source parameters of earthquakes and intraplate deformation beneath the shillong plateau and the northern indoburman ranges we determined the fault plane solutions and focal depths of 17 earthquakes beneath the shillong plateau and the northern indoburman ranges by combining results from the inversion of long period p and sh waveforms and amplitudes from polarities of first motions and from the identification of pp and sp phases on short period seismograms fault plane solutions of 15 earthquakes show mixtures of thrust and strike slip faulting but the p axes for these events are nearly horizontal and consistently oriented north northeast south southwest all of these earthquakes occurred at depths greater than 29 km beneath the shillong plateau one event occurred at a depth of 52 km the relatively large depths for earthquakes in an intraplate setting suggest that the indian lithosphere in this area is especially cold earthquakes beneath the northern indoburman ranges define a gently east southeast dipping zone from 30 to 45 km beneath the bengal basin to 40 to 90 km beneath the ranges this zone seems to steepen and connect with the zone of intermediate depth seismicity that dips eastward beneath burma these earthquakes cannot have occurred along the interface between a subducting indian plate and the overriding indoburman lithosphere because the p axes not the nodal planes are parallel to the north south trending seismic zone although a couple of the earthquakes might have occurred within the indoburman lithosphere most of this seismicity seems to have occurred within the indian plate recently and currently being subducted eastward beneath the indoburman ranges the consistent north northeast trend of the p axes implies that the orientation of maximum compressional strain in the indian plate throughout its northeastern pan is nearly perpendicular to that responsible for roughly north south trending folds of the indoburman ranges thus either recently in geologic time since 1 ma the orientation of maximum compression changed dramatically or more likely the deformation in the indoburman ranges is decoupled from that in the underlying indian plate meanwhile the seemingly identical northward displacement of india and the indoburman ranges with respect to south china must be accommodated farther east along the sagaing and other faults

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

crustal deformation at the southernmost part of the ryukyu subduction east taiwan as revealed by new marine seismic experiments abstract the southernmost part of the ryukyu subduction where the philippine sea plate is subducting under the eurasian plate is known to be a very seismically active region of transition from a north dipping subduction along the ryukyu subduction to an se nw collision along the taiwanese orogenic wedge in this paper we will focus on the ryukyu forearc area close to taiwan where the deformation is paroxysmal in order to decipher the nature of the seismic deformation in this region a three month passive experiment combining 22 ocean bottom seismometers and 51 onland stations has been led starting from an a priori heterogeneous model we have obtained 801 well located earthquake hypocenters a precise p wave tomography model and 14 focal mechanisms the seismicity along the ryukyu forearc is mainly located not only in the vicinity of the interplate seismogenic zone isz but also within both the subducting psp and the overriding plate seismicity within the upper plate is essentially localized east of nanao basin where nw se extension occurs and northwest of the hoping basin where strike slip dominates as revealed by both the p wave velocity structure and the newly derived seismicity we argue that a sub vertical step offsetting the subducting psp around 10 km may support the presence of a trench parallel tear the psp also undergoes extension in its upper part that is probably caused by buckling and slab pull the p wave velocity structure reveals three other major features 1 a continuity between the central range and the ryukyu arc with a shallower moho 30 km depth between 122 3 n and 122 5 n along the ryukyu arc 2 high p wave velocities along the eastern side of the central range and 3 two bodies with similar high crustal velocities 6 5 7 0 km s at 12 18 km depths embedded within the ryukyu arc basement just north of hoping basin and north of the nanao basin

plate coupling in the northern south island and southernmost north island new zealand as illuminated by earthquake focal mechanisms subduction of the pacific plate in the northern south island and southernmost north island of new zealand is transitional insofar as the crustal thickness of the pacific plate increases significantly along strike in the northern south island focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes the stress regime in the subducted plate appears to be dominated by slab pull t axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone and the average azimuth of these t axes is rotated some 25 clockwise out of the direction of dip of the subducted plate this can be related to the asymmetrical shape of the subducted slab in contrast the stress regime in the overlying plate appears to be dominated by subhorizontal compression low angle thrust events near the plate interface in cook strait and the southernmost north island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from global positioning system gps observations an absence of low angle thrust events near the plate interface in the northern south island and the tendency of p axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost north island differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern south island

structure and tectonics in the region of flat subduction beneath central peru crust and uppermost mantle seismic waveform modeling of boundary interaction phases is used to determine the discontinuity structure of the crust in the subandean and foreland basin regions overlying the zone of flat subduction beneath east central peru the data analyzed are from intermediate depth earthquakes 110 to 155 km recorded on an array of three component short period 1 hz digital seismographs deployed in the epicentral region full use is made of both p to s and s to p converted phases in the modeling results from the determination of crustal structure in the subandean and foreland basin region of east central peru confirm the presence of vast depositional basins comprised of low velocity sediments up to at least 8 km thick which flank the andean orogen to the east and correlate with a substantially thickened crust atop the brazilian shield crustal thickness in the foreland basin varies from about 35 km or less where sedimentary cover is minimal to 44 km in regions of maximum sedimentary deposition there is some evidence that the crust thins slightly on the western side of the foreland basin in the gap between basin and subandean fold and thrust belt but it rapidly thickens to 45 50 km beneath the subandes proper and to more than 50 km in the southern part of the subandean belt the results are consistent with but do not require a thick skinned model of foreland crustal deformation similar to that found for the block faulted terrane in argentina above the zone of flat subduction there at least some basin formation appears to be due to block faulting where faults may penetrate into the mantle

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

double seismic zone in tonga the clear evidence of a double seismic zone beneath japan1 2 has led seismologists to search for other such zones elsewhere because they have been found in few other areas3 6 double seismic zones are thought to be rare and to depend strongly on a peculiar combination of external conditions such as age of the descending slab and the convergence rate 7 we report here a new double seismic zone which has been found beneath the tonga arc8 where the stress state in the subducting slab has been described as strongly down dip compressional7 9 10 this discovery suggests that the double seismic zone is a general feature of subducting slabs and may be found in many other subduction zones

global prevalence of double benioff zones double benioff zones provide opportunities for insight into seismogenesis because the underlying mechanism must explain two layers of deep earthquakes and the separation between them we characterize layer separation inside subducting plates with a coordinate rotation to calculate the slab normal distribution of earthquakes benchmark tests on well established examples confirm that layer separation is accurately quantified with global seismicity catalogs alone global analysis reveals double benioff zones in 30 segments including all 16 subduction zones investigated with varying subducting plate ages and stress orientations which implies that they are inherent in subducting plates layer separation increases with age and is more consistent with dehydration of antigorite than chlorite

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

transition from interplate slip to double seismic zone along the kuril kamchatka arc during subduction the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth to characterize this transition we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the kuril kamchatka arc precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances at depths of 30 50 km our results delineate a transitional thrust zone characterized by earthquakes showing thrust faulting on moderately dipping nodal planes the northwesterly dipping nodal plane dips approximately 40 some 10 15 steeper than the dip of low angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km for earthquakes along the transitional thrust zone secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds in the former case the transitional thrust zone is interpreted as the deep seated portion of the plate interface whose dip increases with depth the alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km the inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region however a locked interface does not seem to affect the state of strain in the double seismic zone the upper compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km at this depth the two layers of seismicity in the double seismic zone are separated by as much as 30 40 km too large a distance to be accommodated by models of seismogenesis invoking either the gabbro eclogite transition or dehydration of subducted oceanic crust

a double seismic zone in new britain and the morphology of the solomon plate at intermediate depths we have studied the spatial configuration of earthquakes in the new britain subduction zone by inverting regional and teleseismic p pkp and pp arrival times using a hypocentroidal decomposition method a previously unreported intermediate depth double seismic zone exists along a small 70 km long segment of the slab in this region centroid moment tensor mechanisms suggest that the upper zone of seismicity shows horizontal along strike compression and the lower zone shows downdip tension this contrasts with most previously studied double seismic zones which generally show downdip compression in the upper zone we propose that along strike rather than downdip compressional stresses exist in the upper seismicity plane due to membrane and lateral bending stresses resulting from the shape of the new britain trench which undergoes a 40 change in strike between 148 and 152 longitude the double seismic zone is bounded by a tear in the slab which accommodates the majority of the change in strike that the slab must undergo these results suggest that double seismic zones can show very rapid lateral variations controlled by the local tectonics and that along strike membrane and bending stresses resulting from slab morphology can be significant in producing double seismic zones

