numerical modelling of arc continent collision application to taiwan two dimensional finite element numerical modelling is applied to study the deformation and failure of the overriding plate during arc continent collision continental margin subduction this plate has elasto plastic rheology with strain weakening and contains a volcanic arc with thinned and weakened lithosphere the overriding plate deforms due to the normal and tangential stresses applied along the interplate surface these boundary conditions represent the friction and pressure between the plates the latter is due to the flexural rigidity of the subducting lithosphere and the buoyancy force generated by the subducting continental crust the modelling shows that continental margin subduction results in increasing compression and failure of the overriding plate which occurs along the surface dipping under the arc in either of two possible directions the failure mode is largely controlled by the two competitive factors the rigidity of the subducting plate and thickness gradient of the subducted continental crust a high rigidity favors failure along an ocean vergent fault which is followed by a subduction reversal while a high thickness gradient favors failure in the opposite direction which is followed by a fore arc block underthrusting beneath the arc both scenarios seem to have natural analogs we consider one of them the ongoing arc continent collision in taiwan and argue that this process occurs according to the second scenario corresponding to the fore arc underthrusting wavelet statistical analysis of the seismicity distribution to the south of taiwan has clearly displayed a shallow 0 40 km zone of high density coherently distributed seismicity beneath the luzon arc this zone interpreted as a lithospheric scale fault dips from the forearc basin to the east and corresponds to the initiation of the forearc block subduction a self consistent combined mechanical gravity topography model is used to see whether failure of the overriding plate to the south of taiwan can be captured by this model by tuning diverent controlling parameters we did not succeed in obtaining realistic topography and gravity field in a model where failure of the overriding plate was not allowed introduction of this failure and underthrusting of the forearc block under the luzon arc allowed us to fit both topography and gravity data 2000 elsevier science b v all rights reserved

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

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

spatial variations in stress within the 1987 whittier narrows california aftershock sequence new techniques and results the ml 5 9 whittier narrows california earthquake of october 1 1987 triggered a complex aftershock sequence the aftershocks had many different focal mechanisms including thrusting on e w striking north dipping planes similar to the main shock right lateral motion on nw se striking planes similar to the ml 5 3 aftershock on october 4 1987 thrusting on n s striking planes and left lateral motion on n s striking planes i attempt to interpret these mechanisms in terms of a stress field that may have caused them previously stress inversion methods based on focal mechanisms assumed that the stress field was spatially uniform in order to test for the spatially varying effect of the main shock s dislocation on the aftershocks focal mechanisms a new inversion technique was developed that solves for a uniform stress field that is superimposed on a given spatially varying stress field a series of simulations were performed to test both the new method and an older method that uses only the uniform component of the stress field these simulations show that both techniques work but will be limited by the amount of noise in the data and the accuracy to which the given spatially varying stress field resembles the actual spatially varying stresses most importantly the simulations show that the older methods do work when the stress field has a uniform component that is at least as large as the spatially varying component the simulations also show that when applying the older stress inversion techniques one can determine if the results are valid and the amount of spatially varying stresses present by examining the misfit between the data and the model application of these techniques to the whittier narrows aftershocks suggests that the uniform component of the stress field and the complex faulting corresponds to the response of an elastic halfspace to a simple regional n s compression the misfit between the data and the uniform model demonstrate the presence of spatially varying stresses at whittier narrows however their form as shown by the simulations can not be determined with this data set and these techniques

collision and stress trajectories in taiwan a finite element model abstract field analyses of plio quaternary compressional deformations in taiwan have enabled us to reconstruct the paleostress trajectories resulting from the collision of the luzon arc philippine sea plate with the chinese continental margin eurasian plate the direction of the maximum compressional stress 1 shows a fan shaped pattern that we interpret as resulting from the collision of a rigid body the luzon arc indenting a more deformable material the thick sediments of the chinese continental margin simple analytical models qualitatively explain the fan shaped pattern but the influence of various parameters such as boundary conditions and rheology cannot be quantitatively accounted for by this approach consequently we have used a finite element technique to compute the stresses and strains induced by the push of a rigid body against a two dimensional viscous material the boundary conditions are the velocities based on plate kinematics a motion in the n300 e direction best explains the stress trajectories observed in central taiwan viscosity contrasts as well as small changes in the shape of the northern edge of the indenter have little influence on the computed stress pattern the most important parameter is the direction of convergence our model quantitatively explains the general pattern of the stress trajectories observed in the collision zone of taiwan between the philippine sea plate and eurasia

