomes rather straightforward. The scenariois analo- testedshould comparable studies become available in the fu- gousto that observedin a continentalsetting [e.g., Chen and ture. Molnar, 1983] and basicallyreflects the variationof rheological strengthof subductedmaterials, with a ductile(aseismic) 4.5. Dip Increase of WBZ and Regional Collision lower crust sandwichedby a strongupper crustand a strong uppermostmantle (both seismic). As the slab penetratesto In addition to the lateral compressiontransmitted within greater depth and reaches the appropriate tempera- the subductedslab, the uniquetectonic setting might also af- ture/pressureconditions for metastablephase transitions, the fect the geometryof the subductedplate as well. From our two layersmerge into oneseismic zone and earthquakes begin results,the dip of the subductedPhilippine Sea plate increases to migrateinto the subductedmantle [Kao and Liu, 1995]. In significantlywith depth, particularlyfor depthsgreater than fact, Wanget al. [1997] specificallycalculates the differential -90 km. This is evident both from the tomographicimages stress distribution for the southernmost Cascadia subduction and the distributionof earthquakehypocenters (Plate 2 and zone nearthe Mendocinotriple junction and confirms that the Figure 5). observedtype II doubleseismic zone is physicallycompatible The gravitational instability is certainly one of the most with such a conclusion. straightforwardinterpretations for the geometricvariation of a It is noted that the separation(15+5 km) betweenthe two subductedslab. On the basisof our previousdiscussion, there layersof the type II doubleseismic zone observed beneath NE are major phasetransitions at this depth rangethat can con- Taiwan is slightlylarger than the thicknessof a normaloce- tribute to the dip increase(e.g., dehydration,basalt/gabbro to anic crust(-10 km). We attributeit to the possiblecrustal eclogite transition,and Al-rich enstatiteto Al-poor enstatite thickening,which in turn, is due to the collisionbetween the plus garnet). In addition,the seismogenicmodel of Kao and Philippine Sea and Eurasiaplates in the region. A recent Liu [1995] suggeststhat intermediate-depthearthquakes earthquakestudy using local broadbanddata confirmsthat would graduallymigrate toward the interiorof the subducted there is significantthrust deformation within the Philippine plate,causing an apparentdip increaseof WBZ that is slightly Seaplate offshore east of Taiwan[Kao et al., 1998b]. In ad- largerthan that of the slabitself (Figure6b). KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE 1031

Kao et al. [1998a] studiedlocal seismicityand earthquake nificant lateral compressionbetween the subductedplate and focal mechanismsfor the southernmostRyukyu-NE Taiwan the adjacentlithosphere. region. They concludedthat the interplatethrust zone be- We interpretthe seismogenesisof type II double seismic tween the Philippine Sea plate and Eurasiaplate is greatly zones as reflectingthe lateral compressiveand downdip ex- distortedas the Ryukyu trenchapproaches Taiwan and that tensional stressesoriginating from oblique subductionand the interfacegradually migrates northward from-•23.5øN in slabpulling force, respectively.The upperseismic layer rep- the southernmostRyukyu to -24øN near the coastlineof Tai- resentsseismicity occurringin the upper crust of the sub- wan. Since their resultsshow no significantdistortion in the ducted plate and/or along the plate interface,whereas the geometryof subductedplate at intermediatedepths, such a lower layer is associatedwith eventsin the uppermostmantle. configurationimplies an increasein the dip of WBZ. This The scenariois analogousto that observedin a continental scenariois in a way similar to the sea anchoringeffect ob- setting and basically reflects the variation of rheological servedin Mariana exceptthat here the arcwardmigration of strengthof subductedmaterials, with a ductile (aseismic) plate interfaceis driven by the push of the regional collision lower crust sandwichedby a strongupper crustand a strong insteadof the overridingplate moving away from the trench uppermostmantle (both seismic). [e.g., Scholzand Campos,1995; Uyeda and Kanamori, 1979; Wu, 1978a]. Thus we proposethat the regional collision is Acknowledgments. We appreciatethe personnelin the Seismol- another,and probablysignificant, contributor to the steepen- ogical ObservationCenter of the Central Weather Bureau, Taiwan, for maintainingthe Taiwan SeismicNetwork and making the data ing of the subductedPhilippine Sea plate. easily accessible. We benefited from discussionwith Wang-Ping Notice the collision is most effective within the litho- Chen, Ling-Yun Chiao, Shu-Kun Hsu, Jyr-ChingHu, Jian-Cheng sphere,hence it is expectedto find the biggestdip increase Lee, Wen-TsungLiang, Char-ShineLiu, Lin-gun Liu, Chia-Yu Lu, immediatelybelow the bottom of the overridingplate (pre- Chi-Yuen Wang, and FrancisT. Wu. Commentsfrom DonnaJurdy, sumablyat -90-100 km). This is consistentwith our obser- SergeLallemand, Dapeng Zhao, and an anonymousreviewer significantly improve the manuscript.Some figures are generatedby the vation in NE Taiwan (Plate 2 and Figure 5), New Zealand, GMT softwarewritten by PaulWessel and Walter H. F. Smith. This and Alaska (Figure 7). As for the subductionzone in Cas- researchwas partially supportedby the National ScienceCouncil, cadia near Mendocino triple junction, the systemdoes not ROC, grantsNSC85-2111-M-001-020-Y and NSC86-2116-M-001- seemto reach the scale of regional collision. Also, the sub- 005-Y. ductedslab does not extendto the intermediatedepth. There- fore we expect no obviousgeometric change to be found References there. Chen, W.