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

slab pull and the seismotectonics of subducting lithosphere this synthesis links many seismic and tectonic processes at subduction zones including great subduction earthquakes to the sinking of subducted plate earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity providing details on how subduction occurs compression is localized across a locked interface thrust zone because both the ridge push and the slab pull forces are resisted there the slab pull force increases with increasing plate age thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate the 1960 chile earthquake mw 9 5 the largest earthquake to occur in historic times began its rupture at an interface bounding oceanic plate 30 m y old however this rupture initiation was associated with the locally oldest subducting lithosphere weakest coupling the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere terminating near the subducting chile rise prior to a great subduction earthquake the sinking subducted slab will cause increased tension at depths of 50 200 km with greatest tension near the shallow zone resisting plate subduction plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260 350 km provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle this explains k mogi s observations of m 7 thrust earthquakes at depths of 260 350 km immediately downdip and within 3 years prior to five great shallow earthquakes of northern japan the slab pull model explains the lower layer of double seismic zones as due to tension from the deeper sinking plate and the upper layer as due to localized in plate compression as plate motion is resisted by the bounding mantle just downdip of the interface thrust zone there occurs an aseismic 20 50 dip increase of subducted plate this slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone

detailed structures of the subducted philippine sea plate beneath northeast taiwan a new type of double seismic zone we studied the detailed structure of the subducted philippine sea plate beneath northeast taiwan where oblique subduction regional collision and back arc opening are all actively occurring simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast high quality data recorded by the taiwan seismic network we further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic p and sh waveforms a critical step to define the position of plate interface and the state of strain within the subducted slab the most interesting feature is that relocated hypocenters tend to occur along a two layered structure the upper layer is located immediately below the plate interface and extends down to 70 80 km at a dip of 40 50 below approximately 100 km the dip increases dramatically to 70 80 the lower layer commences at 45 50 km and stays approximately parallel to the upper layer with a separation of 15 5 km in between down to 70 80 km below that the separation decreases and the two layers seem to gradually merge into one wadati benioff zone we propose to term the classic double seismic zones observed beneath japan and kuril as type i and that we observed as type ii respectively a global survey indicates that type ii double seismic zones are also observed in new zealand near the southernmost north island cascadia just north of the mendocino triple junction and the cook inlet area of alaska all of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction we interpret the seismogenesis of type ii double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere originating from oblique subduction and the downdip extension from slab pulling force the upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and or along the plate interface whereas the lower layer is associated with events in the uppermost mantle

variable spatio temporal clustering of microseismicity in the hellenic subduction zone as possible indicator for fluid migration abstract this study investigates the variability of spatio temporal microseismicity clustering and the occurrence of mutual triggering of events along the subduction interface in south eastern aegean as indication for fluid flow along and above the plate interface we quantify spatial temporal and spatio temporal microseismicity clustering from the outer to the inner forearc and at intermediate depths waveform similarity indicates a decreasing of spatially clustered events from the outer and central towards the inner forearc and at intermediate depths highly similar events cross correlation 0 9 used as proxy for spatial clustering decrease from the outer 30 2 and central forearc 34 9 towards the inner forearc 20 5 and at intermediate depth 6 9 such highly similar events show increasing median inter event times from the outer and central towards the inner forearc and at intermediate depth 0 35 0 34 16 45 and 70 days respectively the epidemic type aftershock sequences etas model employed to investigate microseismicity temporal clustering indicates an increase of the percentage of independent events from the outer 32 and central 46 forearc to the inner forearc 93 and at intermediate depth 93 hence etas results suggest that mutual triggering of events is significant in the outer and central forearc and it is almost absent in the inner forearc and at intermediate depths autocorrelation analysis investigating spatio temporal clustering shows the tendency of earthquakes to occur close in space and time in the outer and central forearc while in the inner forearc and especially at intermediate depth earthquakes are more homogeneously and randomly distributed combining the results from spatial temporal and spatio temporal analysis we suggest that the different spatio temporal patterns hint at systematic variations in the presence of migrating fluids on active faults close to failure triggering of seismicity is significant in the outer and central forearc indicating fluid flow from the subduction interface and it is diminishing towards the inner forearc at intermediate depths the nearly complete absence of mutual triggering of earthquakes indicates that there is little evidence for migration of fluids on active faults close to failure because intermediate depth seismicity in the hellenic subduction zone occurs at p t conditions where dehydration reactions are expected fluids released by dehydration reactions within the slab are very likely migrating directly into the overlying mantle without triggering earthquakes

crust mantle boundaries in the taiwan luzon arc continent collision system determined from local earthquake tomography and 1d models implications for the mode of subduction polarity reversal abstract in order to better understand the mode of subduction polarity reversal in the taiwan luzon arc continent collision zone we mapped its crust mantle boundaries using local earthquake tomography by contouring surfaces of constant vp 7 5 km s 1 we identified three moho discontinuities and the plate interface that juxtaposes eurasian lower crust against mantle lithosphere of the philippine sea plate the plate interface dips to the east under southeastern taiwan and steepens progressively towards north until it becomes vertical at 23 7 n from there it continues northward in a vertical orientation until the limit of the tomographic model inhibited further mapping for the moho additional depth constraints were derived from 1d models using p wave arrivals of local earthquakes the mohos of the eurasian and philippine sea plates are disconnected across the plate interface beneath southern taiwan the eurasian moho dips to the east at 50 60 following the orientation of the plate boundary and continuous with the benioff zone towards north the eurasian moho steepens into subvertical again together with the plate boundary the philippine sea plate moho exhibits a synform like crustal root interpreted as the base of the magmatic luzon arc towards the north this root deepens from 30 to 70 km underneath the ryukyu trench in northernmost taiwan the hinge of the vertically subducting eurasian slab steps westward out of the thrust belt leaving the deformation front to the east inactive and giving way to the north dipping philippine sea plate a subhorizontal moho at 30 35 km depth overlies the north dipping philippine sea slab and is interpreted as a newly formed moho established after westward rollback and delamination of the subducting eurasian slab in combination these data support a model of progressive subduction polarity reversal in which a tear within the eurasian lithosphere propagated southwestward deactivating the deformation front

a double seismic zone in the subducting juan fernandez ridge of the nazca plate 32 s central chile 1 the region of central chile offers a unique opportunity to study the links between the subducting juan fernandez ridge the flat slab the double seismic zone dsz and the absence of modern volcanism here we report the presence and characteristics of the first observed dsz within the intermediate depth nazca slab using two temporary seismic catalogs ovalle 1999 and chile argentina seismological measurement experiment the lower plane of seismicity lp is located 20 25 km below the upper plane begins at 50 km depth and merges with the lower plane at 120 km depth where the slab becomes horizontal focal mechanism analysis and stress tensor calculations indicate that the slab s state of stress is dominantly controlled by plate convergence and overriding crust thickness above 60 70 km depth the slab is in horizontal compression and below it is in horizontal extension parallel to plate convergence which can be accounted for by vertical loading of the overriding lithosphere focal mechanisms below 60 70 km depth are strongly correlated with offshore outer rise bend faults suggesting the reactivation of preexisting faults below this depth the large interplane distances for all nazca dszs can be related to the slab s unusually cold thermal structure with respect to its age since lps globally seem to mimic mantle mineral dehydration paths we suggest that fluid migration and dehydration embrittlement provide the mechanism necessary to weaken the rock and that the stress field determines the direction of rupture

microseismicity and faulting in the southwestern okinawa trough abstract in november 2003 15 ocean bottom seismometers were deployed in the southwestern okinawa trough more than 3300 microearthquakes were located during the 10 days passive seismic experiment the earthquake activity is characterized by the ceaseless occurrence of small earthquakes in the vicinity of all the instruments the seismicity is essentially restricted to the central part of the southwestern okinawa trough except for one cluster of events situated in the southern part of it cluster 2 the seismic activity terminates abruptly against the ne sw trending prolongation of the lishan fault most of the microearthquakes are aligned along the e w trending normal faults showing where the present day active normal faulting occurs and how it accounts for the n s extension in the okinawa trough according to the p wave velocity spectra estimated from some deep earthquakes located beneath the cross backarc volcanic trail area the existence of a lower crustal upper mantle magma chamber is confirmed by the presence of low frequency earthquakes in the 3 10 hz bandwidth