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

seismogenic patterns in the taiwan region insights from source parameter inversion of bats data abstract we systematically invert source parameters of 96 earthquakes that occurred in the taiwan region between july 1995 and june 1998 using waveforms recorded by the newly established broadband array in taiwan for seismology bats the results are utilized to delineate the seismogenic patterns associated with the regional tectonic processes between the eurasia and philippine sea plates in general high seismicity is observed at five locations including the nan ao basin 24 n 122 4 e near the hualien area 24 2 n 121 7 e east of the longitudinal valley and within the philippine sea plate the forearc region to the west of lanshu and the okinawa trough events with focal depths less than 25 km scatter across a wide region from the taiwan strait to the ryukyu and luzon arcs whereas events between 25 and 50 km concentrate offshore east of taiwan most earthquakes that occurred beneath the nan ao basin are consistent with the northward subduction of the philippine sea plate along the plate interface events offshore east of the longitudinal valley and within the philippine sea plate are dominated by a compressional regime along the nw nww directions whereas the lanshu lutao forearc is characterized by e w compression normal faults showing n s extension are observed in the okinawa trough where we observed a successive rotation of t axes from n s to nw se as the epicenters approach taiwan indicating a possible interaction between the extensional and compressional strain regimes there most events between 50 and 65 km depth within the subducted philippine sea plate beneath ryukyu are in downdip extension with lateral compression approximately parallel to the strike of the slab the depth where the state of strain switches from downdip extension to downdip compression is constrained at 110 125 km for the southernmost ryukyu slab based on our results we propose that the subducted philippine sea plate beneath ne taiwan plays a significant role in the overall orogenic process of taiwan the lithospheric collision in taiwan should consist of two major components one is the well recognized arc continent collision that dominates the central and southern taiwan and another the slab continent collision that reigns in northern taiwan a cluster of earthquakes near hualien suggests that an incipient west or northwestward subduction of the philippine sea plate is currently taking place there alternatively they can be interpreted to suggest that the steeply east dipping longitudinal valley fault may have transformed near hualien into a diffuse thrust zone which eventually connects itself to the plate interplate beneath the southernmost ryukyu arc earthquakes beneath taiwan and the taiwan strait are compatible with the overall collision convergence and lateral extrusion nonetheless the lateral extrusion is probably only a secondary feature in the tectonic processes of taiwan

nied seismic moment tensor catalogue for regional earthquakes around japan quality test and application abstract we have examined the quality of the national research institute for earth science and disaster prevention nied seismic moment tensor mt catalogue obtained using a regional broadband seismic network freesia first we examined using synthetic waveforms the robustness of the solutions with regard to data noise as well as to errors in the velocity structure and focal location then to estimate the reliability robustness and validity of the catalogue we compared it with the harvard centroid moment tensor cmt catalogue as well as the japan meteorological agency jma focal mechanism catalogue we found out that the nied catalogue is consistent with harvard and jma catalogues within the uncertainty of 0 1 in moment magnitude 10 km in depth and 15 in direction of the stress axes the nied mt catalogue succeeded in reducing to 3 5 the lower limit of moment magnitude above which the moment tensor could be reliably estimated finally we estimated the stress tensors in several different regions by using the nied mt catalogue this enables us to elucidate the stress deformation field in and around the japanese islands to understand the mode of deformation and applied stress moreover we identified a region of abnormal stress in a swarm area from stress tensor estimates