-P., and P. Molnar, Focal depthsof intracontinentalearth- quakes and their implicationsfor the thermal and mechanical 5. Conclusion propertiesof the lithosphere,d. Geophys.Res., 88, 4183-4214, 1983. The tectonicsetting of northeastTaiwan is complicatedby Chen, W.-P., and P. Molnar, Sourceparameters of earthquakesand the simultaneouspresence of obliquesubduction, region colli- intraplatedeformation beneath theShillong plateau and the northern Indoburman ranges,d. Geophys.Res., 95, 12,527-12,552, sion, and back arc opening. Detailed structuresof the sub- 1990. ductedPhilippine Sea plate beneath the regionare mappedby Curray,J.R., D.G. Moore, L.A. Lawver, F.J. Emmel, R.W. Raitt, M. seismictomography using high-qualitydigital data recorded Henry, and R. Kieckhefer, Tectonicsof the Andaman Sea and by the Taiwan SeismicNetwork. We further supplementthe Burma, in Geological and GeophysicalInvestigations of Conti- nental Margins, edited by J.S. Watkins, L. Montadert,and P. inversion results with earthquakesource parametersdeter- Dickerson,AAPG Mem, 29, 189-198, 1979. mined from inversion of teleseismic P and SH waveforms. DeMets, C., R. Gordon,D. Argus, and S. Stein, Effect of recentrevi- This stepis criticalto definethe positionof the plate interface sionsto the geomagneticreversal time scaleon estimatesof cur- and hence avoid the possibilityof mistakingthe interplate rent plate motions,Geophys. Res. Lett., 21, 2191-2194, 1994. thrustzone or structuresin the overridingplate as the WBZ. Dziewonski, A.M., J.E. Franzen, and J.H. Woodhouse, Centroid- moment tensor solutionsfor January-March, 1985, Phys. Earth The most interestingfeature is that earthquakeswithin the Planet. Inter., 40, 249-258, 1985. subductedPhilippine Sea plate tend to occur along a two- Dziewonski, A.M., G. Ekstr6m, J.E. Franzen, and J.H. Woodhouse, layeredstructure (a doubleseismic zone). The locationof the Centroid-moment tensor solutions for January-March, 1986, upper layer is immediatelybelow the interplatethrust zone Phys.Earth Planet. Inter., 45, 1-10, 1987a. Dziewonski, A.M., G. Ekstr6m, J.E. Franzen, and J.H. Woodhouse, and extendsdown to 70-80 km at a dip of 400-50ø. Below Global seismicityof 1978: Centroid-momenttensor solutionsfor approximately100 km, the dip increasesdramatically to 70ø- 512 earthquakes,Phys. Earth Planet. Inter., 46, 316-342, 1987b. 80ø. The lower layer commencesat 45-50 km and maintains Dziewonski, A.M., G. EkstrOm, J.E. Franzen, and J.H. Woodhouse, approximatelyparallel to the upperlayer with a separationof Global seismicityof 1981: Centroid-momenttensor solutions for 15ñ5 km in between. Below-70-80 km, the separationde- 542 earthquakes,Phys. Earth Planet.Inter., 50, 155-182, 1988. Eberhart-Phillips,D., and M. Reyners,Continental subduction and creasesand they seemto graduallymerge into one WBZ. three-dimensional crustal structure: The norhem South Island, To avoid furtherconfusion, we proposeto term the classic New Zealand,d. Geophys.Res., 102, 11,843-11,861, 1997. double seismiczones observedbeneath Japan and Kuril as Engdahl,E.R., Seismicityand plate subductionin the central Aleu- "type I" and that we observedas "type II," respectively. A tians, in Island Arcs, Deep Sea Trenches,and Back-Arc Basins, Maurice Ewing Set., vol. 1, editedby M. Talwani and W.C. Pit- globalsurvey of major subductionzones indicates that type II man III, pp. 259-271, AGU, Washington,D.C., 1977. double seismic zones were also documented in three other re- Engdahl,E.R., and C.H. Scholz, A doubleBenioff zone beneaththe gions:New Zealand near the southernmostNorth Island, Cas- central Aleutians: An unbendingof the lithosphere,Geophys. cadiajust north of the Mendocinotriple junction, and the Res. Lett., 4, 473-476, 1977. Cook Inlet area of Alaska. All of them are located within a Evans,J.R., D. Eberhart-Phillips,and C.H. Thurber,User's manual for SIMULPS12 for imagingVp and Vp/Vs: A derivative of the short distance from the termini of subducted slabs and share a "Thurber"tomographic inversion SIMUL3 for local earthquakes characteristictectonic setting of obliquesubduction with sig- and explosions,U.S. Geol. Surv. OpenFile Rep., 94-431, 1994. 1032 KAO AND RAU: NEW TYPE OF DOUBLE SEISMIC ZONE

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