focal mechanism determination in taiwan by genetic algorithm we determined the focal mechanism solutions for earthquakes with mag nitude ml 4 0 that occurred in the taiwan region between 1991 and 2005 first motion polarities of p waves recorded at over 700 seismic stations in taiwan were used because of the large number of events and stations involved we implemented the genetic algorithm in a nonlinear global search for the focal mechanism solutions the algorithm was tuned and validated through synthetic tests we finally determined the focal mechanisms of 1635 events with good qualities among 4188 earthquakes focal mechanism solutions for a majority of the earthquakes display a dominant pat tern of thrust fault type reflecting the compressive stress field due to the plate colli sion normal fault events occurred at intermediate depths in subduction zones which is likely the result of the bending of the subducting slabs strike slip faults are also found within the eurasia plate around the peikang basement high and in collision zones near ilan where the geometry of the colliding plates is complex our study pro vides a database of focal mechanisms for studying seismogenic structures and plate tectonics this database can also be used by structural seismologists to compute syn thetics for waveform tomography studies

subsurface imaging taiger experiments and tectonic models of taiwan abstract the seismicity deformation rates and associated erosion in the taiwan region clearly demonstrate that plate tectonic and orogenic activities are at a high level major geologic units can be neatly placed in the plate tectonic context albeit critical mapping in specific areas is still needed but the key processes involved in the building of the island remain under discussion of the two plates in the vicinity of taiwan the philippine sea plate psp is oceanic in its origin while the eurasian plate eup is comprised partly of the asian continental lithosphere and partly of the transitional lithosphere of the south china sea basin it is unanimously agreed that the collision of psp and eu is the cause of the taiwan orogeny but several models of the underlying geological processes have been proposed each with its own evolutionary history and implied subsurface tectonics taiger taiwan integrated geodynamics research crustal and mantle imaging experiments recently made possible a new round of testing and elucidation the new seismic tomography resolved structures under and offshore of taiwan to a depth of about 200 km in the upper mantle the steeply east dipping high velocity anomalies from southern to central taiwan are clear but only the extreme southern part is associated with seismicity toward the north the seismicity disappears the crustal root under the central range is strongly asymmetrical using 7 5 km s as a guide the steep west dipping face on the east stands in sharp contrast to a gradual east dipping face on the west a smaller root exists under the coastal range or slightly to the east of it between these two roots lies a well delineated high velocity rise spanning the length from hualien to taitung the 3 d variations in crustal and mantle structures parallel to the trend of the island are closely correlated with the plate tectonic framework of taiwan the crust is thickest in the central taiwan collision zone and although it thins toward the south the crust is over 30 km thick over the subduction in the south in northern taiwan the northward subducting psp collides with taiwan and the crust thins under northern taiwan where the subducting indenter reaches 50 km in depth the low vp vs ratio of around 1 6 at a mid crustal depth of 25 km in the central range indicates that current temperatures could exceed 700 c the remarkable thickening of the crust under the central range its rapid uplift without significant seismicity its deep exhumation and its thermal state contribute to make it the core of orogenic activities on taiwan island the expanded network during the taiger deployment captured broadband seismic data yielding enhanced s splitting results with mainly sks skks data the polarization directions of the fast s waves follow very closely the structural trends of the island supporting the concept of a vertically coherent taiwan orogeny in the outer few hundred kilometers of the earth

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

microseismicity in the middle okinawa trough we conducted a microearthquake survey with ocean bottom seismometers obs in the middle okinawa trough in 1984 during about one month of operation very active microseismicity was observed the microearthquake activity was characterized by the ceaseless occurrence of numerous small earthquakes in the vicinity of obs stations this activity was recorded by almost every obs station but not by land based instruments swarm activities also occurred at many places average activity levels of earthquakes whose durations are longer than 30 s recorded at individual obss are from 4 to 20 events per day which seem higher than those of ordinary mid oceanic ridge fracture zone systems and other back arc basins apparently the number of smaller size earthquakes is fewer than expected this might indicate the rather high attenuation of seismic waves or low q of the crust around obs stations which makes it difficult to detect small size events at a distant range attenuation of seismic waves was large beyond the epicentral distance of about 120 km this seems to indicate low q in the lower crust also a particularly low q area was found at the northern most part of the research area where high heat flow measurements were obtained this suggests that igneous activity reaches to the surface in that vicinity from the bottom of the crust where magmatic activity has been implied in other studies at least two types of swarm activities are indicated for this area some have no dominant events show high b value properties and seem to be volcanic others have dominant events and appear to be tectonic in nature among the swarm activity two kinds of signals closely resembling volcanic tremors were observed from these features we conclude that the crust of this area is highly fractured and subject to intense tectonic activities as well as some kind of igneous processes

seismogenic structures along continental convergent zones from oblique subduction to mature collision we summarize seismogenic structures in four regions of active convergence each at a different stage of the collision process with particular emphases on unusual deep seated seismogenic zones that were recently discovered along the eastern hellenic arc near crete an additional seismogenic zone seems to occur below the seismogenic portion of the interplate thrust zone a configuration found in several other oblique subduction zones that terminate laterally against collision belts the unusual earthquakes show lateral compression probably reflecting convergence between the subducting lithosphere s flank and the collision zone nearby along oblique zones of recent collision the equivalence between space and time reveals the transition from subduction to full collision in particular intense seismicity beneath western taiwan indicates that along the incipient zone of arc continent collision major earthquakes occur along high angle reverse faults that reach deep into the crust or even the uppermost mantle the seismogenic structures are likely to be reactivated normal faults on the passive continental margin of southeastern china since high angle faults are ineffective in accommodating horizontal motion it is not surprising that in the developed portion of the central taiwan orogen 5 ma seismogenic faulting occurs mainly along moderate dipping 20 30 thrusts this is probably the only well documented case of concurrent earthquake faulting on two major thrust faults with the second seismogenic zone reaching down to depths of 30 km furthermore the dual thrusts are out of sequence being active in the hinterland of the deformation front along the mature himalayan collision zone where collision initiated about 50 ma ago current data are insufficient to distinguish whether most earthquakes occurred along multiple out of sequence thrusts or along a major ramp thrust intriguingly a very active seismic zone including a large mw 6 7 earthquake in 1988 occurs at depths near 50 km beneath the foreland such a configuration may indicate the onset of a crustal nappe involving the entire cratonic crust in all cases of collision discussed here the basal decollement a key feature in the critical taper model of mountain building appears to be aseismic it seems that right at the onset of collision earthquakes reflect reactivation of high angle faults for mature collision belts earthquake faulting on moderate dipping thrust accommodates a significant portion of convergence a process involving the bulk of crust and possibly the uppermost mantle

the focus thermal study around the spreading center of southwestern okinawa trough abstract voluminous hydrothermal systems are corresponding to the heat flow anomaly discovered from previous geophysical surveys in the southern okinawa trough sot which is an area with a high sedimentation rate supplied by the east china sea shelf and the island of taiwan our latest fifteen field measurements around the geolin mound glm 24 36 6 n 122 53 e are located at a sedimentary basin west of the yonaguni rift which is nearby the southwestern tip of a ne sw trending volcano trail in the sot the heat flow results illustrate a concentric decrease from 31 477 to 180 mw m2 in a 1 km radius around the glm evidently proving the focus fluid migration along the major fluid conduit the ultrahigh temperature observations revealed the existence of magmatic components in shallow sediments combining the subsurface profiles with the temperature fields around the glm a 0 16 km2 acoustic blanking area of the g1 fluid conduit was recognized from the chirp sonar image and numerous interpreted igneous reflections with fluids related features from seismic profiles as a result we proposed a conceptual model which simultaneously illustrates the hydrothermal circulation pattern from the previous general surveys and the enormous thermal anomaly from current detailed measurements the sill to sill feeding sedimentary basin of the glm is capable of demonstrating the enormous thermal anomaly around the spreading center of the sot and appropriately represents the potential hydrothermal discharge and recharge by heat flow distribution around the glm