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

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

subducting oceanic high causes compressional faulting in southernmost ryukyu forearc as revealed by hypocentral determinations of earthquakes and reflection refraction seismic data absolute earthquake hypocenter locations have been determined in the area offshore eastern taiwan at the southernmost ryukyu subduction zone location process is run within a 3d velocity model by combining the taiwanese and neighboring japanese networks and using the 3d maxi technique the study focuses on the most active seismic cluster in the taiwan region that occurs in the forearc domain offshore eastern taiwan earthquakes distribute mainly along 2 active planes the first one aligns along the subduction interface and the second one shallower affects the overriding margin focal mechanisms within the shallow group indicate that nodal planes are either compatible with high angle back thrusts or low angle thrusts the active seismic deformation exclusively indicates reverse faulting revealing that the forearc basement undergoes trench perpendicular strong compression by integrating the seismological image into the regional context we favor the hypothesis in which the dense seismicity occurring offshore marks the activity of en echelon high angle reverse faults accommodating the uplift of a broken piece of ryukyu arc basement called hoping basement rise the uplift is inferred to be caused by the subduction of an oceanic relief either exotic block seamount or oceanic crust sliver our favored solution satisfies the narrowness of epicenter s cluster along the hoping canyon and the observation of high angle active faults on seismic lines crossing the area furthermore this solution is compatible with the active uplift of the hoping rise demonstrated from morphological and sedimentological data we do not exclude the branching of the high angle reverse faults system onto a splay fault connected with the subduction interface but further investigations are needed to map precisely the 3d distribution of active faults that break the margin

kinematics of convergence deformation and stress distribution in the taiwan collision area 2 d finite element numerical modelling abstract using a 2 d plane stress finite element model with elastic and elasto plastic rheologies appropriate for deformation within the brittle upper crust we analyse the relationship between kinematics of convergence deformation and stress distribution in the present taiwan collision occurring between the ryukyu and luzon subduction zones the distribution of stress trends calculated in our models is compared with a synthetic map of actual stress trajectories based on the most recent data available in the collision zone these data combine present day sources from borehole breakouts and earthquake focal mechanism with the reconstruction of quaternary palaeostress from fault slip data analyses resulting in a complete map of compressional stress trajectories which is used to constrain our models we show that the distribution of stress trajectories in the active taiwan collision is principally controlled by 1 the geometric configuration of the boundary between eurasia and the philippine sea plate 2 the shape and rheological properties of major structural units 3 the direction of convergence of the philippine sea plate relative to eurasia and 4 the influence of the opening of the okinawa trough the study of a two dimensional elastic and elasto plastic finite element modelling of the subduction collision in and around taiwan allows us to estimate the influences of these different parameters in the stress pattern taking into account both the simplifying assumptions of the numerical modelling and the angular uncertainties of field determinations the fit between the calculated stress pattern of the finite element model and that determined based on the geometrical synthesis of field analyses is rather good in general indicating that our model is valid to first approximation misfits remain minor and can be explained by data uncertainties and simplifying modelling assumptions for instance the shape of the corner of the collision zone is critical but is not accurately known also limited decoupling in the longitudinal valley collision zone was not considered in our models although it certainly plays a role some interesting features of our model are 1 the greater influence of the shape of the collision zone in comparison with that of the direction of convergence 2 the requirement for a trench retreat related to suction force in the ryukyu arc and 3 the crucial role of the interaction between okinawa trough opening and collision at the sharp northwestern corner of the philippine sea plate including its influence on the geological evolution of northeastern taiwan

cmt data inversion using a bayesian information criterion to estimate seismogenic stress fields summary we developed an inversion method to estimate the stress fields related to earthquake generation seismogenic stress fields from the centroid moment tensors cmt of seismic events by using akaike s bayesian information criterion abic on the idea that the occurrence of an earthquake releases some part of the seismogenic stress field around its hypocentre we define the cmt of a seismic event by a weighted volume integral of the true but unknown seismogenic stress field representing each component of the seismogenic stress field by the superposition of a finite number of 3 d basis functions tri cubic b splines we obtain a set of linear observation equations to be solved for the expansion coefficients model parameters we introduce prior constraint on the roughness of the seismogenic stress field and combine it with observed data to construct a bayesian model with hierarchic highly flexible structure controlled by hyper parameters the optimum values of the hyper parameters are objectively determined form observed data by using abic given the optimum values of the hyper parameters we can obtain the best estimates of model parameters by using a maximum likelihood algorithm we tested the validity of the inversion method through numerical experiments on two synthetic cmt data sets assuming the distribution of fault orientations to be aligned with the maximum shear stress plane in one case and to be random in the other case then we applied the inversion method to actual cmt data in northeast japan and obtained the pattern of the seismogenic stress field consistent with geophysical and geological observations

spatial variation of the crustal stress field along the ryukyu taiwan luzon convergent boundary 1 we applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the philippine sea plate our results indicate that the crustal stress along the ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction including the tokara channel an extensional stress regime is observed along the entire ryukyu back arc implying that back arc rifting may have extended northward to kyushu a triangular area near the southernmost terminus of the ryukyu arc is characterized by a unique stress signature the eastern boundary of this ryukyu taiwan stress transition coincides with the 123 e meridian where the gagua ridge intercepts the ryukyu trench whereas its western boundary agrees remarkably well with the border between the postcollision and waning collision domains in northern taiwan the taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the lukang magnetization high lmh suggesting that the lmh may be critical in controlling the local stress distribution the stress signature of the luzon arc taiwan collision reaches as far south as 19 5 n the tectonic stress along the manila trench luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in slab membrane stress since this extensional regime is observed only south of 22 n it probably marks the northern limit of the contemporary boundary between the subduction along the manila trench and the collision in taiwan

stress field in the ryukyu kyushu wadati benioff zone by inversion of earthquake focal mechanisms abstract the study addresses the space distribution of the stress field in the kyushu ryukyu wadati benioff zone based on homogeneous data of earthquake focal mechanisms and the inverse technique by gephart and forsyth j geophys res 89 1984 9305 the used data set consists of 148 harvard cmt solutions and 22 earthquake focal mechanisms listed in previous studies the stress field parameters are determined for 0 40 41 100 and h 100 km depth ranges the top 100 km layer of the wadati benioff zone wbz is characterized by strike normal maximum compression 1 and steeper than the slab minimum compression 3 the last indicating for unbalanced slab pull force the tokara channel divides the subduction into two parts of different stress regime at depth greater than 100 km to the south of the channel the slab is under slab parallel 1 and slab normal 3 while its northern part beneath kyushu is under slab parallel extension and slab normal compression the results of recent studies on the regional velocity structure and geochemistry of the volcanic lava indicate that the most plausible reason for the observed stress field difference below 100 km in the northern and rest part of the arc is the presence of hot low viscosity upper mantle west of kyushu the results of this study indicate that the forces involved in the contemporary subduction dynamics in the ryukyu kyushu wadati benioff zone are related to the convergence between the philippine sea plate and the eurasian plate the trench suction force slab pull the slab anchor force and in the southern central part of the arc mantle resistance

rapid subsidence of the kikai seamount inferred from drowned pleistocene coral limestone implication for subduction of the amami plateau northern philippine sea abstract carbonate rock cores drilled on the kikai seamount northern philippine sea are examined for better understanding of tectonic history of the northern philippine sea the kikai seamount the summit of which is at 1960 m water depth is an isolated high on the northwestern part of the amami plateau formed by subduction related arc volcanism and is situated close to the axis of the ryukyu trench in front of the ryukyu arc sw japan the seamount is capped with shallow water carbonates such as coral rudstone detailed examinations of lithology larger foraminiferal assemblages and sr isotope composition reveal that the core material comprises miocene carbonates unconformably overlain by early pleistocene carbonates it indicates rapid subsidence of the kikai seamount since the early pleistocene the most probable cause of rapid subsidence is collision and subduction of the amami plateau laden with the kikai seamount the rapid subsidence may have started when the western corner of the plateau reached the ryukyu trench and began subduction beneath the ryukyu arc the onset of the subsidence is likely to be controlled by a motion change in the philippine sea plate the latest change in subduction direction from north to northwestward into northwestward to west has been believed to have occurred at 1 2 ma during latest pliocene to early pleistocene time the change of direction resulted in the shift from oblique into right angle subduction of the plate beneath the ryukyu arc and also the onset of the collision and subduction of the amami plateau