a hypothesis for the seismogenesis of a double seismic zone summary the seismogenesis of a double seismic zone in particular the lower layer of a double seismic zone has not been adequately explained in the literature on the basis of seismic data and geothermal structures along three well studied cross sections in the kuril kamchatka and japan subduction zones we investigate the temperature pressure conditions associated with seismogenic structures of the double seismic zones the corresponding tip loci seem to suggest that earthquakes observed in the lower layer and in the lower part below approximately 130 20 km of the top layer of a double seismic zone were caused by metastable phase transition a mechanism similar to that responsible for deep focus earthquakes only at lower temperature pressure conditions under this hypothesis the wedge shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition according to theoretical experimental studies and the constraints imposed by our observations a likely candidate for such a phase transition is the metastable al rich enstatite decomposing into the assemblage of al poor enstatite plus garnet earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and or facies change from basalt to eclogite that the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 f 20 km because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition it is inferred that single seismic zones are also caused by the same mechanisms except that the implicit layer of a supposed double seismic zone is missing due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region

details of stress directions in the alaska subduction zone from fault plane solutions we used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions we propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses using a synthetic data set of 50 fault plane solutions composed of two halves corresponding to two incompatible stress tensors we tested the validity and efficacy of the cumulative misfit method our acceptance criteria for results of stress tensor inversions are 1 the directions must be well constrained that is the 95 confidence regions of the greatest and least principal stresses do not overlap and 2 there must be evidence for homogeneity in the sample that is the average misfit of the inversion is less than 6 we estimated stress directions in the alaska wadati benioff zone wbz using 470 fault plane solutions which we determined for earthquakes with ml 3 and 67 published fault plane solutions for earthquakes with ms 5 we succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets with the average misfit ranging from 3 2 to 5 5 the average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set f 10 by factors of 2 to 3 we estimated the stress fields at two scales along the alaska wbz the stress directions measured by the large earthquakes ms 5 were homogeneous over large volumes with extension downdip and the direction of greatest compression along strike this unusual orientation of the greatest principal stress is attributed to the bend of the slab under central alaska which generates compressive stresses along strike the stress orientations revealed by small earthquakes in contrast exhibited a great deal of heterogeneity as a function of space although their general trend confirms the overall stress directions obtained from the large events we propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50 the estimated stress directions of the crustal earthquakes corresponded to the following mechanisms 1 strike slip faulting with the greatest principal stress oriented n s near fairbanks and 2 thrusting with the greatest principal stress oriented nw se near mount mckinley

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

large earthquakes and crustal deformation near taiwan source parameters of 15 large mb 6 earthquakes which occurred near taiwan during the last 25 years are determined by synthetic seismogram analysis of teleseismic p and s waveforms source depths range from 13 to 88 km three earthquakes of intermediate depth are located within the southernmost limit of the ryukyu arc and show thrust type mechanisms with t axes subparallel to a northward dipping wadati benioff zone shallow earthquakes 30 km are distributed over a broad region along and off the east coast of taiwan and show both high angle reverse and strike slip mechanisms the p axes of the shallow events located off the east coast of northern taiwan are oriented near horizontal trend south to southwest and may reflect relative motion between the okinawa platelet and taiwan or complex deformation at the southwest limit of the ryukyu trench in contrast p axes of shallow earthquakes both onshore and off the east coast of central and southern taiwan consistently trend northwest and are interpreted to reflect relative convergence between the philippine sea and eurasian plates summation of the seismic moment tensors of eight large shallow earthquakes that occurred along and off the central and southern coast of taiwan shows that relative plate motion between the eurasian and philippine sea plates when averaged over the last 25 years has been accommodated by crustal contraction at a rate of 2 6 5 4 cm yr along an azimuth of about 290 o

global plate velocities from the global positioning system we have analyzed 204 days of global positioning system gps data from the global gps network spanning january 1991 through march 1996 on the basis of these gps coordinate solutions we have estimated velocities for 38 sites mostly located on the interiors of the africa antarctica australia eurasia nazca north america pacific and south america plates the uncertainties of the horizontal velocity components range from 1 2 to 5 0 mm yr with the exception of sites on the pacific and nazca plates the gps velocities agree with absolute plate model predictions within 95 confidence for most of the sites in north america antarctica and eurasia the agreement is better than 2 mm yr we find no persuasive evidence for significant vertical motions 3 standard deviations except at four sites three of these four were sites constrained to geodetic reference frame velocities the gps velocities were then used to estimate angular velocities for eight tectonic plates absolute angular velocities derived from the gps data agree with the no net rotation nnr nuvel 1a model within 95 confidence except for the pacific plate our pole of rotation for the pacific plate lies 11 5 west of the nnr nuvel 1a pole with an angular speed 10 faster our relative angular velocities agree with nuvel 1a except for some involving the pacific plate while our pacific north america angular velocity differs significantly from nuvel 1a our model and nuvel 1a predict very small differences in relative motion along the pacific north america plate boundary itself our pacific australia and pacific eurasia angular velocities are significantly faster than nuvel 1a predicting more rapid convergence at these two plate boundaries along the east pacific rise our pacific nazca angular velocity agrees in both rate and azimuth with nuvel 1a

double seismic zone beneath the mariana island arc a double zone of seismicity has been found beneath the northern marianas in the depth range 80 120 km the benioff zone undergoes a pronounced bend with radius of curvature about 200 km before straightening and descending nearly vertically to a depth of more than 600 km thus this is an ideal area in which to search for evidence of stresses associated with unbending of the subducted oceanic plate we relocated all teleseismic events from 1963 through 1975 by using a regional composite master event technique which significantly decreased the scatter of the hypocenters the three events of the lower seismic zone are separated from the upper zone by about 30 35 km the true thickness of the upper seismic zone is probably 20 km or less the limited focal mechanism data indicate that there is downdip tension in the lower zone and downdip compression in the upper zone the seismicity patterns could be generated either by thermal stresses or by unbending a plate which was originally deformed anelastically we show that an elastic perfectly plastic model of the rheology of the plate can account for the separation of the two seismic zones the focal mechanisms and the greater activity of the upper seismic zone

microseismicity of back arc rifting in the middle okinawa trough a one month microearthquake survey was conducted at iheya knolls in the middle okinawa trough in 1992 to reveal microearthquake characteristics of back arc rifting we deployed five ocean bottom seismometers and observed over 1700 events during 30 days hypocenters were determined for 48 earthquakes with good resolution from this survey we conclude that the survey area with 100km 100km has high seismicity many earthquakes occur in the western part of iheya knolls and few occur in the central part the earthquakes are distributed in an en echelon shape and throughout the entire depth of the crust there is no evidence for the existence of transform faults suggesting that the middle okinawa trough is in a stage of back arc rifting

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

cenozoic plate reconstruction of the south china sea region abstract reconstructions of the south china sea region at 60 ma 40 ma 30 ma 20 ma 10 ma and 5 ma are presented we have attempted to place the south china sea basin in a regional tectonic framework the tectonic evolution of the major blocks surrounding the south china sea were analyzed as well as the relative motions of the indian and australian plates we have tried to correct the tectonic models available in this region a 3 d graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self consistency when the model conflicted with previous interpretations the input data were evaluated for alternative explanations at least two and possibly three stages of extension can be recognized in this region the earliest one active in the late cretaceous to eocene involved nw se extension the second one active from the late eocene to early miocene involved north south extension the third stage of extension which probably trended nw se can be dated as post oligocene the first extensional event produced the ne sw trending proto south china sea and a series of sedimentary basins along the south china margin following the southeastward extrusion of indochina the proto south china sea was mostly consumed at the palawan trough renewed north south extension in the south china continental margin started the present day south china sea spreading in the oligocene the southeastward extrusion of indochina blocked by sundaland resulted in the nw se trending opening of the south china sea basin in the early miocene collision of the north palawan microcontinental block with the west philippines block stopped the opening of the south china sea at the end of early miocene spreading activity switched to the sulu sea basin in the middle miocene but collision between the sulu ridge and the west philippines at mindanao halted the opening of the sulu sea at the end of the middle miocene in the late miocene greater india continued its northward path and seems to have ripped open the andaman sea in the pliocene subduction along the northern manila trench placed the north luzon arc on a collision path with the east asia continental margin at taiwan our reconstructions along with detailed geological and geophysical information may be used as a predictive tool for basin evolution models and block interactions in this region the development of the south china sea basin the gulf of thailand the malay basin and the central thailand basins are the result of collision induced extensional forces the sulu celebes and sumatra basins were formed as a consequence of prolonged subduction the opening of the pearl river mouth west natuna south china sea sulu and possibly celebes basins were terminated by various plate collisions during the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