plate convergence at the westernmost philippine sea plate abstract to understand the convergent characteristics of the westernmost plate boundary between the philippine sea plate psp and eurasian plate ep we have calculated the stress states of plate motion by focal mechanisms cataloged by the harvard centroid moment tensor solutions harvard cmt and the broadband array in taiwan bats moment tensor 251 focal mechanisms are used to determine the azimuths of the principal stress axes we first used all the data to derive the mean stress tensor of the study area the inversion result shows that the stress regime has a maximum compression along the direction of azimuth n299 this result is consistent with the general direction of the rigid plate motion between the psp and ep in the study area in order to understand the spatial variation of the regional stress pattern we divided the study area into six sub areas blocks a to f based on the feature of the free air gravity anomaly we compare the compressive directions obtained from the stress inversion with the plate motions calculated by the euler pole and the global positioning system gps analysis as a result the azimuth of the maximum stress axis 1 generally agrees with the directions of the theoretical plate motion and gps velocity vectors except block c lanhsu region and block f ilan plain region the discrepancy of convergent direction near the ilan plain region is probably caused by the rifting of the okinawa trough the deviation of the 1 azimuth in the lanhsu region could be attributed to a southwestward extrusion of the luzon arc la block between 21 n and 22 n whose northern boundary may be associated with the right lateral ne sw trending fault i e huatung fault hf along the taitung canyon comparing the 1 stress patterns between block c and block d great strain energy along hf may not be completely released yet alternatively the upper crust of block c may significantly have decoupled from its lower crust or uppermost mantle

the southwest ryukyu arc is a migrating microplate forearc sliver abstract we propose a new interpretation for the tectonism of the southwest ryukyu arc based on inland stress field analyses and the interpretation of seismic reflection profiles the recent stress field in the southwest ryukyu arc is under tension the axial direction of which is parallel to the trench axis but an e w dextral slip shear zone is developing at its western extremity yonaguni and westwards to taiwan these features are closely related to the degree of the oblique subduction of the philippine sea plate and the strength of coupling between it and southwest ryukyu arc we regard the southwest ryukyu arc as a migrating microplate the neotectonics of the southwest ryukyu arc are attributed to the westward migration of the microplate which is bounded by the miyako depression to the east and by taiwan to the west the microplate obducting onto the eurasian plate at taiwan started to migrate westward at about 4 ma when the relative motion between two plates philippine sea and eurasian changed direction from s n to se nw in region

fluid inclusion and carbon isotope studies of quartz graphite veins black hills south dakota and ruby range montana abstract fluid inclusions and graphite are intimately associated in quartz veins that cut high grade metamorphic rocks in the black hills south dakota and at the crystal graphite mine in the southwestern ruby range montana measured fluid inclusion compositions and volumetric properties were compared with calculated compositions of graphite saturated fluids and with estimates of metamorphic p t conditions and carbon isotope ratios of graphite were measured to evaluate possible sources of carbon in veinforming fluids fluid inclusions from the two areas contrast markedly in their reliability as recorders of metamorphic fluid compositions and metamorphic conditions the 13 c of graphite associated with the veins indicates that the source of carbon was also different in the two areas in the black hills veins fluid inclusions are dominantly h 2 o co 2 mixtures with 24 96 mol co 2 and a maximum of 5 mol n 2 and 13 mol ch 4 isochores for the highest density inclusions pass near estimated peak metamorphic conditions 550 600 c 4 5 6 5 kbar and fluid inclusion compositions are compatible with thermodynamic predictions for fluids in equilibrium with graphite in the stated p t range at geologically reasonable o 2 graphite in a 12 cm wall rock alteration zone adjacent to one of the veins has uniform 13 c of 20 8 0 2 indicating that carbon in the vein forming fluid was derived largely from reduced organic carbon in the ruby range peak metamorphic conditions were higher 750 850 c 5 8 kbar in contrast to the black hills veins fluid inclusions are almost all co 2 ch 4 mixtures with unknown n 2 content many contain 20 equivalent mol ch 4 and mixed h 2 o co 2 inclusions were observed in only one sample inclusions in one vein have 84 97 mol ch 4 virtually all inclusion compositions are incompatible with computed graphite equilibria and inclusion isochores likewise do not pass through estimated metamorphic conditions the density and composition of most if not all inclusions have been modified subsequent to original trapping possibly through h 2 o loss the range of 13 c values of vein graphites 5 8 to 8 6 is nearly indistinguishable from values for graphite in dolomitic marble near the veins 4 8 to 7 1 carbon was probably mobilized through devolatilization reactions in the marble and precipitated as 13 c rich graphite in the veins at fairly constant temperature and from fluid of fairly constant composition

a model with rigid rotations and slip deficits for the gps derived velocity field in southwest japan abstract we interpret the gps derived velocity field in southwest japan by a superposition of the elastic deformation caused by fault interactions slips or slip deficits on the rigid motion of tectonic blocks or plates based on the strain rate field and crustal seismicity we apply a model with three blocks inner arc outer arc and the northern ryukyu block and slip deficits along the block boundaries several characteristics of the synthesized contributions are found 1 westward motion of the outer arc relative to the amurian plate and the inner arc 2 southeastward motion of the northern ryukyu block relative to the amurian plate 3 2 4 mm yr deficits of left lateral slip rates along the boundary at 32 n in southern kyushu 4 0 8 mm yr deficits of right lateral slip rates along the median tectonic line and the beppu shimabara graben 5 slip deficit rates on the plate interface smaller than those in the case without any consideration for rigid block motions 6 clockwise deflection of slip deficit rate vector on the plate interface from that estimated when not taking rigid block motions into consideration

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

stress tensor analysis in the taiwan area from focal mechanisms of earthquakes abstract we produce a map of the stress pattern in and around taiwan based on 200 earthquake focal mechanism solutions these solutions were determined by using data from taiwan telemetered seismographic network microearthquake surveys and wwssn the stresses are derived through a minimization of angles between the slip vector and the shear stress on each nodal plane considered as a fault employing appropriate weighting factors the whole set of focal mechanisms is divided into several groups mainly according to apparent clustering of the event locations the results show that the direction of maximum principal stress in taiwan area is nearly horizontal and se nw on average this is in good agreement with the direction of relative motion between the philippine sea plate and the eurasian plate in western taiwan the fan shaped distribution of the maximum principal stress is consistent with the direction of philippine sea eurasian plate convergence through a simple model of viscous material indented by a rigid wedge in the northeastern part of taiwan a nearly horizontal minimum principal stress oriented n s is found for shallow depths it occurs in a region of low seismic velocities probably related to the back arc activity of the okinawa trough down dip compressional and down dip extensional stresses have been identified in different depth ranges within the subducting slab of the philippine sea plate in the northern taiwan this may reflect the slab characteristics in this area a complex stress pattern prevails in the hualien area at the junction between the ryukyu subduction system and the taiwan collision zone

rapid microplate rotations and backarc rifting at the transition between collision and subduction using global positioning system velocities from convergent plate boundaries in papua new guinea new zealand tonga vanuatu and the marianas we note a spatial correlation between rapid tectonic block rotations and the transition from subduction to collision we present a mechanism for the block rotations in which the change from collision of a buoyant indentor to normal subduction exerts a torque on the upper plate microplate this work improves our understanding of the causes of rapid vertical axis rotations often observed in paleomagnetic studies we also show how collision induced rotations may lead to backarc rifting

regional seismicity and on land deformation in the ryukyu arc implications for the kinematics of opening of the okinawa trough the stress field evolution and the kinematics of opening of the okinawa trough are investigated on the basis of earthquake focal mechanisms and structural data in the ryukyu arc and the okinawa trough focal mechanisms show that the crust underlying the arc and the trough undergoes extension along two suborthogonal directions a regional arc perpendicular extension and a local arc parallel extension both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal 2 and 3 axes earthquake slip vectors reveal a southward motion of the ryukyu arc with respect to the south china block the current pole of opening of the okinawa trough is located around 16 n and 50 e fracture analysis in okinawa island allows identification of three episodes of extension a late miocene n40 w to n20 e extension episode i a late pliocene to early pleistocene n20 e extension episode ii and a latest pleistocene to present day n20 w extension episode iii episodes ii and iii are characterized by permutations between the two horizontal 2 and 3 axes by synthesizing regional deformation data and by comparing the geometry of the deformation with analogue models of oblique rifting we reconstruct the kinematics of opening of the okinawa trough since the late miocene the direction of divergence of the ryukyu arc has rotated clockwise from n150 e in the late miocene to nearly n s today