an alternative interpretation for slip vector residuals of subduction interface earthquakes a case study in the westernmost ryukyu slab abstract slip partitioning along oblique subduction zones has conventionally been examined by slip vector residuals determined from interface earthquakes it is measured through contrasting the slip vector with respect to the local plate convergence direction defined on the surface of the earth by the euler vector interpretation of regional plate kinematics based on slip vector residuals thus defined may be misleading besides the apparent discrepancy attributable to the surface projection of vectors on an obliquely plunging surface a more fundamental problem is the construction of the reference subduction flow field for the subducted slab based upon the known plate convergence it can be shown that for complicated slab geometry the conventional practice of intuitively rotating the euler kinematics onto the subducted slab leads to unrealistic intraplate deformation that invalidates the mechanical integrity of the oceanic lithosphere alternatively we propose to calculate the velocity field for the specific slab geometry based on the rationale that the subduction flow field should be the one that endures the least amount of intraplate deformation the calculated flow field for the westernmost ryukyu slab that is in transition to the philippine sea plate psp eurasian plate ep collision near taiwan reveals quite distinct subduction flow field the particle paths associated with the calculated flow field are in better accordance with the observed slip vectors while the corresponding strain rate field is consistent with the previously reported lateral compression seismogenic zone furthermore it is speculated that the plate kinematics of the northwestern corner of psp that is impinging the vicinity of the collision might have also undergone significant readjustment to accommodate the potential intraplate deformation it is noted that the westernmost subducted ryukyu slab and the northwestern corner of psp belong to a mechanically coherent tectonic unit the 3d geometric constraints set up by the subducted slab and the unfriendly kinematic constraints imposed by the collision boundary both play important roles in configuring the intraplate deformation within psp manifestation in the local kinematics might indicate deviation from the general psp ep convergence and is thus of crucial importance to tectonic interpretations of geophysical observations in this area

aftershock distribution of the february 27 2010 okinawa honto kinkai earthquake japan using sp depth phase abstract a major earthquake moment magnitude mw 7 0 occurred on february 27 2010 in the central ryukyu trench southwestern japan a slab parallel extension type mainshock and its aftershocks occurred on a strike slip fault whose extension axis was parallel to the dip of the philippine sea plate the epicenters of the mainshock and the aftershocks were located in the cluster of slab parallel compression type earthquakes whose compression axes were parallel to the dip of the philippine sea plate to clarify the relationship between the slab parallel extension type mainshock and aftershocks and the slab parallel compression type earthquakes i determined the depths of the hypocenters of 114 aftershocks m 3 0 of the mainshock and 29 background earthquakes m 3 5 from 2006 to 2009 near the central ryukyu trench using the arrival times of the depth phase then i determined the distribution of the aftershocks using the double difference method the relocated aftershock distribution extends along the ene wsw direction this suggests that the mainshock fault lies on a left lateral strike slip fault extending along ene wsw direction on the basis of the relocated hypocenters it is found that reverse fault type slab parallel compression type earthquakes occur in the oceanic crust of the subducted philippine sea plate and strike slip fault type slab parallel extension type earthquakes occur in the mantle of the subducted philippine sea plate this suggests that a double seismic structure exists locally in the shallow part of the subducted philippine sea plate of the central ryukyu trench

seismicity of the gorda plate structure of the continental margin and an eastward jump of the mendocino triple junction analysis of 9 years of data from the humboldt bay seismic network sheds new light on the structure and evolution of the gorda plate and mendocino triple junction significant findings include the buttressing effect of the pacific plate which demonstrates that there is no underthrusting of the gorda plate along the mendocino fault and the pattern of left lateral northeast trending faults which demonstrates how the gorda plate accommodates n s shortening without such underthrusting focal mechanisms are consistent with northward compression of the gorda plate by the pacific plate until the gorda plate passes the triple junction beyond which the n s compressive stress is effectively removed and the focal mechanisms show a change from strike slip to normal faulting with downslab tension another significant feature is the shallow benioff zone 10 40 km which shows a double seismogenic layer unlike most other double seismic layers in deep subduction zones this one appears to be due to underplating with a new subduction zone being developed east of the former one a corner of continental margin material has been partially subducted we attribute this double seismic zone to reactivation of both the old and new subduction boundaries under n s compression produced by pacific gorda plate interaction at the triple junction because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the san andreas fault has migrated eastward we have constructed a model that accounts for both the eastward migration of the san andreas fault and the doubling of the seismic zone in the gorda plate this can be done by assuming that at the time of the last eastward jump of the fault the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the san andreas fault other scenarios for the evolution of the mendocino triple junction do not require that the subduction zone migrate eastward however the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place this is consistent with other evidence for an eastward jump of the san andreas fault

role of shallow phase changes in the subduction of oceanic crust detailed studies of the seismicity of several subduction zones demonstrate that shallow dipping thrust zones turn to steeper angles at depths of about 40 kilometers an increased downward body force resulting from shallow phase changes in subducted oceanic crust may be the cause of this increased dip angle in addition the volume reduction associated with phase changes may produce sufficiently large stresses in neighboring rocks to cause the seismicity of the upper benioff zone

double seismic zones kinematics the kinematics of a double seismic zone of subducting lithosphere at an intermediate depth is studied a simple geometrical argument shows that the strain rate due to unbending of subducting lithosphere can be as large as 10 15 s 1 and large enough to account for the seismic strain rate in this depth range the geometrical unbending of subducting lithosphere in the mantle and the presence of a double seismic zone there can be therefore considered essentially equivalent phenomenon we model a double seismic zone required from this unbending in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere the thickness of a double seismic zone is controlled by the thermal structure and the rheological properties of the subducting slab a preliminary result for the double seismic zone beneath tohoku japan indicates a possibility that the currently available flow law of dry olivine does not properly describe the double seismic zone or at least presents some inconsistency in our knowledge of the mechanical properties e g thermal structure rheology of oceanic lithospheres

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate 1 we investigate the relationship between high precision hypocentres in the subducting juan de fuca plate beneath southwest british columbia and the velocity structure from receiver function analysis and reflection seismic data 108 earthquakes at depths of 40 to 65 km were relocated using a double difference algorithm to obtain precise earthquake locations correlation of the relocated seismicity with structural information shows a concentration of earthquakes near the top of the subducting oceanic crust and a deeper layer of seismicity in the uppermost mantle of the subducting juan de fuca plate the strong correlation of the seismicity with the structure of the subducting plate suggests that seismic failure is caused by changes in mechanical strength within the plate supporting the hypotheses that phase transformations and thermal petrological conditions play an important role in the seismogenesis of double seismic zones index terms 7209 seismology earthquake dynamics and mechanics 7218 seismology lithosphere and upper mantle 7230 seismology seismicity and seismotectonics 7220 seismology oceanic crust citation cassidy j f and f waldhauser evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate geophys res lett 30 2 1095 doi 10 1029 2002gl015511 2003

thermal stresses in subducting lithosphere can explain double seismic zones the discovery of double planed zones of intermediate depth seismicity associated with subducting oceanic lithosphere refs 1 7 and l s h and k h j in preparation has generated much interest in the origin and distribution of stresses in descending plates several mechanisms have been proposed to explain why double zones occur8 here we investigate the role of thermal stresses for the state of stress in a descending plate and test whether they can account for the observed characteristics of double seismic zones by modelling the descending slab as a thin elastic plate and using the temperatures in the slab estimated by toksoz et al 9 we find that the polarities of thermally generated stresses agree with those obtained from the focal mechanisms of earthquakes in the two zones thermal stresses also can explain the separation between the two zones of seismicity at shallow depths and their gradual merging at greater depth

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

global seismicity of 1981 centroid moment tensor solutions for 542 earthquakes abstract centroid moment tensor solutions are presented for 542 earthquakes that occurred during 1981

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

moment tensor inversion for offshore earthquakes east of taiwan and their implications to regional collision reliable determination of source parameters for offshore earthquakes east of taiwan with mb 5 5 was a difficult task because of the poor azimuthal coverage by local network and the lack of signals at teleseismic distances we take advantage of the recently established broadband array in taiwan for seismology bats to invert seismic moment tensors for 7 such events occurred in 1996 to cope with different patterns of background noise and unknown structural details we utilize variable frequency bands in the inversion and adapt a two step procedure to select best velocity models for individual epicenter station paths our results are consistent with the overall patterns of regional collision and indicate that the resulting compressive stress has caused significant intraplate deformation within the philippine sea plate simulation of the region s geological evolution and orogenic processes should take this factor into account and allow the philippine sea plate to deform internally

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

significant crustal structural variation across the chaochou fault southern taiwan new tectonic implications for convergent plate boundary abstract the chaochou fault a major geological boundary in southern taiwan is considered to be a part of the convergent plate boundary between the eurasia plate and the philippine sea plate we applied the common conversion point stacking technique to teleseismic radial receiver functions and obtained moho variation and crustal structure across the chaochou fault in the eurasia plate to its west the moho depth is about 37 km and the crust is subducting to the east beneath the philippine sea plate with a dip angle of about 30 between the backbone belt and the tananao schist in the philippine sea plate the moho depth is about 17 km the longitudinal valley marks the collision boundary between the eurasia plate and the philippine sea plate the results suggest that the depth extent of the chaochou fault is about 30 35 km and the fault becomes a shallow angle thrust fault at depth the common conversion point image also shows several bending interfaces of velocity contrast in the crust we proposed a simple model to explain the philippine sea plate and eurasia plate collision process and the observed crustal deformations

subduction continent collision in southwestern taiwan and the 2010 jiashian earthquake sequence abstract we integrated a regional tomography model 20 year seismicity and earthquake focal mechanisms of the 4 march 2010 m l 6 4 jiashian earthquake source region to delineate the seismogenic structure and mechanics of a relatively rare damaging inland earthquake that occurred in the southwestern fold and thrust belt of taiwan the main shock of the jiashian earthquake occurred at a depth of 23 km beneath the slate belt of the southern central range with a sinistral thrust mechanism based on seismic tomography and seismicity the jiashian earthquake sequence occurred in the chishan transfer fault zone which we interpret to be located near the transition zone between the subducted eurasian plate to the south and the arc continent taiwan collision to the north the distribution of the jiashian aftershocks define a wnw ese striking fault plane that dips 30 40 nne which is consistent with the optimal fault plane of the gps derived coseismic slip model focal mechanisms and inverted stress results of the earthquake sequence indicate both thrust and strike slip motions with the maximum compressive stress at nearly 90 to the regional compressive stress and thus sub parallel to the strike of the taiwan orogen we propose that the 2010 jiashian earthquake resulted from rupturing of a buried oblique fault within the chishan transfer fault zone at the subduction collision transition zone in southern taiwan the orogen parallel p axis of the 2010 jiashian earthquake represents collision related lateral compression although a local stress deviation is also possible

monitoring interseismic activity on the ilan plain ne taiwan using small baseline ps insar gps and leveling measurements partitioning from arc continent collision and backarc extension

a study on fault type and site effect vs30 parameters in the attenuation relationships of peak ground acceleration and peak ground velocity in ilan taiwan the purpose of this study is to derive attenuation relationships of the peak ground acceleration pga and peak ground velocity pgv in ilan taiwan including factors of fault type and site effect a total of 2852 accelerograms recorded from 92 shallow earthquakes with mw magnitude ranging from 4 0 to 7 7 are used to develop theregionalattenuationrelationships twomodelsareused model1usesrecordsfrom 65 strong motion sites the results will be especially useful for the central weather bureau cwb early warning system to quickly assess and report the pga and pgv maps for effective emergency response operations model 2 uses records from the 46 strong motion sites with available vs30 to incorporate a site effect term aiming to reduce the standard deviation of the predicted ground motion for engineering appli cations first the results show that the fault type amplification factor decreases faster with increasing magnitude for the strike slip fault type than the reverse or normal fault types in addition from the intraevent residuals as function of distance of horizontal pga for reverse earthquakes we found clusters of higher pgavalues at a distance of 110 130 km from events in southwestern areas this was probably due to moho re flection along paths crossing the central mountain range second the pgvintraevent residual is more closely correlated with vs30 than that of the pga because the latter is nota simple function ofvs30 finally bycomparing thestandard deviations ofthetotal residuals between the observed and predicted values before and after incorporating the fault type and site effect vs30 terms the change of standard deviation for pga is 2 3 in contrast the pgv standard deviation is significantly decreased by about 11 6 evidently the model 2 attenuation relationships especially for pgv would be better suited for engineering applications

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

accretionary tectonics of burma and the three dimensional geometry of the burma subduction zone the geometry of the burma wadati benioff zone wbz has been determined by fitting a trend surface parameterized with eight effective degrees of freedom to 184 well located hypocenters the dip of this surface which passes through the middle of the wbz varies from about 50 in the north near the eastern himalayan syntaxis to about 30 in the bay of bengal area the eastern edge of the indo burman ranges closely follows the map projection of the 60 km depth contour of the wbz the curvature of the indo burman ranges is controlled by the geometry of the interface between the more steeply dipping part of the indian plate and the leading edge of the overriding burma platelet shallow earthquakes beneath the indo burman ranges are primarily confined to the underthrusting indian plate their focal mechanisms indicate strike slip faulting and north south shortening parallel to the eastern margin of the indian plate

relocation of earthquakes in the cook inlet area south central alaska using the joint hypocenter determination method abstract the joint hypocenter determination jhd method was used to relocate 1604 earthquakes that occurred during 1988 to 1993 in the wadati benioff zone wbz beneath the cook inlet in south central alaska the study utilizes both p and s wave arrivals recorded by a recently improved regional seismic network and it provides an improvement over previous studies the jhd locations of the events allowed improved delineation of the geometry of the subducted slab and the detection of a double seismic zone within the wbz in the 40 to 100 km depth range the jhd station corrections obtained correlate with the major surface geological features in the study area slow velocity sedimentary basins as well as with crustal and mantle structure high velocity subducting slab and low velocity anomalies beneath the active volcanoes to investigate the dependence of the jhd relocation procedure on factors such as the hypocentral depths of the events used the velocity model the p s ratio and random and systematic errors in travel time readings synthetic travel times were calculated using an exact raytracing technique a three dimensional tomographic p wave velocity model the present network configuration and the actual seismicity distribution then the synthetic events were located with the jhd algorithm the synthetic experiments showed that the jhd procedure is rather insensitive to the choice of plane layered velocity model and p s ratio preserves relative hypocenter locations well and can handle systematic and large random reading errors effectively

p wave velocity and gradient images beneath the okinawa trough abstract we determined the high resolution 3 d p wave velocity structure under the okinawa trough in the region between south taiwan and north kyushu japan by inverting 102 511 absolute travel time data from 5012 local earthquakes and 29 153 relative travel time residuals calculated from 1873 teleseismic events for the first time a combined data set recorded simultaneously by three seismographic networks located in kyushu okinawa trough taiwan and fujian province of china has been used which enables us to determine the high resolution p wave velocity structure in the crust and upper mantle under the entire arc system the resulting tomographic and gradient images provide a new insight into the structural heterogeneities related to the opening of the okinawa trough and dehydration of the philippine sea slab phs and confirm some of the features proposed by previous studies a high velocity zone is imaged clearly at depths of 50 300 km along the ryukyu arc system from south kyushu to north taiwan showing good agreement with the spatial distribution of seismicity under the subduction zone the relative residuals from different quadrants also exhibit early relative arrivals at the stations corresponding to the high vp regions and we therefore interpret the high vp zone as the subducting phs slab along the volcanic front from south kyushu to north taiwan low vp areas are visible which are mainly caused by the fluids liberated by the extensive dehydration process of the subducting phs slab this fluid related low velocity zone is located at depths of 20 120 km along the arc with a total length of 1200 km from north taiwan to south kyushu using the inverted p wave velocity model we also calculated the gradient images which are independent of the selection of reference model or color scale to provide further information and help us to understand and explain the tomoagraphic structures three sub blocks of the ryukyu arc system are revealed from the inverted velocity images and the calculated gradient maps these features of the velocity anomalies together with other parameters determined by the present study suggest that the okinawa trough extension arc magmatism and seismicity are mainly attributed to the dehydration process associated with the phs slab subduction and the oblique subduction direction of the plate

triple seismic zone and the regional variation of seismicity along the northern honshu arc the regional variation of seismicity along the northern honshu arc japan is studied using accurate focal depths and focal mechanism types we use focal depths determined from pp p time intervals reported in the isc bulletins for submarine earthquakes depths are corrected by considering the pp phase reported in the bulletins as the pwp phase the reflection from the ocean surface out of more than 600 well located earthquakes selected from the isc bulletins we determine the types of the focal mechanisms of 184 events using p wave first motion data based on historical seismicity of great and large earthquakes we divide the zone of thrust type earthquakes at the plate interface into two regions the shallow thrust zone 0 40kin where great earthquakes ms 8 0 occur and the deep thrust zone 40 60km where large earthquakes ms 7 4 occur the activity of great or large earthquakes shows a variation along the are in some regions both the shallow and deep thrust zones are active and in other regions only one of the thrust zones is active the seismicity of recent moderate size earthquakes mb 4 combined with the focal mechanism type shows a variation along the are which reflects the variation of the activity of great or large earthquakes where large earthquakes do not occur in the deep thrust zone neither thrust type nor down dip compression tension type events occur in and beneath the deep thrust zone where large earthquakes do occur in the deep thrust zone we find a number of thrust type earthquakes further in the latter case in some regions the down dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple planed structure of seismicity the triple seismic zone this study confirms the hypothesis of previous workers seno and pongsawat 1981 on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone i e the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling respectively here the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone triple seismic zones are found offshore of miyagi prefecture where the deep thrust zone has been broken recently in 1978 and offshore of fukushima prefecture we expect a future large earthquake at the deep thrust zone offshore of fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938

model calculations of regional network locations for earthquakes in subduction zones algorithms for locating earthquakes recorded by local networks commonly assume a flat layered velocity structure even in island arc regions where the presence of a dipping lithospheric slab introduces large lateral velocity variations moreover because seismic stations occur along the narrow island arc the resulting linear network geometry degrades earthquake location to model this situation we trace rays in a spherical earth from hypothetical earthquakes through appropriate velocity models to networks of various geometries to isolate the effect of the slab most of the velocity models consist simply of a planar dipping constant velocity region embedded in a sphere with a constant lower seismic velocity we then use the calculated p and s travel times to relocate the hypothetical earthquakes with a flat layered velocity structure we perform calculations of this kind in a variety of models varying the dip of the slab the velocity contrast between the slab and the mantle the station geometry and the distance between the earthquakes and the slab mantle boundary we also perform calculations in more realistic models with geometry closely resembling two seismic networks in alaska these model calculations emphasize the great care that seismologists should exercise when interpreting local network locations in island arc regions in the present study all the relocations incorporate p and s arrivals at all stations and include no random or picking errors nevertheless the relocated hypocenters differ from the actual model hypocenters by up to 100 km or more and produce significant distortions in the wadati benioff zone the most prominent spurious feature is a clear increase in the apparent dip of the wadati benioff zone beneath a certain depth furthermore the relocation reduces the width or apparent thickness of the wadati benioff zone e g they produce an apparent thickness of 5 km or smaller for a zone of earthquakes with an actual thickness of 20 km however if different combinations of stations report different events it is possible to determine an apparent thickness which exceeds the actual thickness for local networks near subduction zones it is useful to estimate the mislocation and rms residual expected for each event recorded and located this can be accomplished by performing model calculations in a simple model with a constant velocity dipping slab embedded in a half space for such a simple model there exists only one ray path from each earthquake within the slab to each station and the travel time for this path can be calculated as a part of the routine network location process a relocation using the routine network location algorithm and these calculated travel times provides an indication of the expected mislocation and rms residual for the actual recorded event

a model for the termination of the ryukyu subduction zone against taiwan a junction of collision subduction separation and subduction boundaries 1 the nw moving philippine sea plate psp collides with the eurasian plate eup in the vicinity of taiwan and at the same time it subducts toward the north along sw ryukyu the ryukyu subduction zone terminates against eastern taiwan while the ryukyu trench is a linear bathymetric low about 100 km east of taiwan closer to taiwan it cannot be clearly identified bathymetrically owing to the deformation related to the collision making the location of the intersection of the ryukyu with taiwan difficult to decipher we propose a model for this complex of boundaries on the basis of seismicity and 3 d velocity structures in this model the intersection is placed at the latitude of about 23 7 n placing the northern part of the coastal range on eup as psp gets deeper along the subduction zone it collides with eup on the taiwan side only where they are in direct contact thus the eurasian plate on the taiwan side is being pushed and compressed by the nw moving philippine sea plate at increasing depth toward the north offshore of northeastern taiwan the wedge shaped eup on top of the ryukyu subducting plate is connected to the eup on the ryukyu side and coupled to the nw moving psp by friction at the plate interface the two sides of the eup above the western end of the subduction zone are not subjected to the same forces and a difference in motions can be expected the deformation of taiwan as revealed by continuous gps measurements geodetic movement along the east coast of taiwan and the formation of the hoping basin can be understood in terms of the proposed model

an explanation for the double seismic layers north of the mendocino triple junction we propose that the gently eastward dipping double planed seismic zone observed at 15 25 km depths in the southern cascadia subduction zone just north of the mendocino triple junction is a direct consequence of the thermally controlled rheology as the oceanic lithosphere subducts to a depth of about 15 km the temperature regime causes a brittle plastic transition to occur within the oceanic crust thus a ductile layer forms in the lower oceanic crust sandwiched between the brittle upper crust and brittle upper mantle the very high strain rates near the triple junction caused by the northward push of the pacific plate on the gorda plate increase the seismicity and thus accentuate the double seismic zone in this region this model explains the focal mechanisms observed in the seismic zone and their spatial change the double seismic layers clearly define the position of the subducting gorda plate previously uncertain in the cape mendocino region

the double seismic zone in kuril kamchatka the tale of two overlapping single zones the classic double seismic zone in the central portion of the kuril kamchatka arc seems to turn into a regular wadati benioff zone with downdip compression and extension in the northeastern and southwestern ends of the arc respectively to delineate this lateral changeover we studied 33 large to moderate sized mb 5 1 earthquakes that occurred at depths shallower than 200 km between 1963 and 1991 these events took place in two regions where the double seismic zone terminates and single seismic zones emerge precise earthquake source parameters including focal depths and mechanisms are determined from a careful analysis of a mixture of analog and digital p and sh waveforms recorded at teleseismic distances our results indicate that the double seismic zone is confined between approximately 46 and 53 n northeast of 53 n intermediate depth earthquakes 70 200 km show only downdip compressional focal mechanisms furthermore the wadati benioff zone there seems to be a continuation of the upper compressional layer of the double seismic zone thus the changeover from a double to a single seismic zone in this case is simply disappearing of the lower extensional sheet of the double seismic zone similarly southwest of 46 n intermediate depth earthquakes show only downdip extensional mechanisms the single seismic zone is unlikely to consist of two layers of extensional seismicity in fact nowhere along the entire arc did we observe two distinct seismogenic layers with a single state of strain in other words these extensional events did not occur in the same seismogenic structure as the upper compressional layer of the double seismic zone however we cannot ascertain whether the single seismic zone in extension is simply a southwestern extension of the lower extensional layer of the double seismic zone lateral variation from a single zone under extension in the southwest to a double seismic zone and finally to a single zone under compression cannot be accounted for by conventional models of a double seismic zone on the other hand the addition of a component of stress transmitted from depths greater than 250 km can explain the observed lateral variation in the state of strain at intermediate depths because the slab between depths of 250 450 km is under extension in the southwestern end of the arc whereas the rest of the arc is under compression vanishing of the lower extensional layer of the double seismic zone toward the northeastern end of the arc is interpreted as a result of compression transmitted from below overshadowing extension caused by effects such as unbending a similar reasoning applies to the southwestern termination of the double seismic zone

collisional tectonics between the eurasian and philippine sea plates from tomography evidences in southeast china abstract the upper mantle structure of southeast china is important for us to understand the deformation and mantle dynamics process associated with the interaction between the eurasian plate and philippine sea phs slab we determined a detailed three dimensional p wave velocity vp structure of the crust and upper mantle down to 400 km depth beneath southeast china by applying teleseismic tomography to 6869 high quality p wave arrival times the data were collected very carefully from the original seismograms of 635 teleseismic events recorded by 65 broadband stations deployed in southeast china our images show that the high vp phs slab subducts toward the north along the ryukyu trench at the latitude of about 24 n and extends down to 350 km depth and even more high vp anomalies are imaged in the upper mantle under central and southern taiwan which represent the subducted eurasian plate break off eurasian plate at a big angle subducting eastward is revealed under central taiwan at depths from the upper mantle to 400 km while continuous eurasian plate under south taiwan is mainly imaged from the moho down to 400 km depth a torn mantle window within the eurasian continent beneath central and northern taiwan created by the northward motion of the philippine sea plate is the upwelling path of the asthenosphere the tomographic images also show the low vp anomalies spread widely under the coastal areas of mainland china and taiwan strait the structure of the crust and upper mantle suggests that the mountain building process in the central part of taiwan is mainly attributed to the subduction collision tectonics at the boundary between the eurasian continental lithosphere and the subducting oceanic lithosphere of the phs slab

a double planed seismic zone in kamchatka from local and teleseismic data the fine structure of a double planed deep seismic zone is studied over a wide area of the kamchatka peninsula this prominent feature of deep seismic zone configuration is ascertained through the analysis of microearthquake hypocenters from the local seismic network of the institute of volcanology of kamchatka and 22 focal mechanism solutions from the formal inversion of long period p and sh waves for events with mh 5 5 additionally 1 1 focal mechanism solutions estimated from the first motion of p waves and 12 centroid moment tensor solutions of harvard university are used the maximum depth of the double seismic zone is 170 180 km the two planes of seismicity are separated by 40 km at a depth of 50 km and by 10 15 km at 180 km depth the focal mechanism solutions of shallow earthquakes show an abrupt change from the thrust events to down dip compressional events at approximately 60 km depth at the upper boundary of the descending slab within the descending slab the earthquakes with down dip tensional axis form the lower plane of the double planed deep seismic zone several earthquakes with down dip tensional axis are discovered in a narrow area of the upper seismic zone at the depth of about 50 km the double seismic zone is revealed clearly in the area between 52 n to 54wn and probably extends up to 56 n

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

transition from oblique subduction to collision earthquakes in the southernmost ryukyu arc taiwan region tectonic characteristics of the region between taiwan and the southernmost ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5 5 mb 6 6 five major seismogenic structures can be delineated the collision seismic zone csz the interface seismic zone isz the wadati benioff seismic zone wbsz the lateral compression seismic zone lcsz and the okinawa seismic zone osz in the csz located along the east coast of taiwan and offshore earthquake focal mechanisms show horizontal p axes distributed in two directions 287 10 and 333 16 possibly reflecting a strain partition associated with the relative plate convergence between the eurasia plate and the philippine sea plate the corresponding seismic strain tensor indicates a maximum compressive strain rate of 1 2 10 7 yr 1 along 293 and a comparable extension in vertical direction presumably resulted from plate collision in the region the geometry of the isz which is distorted significantly at its westernmost end can be approximated by a north dipping plane that is gradually pushed northward with increasing dip the seismogenic portion of the interface spans a short depth range from 10 km to 35 km a clear pattern of earthquake slip partition is observed the average slip vector residual is as large as 35 seismic strain patterns within the subducted philippine sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at 270 km different from the pattern of strain segmentation observed for the rest of the ryukyu arc where the northern and southern portions are dominated by downdip extension and compression respectively owing to the large convergence obliquity the slab is descending at a rate significantly slower near taiwan than in the southern ryukyu thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab s negative buoyancy a number of thrust or oblique strike slip earthquakes between isz and wbsz show a consistent pattern of lateral compression with p axes oriented roughly parallel to the local strike of the trench arc system they are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere shallow normal faulting earthquakes show successive rotation of t axes from approximately n s in the okinawa trough to approximately e w in northeast taiwan possibly as a result of interaction between the extension from the opening of the okinawa trough and the compression from collision one normal event january 18 1991 mb 5 9 occurred in the central range with t axis roughly parallel to the structural trend of taiwan implying that the nature of the orogeny in taiwan has changed from thin skinned deformation to lithospheric collision involving the whole crust and uppermost mantle

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

structure of the benioff zone beneath the shumagin islands alaska relocation of local earthquakes using three dimensional ray tracing seismic rays are traced through a prescribed three dimensional inhomogeneity that simulates the subducted slab below the shumagin islands region to calculate local station delays for a given hypocenter and a slab model hypocenters determined by the shumagin seismic network are then relocated using the station delays a flat layered velocity structure and a standard earthquake location computer program station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data a set of path corrections that is calculated for each grid point station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km application of this method to data from 1982 for the digitally recording shumagin seismic network shows the following results 1 a previously observed apparent increase in dip of the subducted slab at depths of 100 km disappears 2 the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45 toward north northwest at depths between 80 and 250 300 km and has a 7 higher velocity than the surrounding mantle 3 hypocenters determined from shumagin network data are located only 10 20 km south of high quality hypocenters determined from teleseismic data alone 4 qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high frequency coda possible converted phases and low amplitudes of first p arrivals for rays that travel mostly along the slab conversely for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first p arrivals that are followed by an insignificant coda and low amplitude s waves

improved seismic tomography offshore northeastern taiwan implications for subduction and collision processes between taiwan and the southernmost ryukyu summary to improve the resolution in imaging the 3 d vp and vp vs structures in the taiwan region especially offshore eastern taiwan we combine the arrival times from eleven ocean bottom seismometers obss and from seismic stations of the japan meteorological agency jma with those from the permanent stations of taiwan central weather bureau seismic network cwbsn and taiwan strong motion instrumentation program tsmip by doing so we have obtained a new 3 d tomographic model for taiwan and its surrounding regions with a better resolution especially in the area offshore northeastern taiwan we also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern taiwan and determined the focal mechanisms of relatively large events our tomography results indicate that in the region northeast of taiwan the subducting oceanic philippine sea plate beneath the eurasian continental lithosphere is characterized by a high vp layer surrounded by lower vp areas this p wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface especially in its shallower portion in the hangingwall eurasian block above the subduction interface a vertically elongated high vp vs body appears to originate from the interface at depths between 100 and 140 km we suggest that it represents the partially melted materials that are related to the magmatic activity of the ryukyu volcanic arc

source parameters of aftershocks of the 1991 costa rica and 1992 cape mendocino california earthquakes from inversion of local amplitude ratios and broadband waveforms abstract source parameters of aftershocks of the 22 april 1991 m w 7 7 costa rica and the 25 april 1992 m w 7 1 cape mendocino california earthquakes are determined using a grid search inversion of p sh and sv amplitude ratios recorded by sparse local networks of three component broadband and short period stations the inversion procedure consists of computing synthetic seismograms for three fundamental fault orientations for all source receiver pairs over a range of source depths calculating the complex envelopes of the observed and synthetic seismograms to determine peak amplitudes of p sh and sv waves combining the fundamental fault amplitudes for all possible values of strike dip and rake at 10 increments and determining the best fault orientation and depth as the one that yields the smallest misfit between observed and synthetic p sh p sv and sv sh amplitude ratios the ambiguity in the sense of motion on the nodal planes arising due to the use of amplitude ratios is resolved by examining p wave polarities the sensitivity of source parameters to uncertainties in earthquake location and crustal structure is explored for events with good station coverage focal mechanism determinations are stable for a wide range of assumed values of crustal structure earthquake location and depth source parameters for many of the largest events m 3 4 are also determined by inversion of broadband displacement waveforms using a similar grid search technique comparable results were obtained using both broadband waveforms and amplitude ratios focal mechanism solutions for 20 aftershocks of the costa rica earthquake reveal a complicated faulting geometry indicating active thrust normal and strike slip faults in the back arc of costa rica the 1992 cape mendocino earthquake occurred at the intersection of the north american gorda and pacific plates while the mainshock was associated with underthrusting of the gorda plate beneath the north american plate fault plane solutions for 70 of the 38 largest aftershocks indicate that these events result from either motion between the gorda and pacific plates or from internal deformation within the gorda plate