stresses in the subducting slab beneath southwest japan and relation with plate geometry tectonic forces slab dehydration and damaging earthquakes 1 we infer stresses in the subducting slab beneath southwest japan from focal mechanisms and discuss their geodynamical implications the majority of the 824 fault plane solutions used in this study were obtained by the japan meteorological agency for earthquakes that occurred during 1970 2002 and were previously unpublished the focal mechanisms are believed to reflect stresses in both the untransformed slab crust and the slab mantle the stresses are controlled by tectonic forces and the earthquakes are facilitated by local dehydration the slab experiences a regional e w stretch that is downdip and margin parallel tension at the kyushu and nankai margins respectively we propose that the margin parallel component of mantle resistance to the obliquely subducting young slab at nankai provides an easterly stretch acting against the westerly slab pull of the older and colder part of the slab at northern kyushu despite the complex slab geometry along the nankai margin the direction of tension is largely parallel with the local strike of the slab indicating that the slab is a stress guide the complex slab geometry is not related to the present stress regime but likely reflects severe deformation in the past when an extremely young and incompetent plate was forced to subduct against its own buoyancy the maximum margin parallel stretch and a sharp bending of the slab both occur in the area of the 2001 m 6 7 geiyo earthquake using a simple strength envelope model we demonstrate that normal faulting failure of the slab mantle can occur under such conditions

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

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

neotectonic architecture of taiwan and its implications for future large earthquakes the disastrous effects of the 1999 chi chi earthquake in taiwan demonstrated an urgent need for better knowledge of the island s potential earthquake sources toward this end we have prepared a neotectonic map of taiwan the map and related cross sections are based upon structural and geomorphic expression of active faults and folds both in the field and on shaded relief maps prepared from a 40 m resolution digital elevation model augmented by geodetic and seismologic data the active tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the eurasian continental margin have created two neotectonic belts in taiwan in the southern part of the orogen both belts are in the final stage of consuming oceanic crust collision and suturing occur in the middle part of both belts and postcollisional collapse and extension dominate the island s northern and northeastern flanks both belts consist of several distinct neotectonic domains seven domains kaoping chiayi taichung miaoli hsinchu ilan and taipei constitute the western belt and four domains lutao lanyu taitung hualien and ryukyu make up the eastern belt each domain is defined by a distinct suite of active structures for example the chelungpu fault source of the 1999 earthquake and its western neighbor the changhua fault are the principal components of the taichung domain whereas both its neighboring domains the chiayi and miaoli domains are dominated by major blind faults in most of the domains the size of the principal active fault is large enough to produce future earthquakes with magnitudes in the mid 7 values

stress permutations three dimensional distinct element analysis accounts for a common phenomenon in brittle tectonics 1 using three dimensional 3 d distinct element modeling we explored a variety of simulations to characterize and interpret the stress permutations in brittle tectonics stress inversions of fault slip data or earthquake focal mechanisms often revealed such permutations the main aim of our study is to produce simple mechanically consistent 3 d models that account for these switches between the principal stress axes 1 2 or 2 3 even with simple boundary conditions the stress changes induced by variations in rheology are large enough to modify the local tectonic behavior and to produce permutations of principal stress axes rather than simple directional changes of stress axes which exist but often remain limited the relative variations in principal stress magnitudes are the major cause of permutations 1 2 and 2 3 in nature permutations being left apart the orientations of axes often remain tightly clustered in our experiments we adopted a ratio 2 3 1 3 of 0 5 which makes permutations difficult low favors 2 3 permutations high favors 1 2 permutations and we explored a variety of tectonic situations involving compression extension and strike slip our experiments indicate that the major causes of stress permutations are the heterogeneity of the brittle deformation e g intact rock massifs between heavily faulted deformation zones and the anisotropy of the mechanical properties that results from fracturing and faulting which concur to modify the mechanical balance inside the analyzed volume and to produce stress permutations our models show that contrasts and anisotropy in rock properties favor stress permutations of major importance is the existence of relatively resistant zones at tips of the deformed zones acting as channels where stress concentrates and switches occur because in nature such zones move in time and space it is not surprising that stress permutations are so pervasive

strain and stress field in taiwan oblique convergent system constraints from gps observation and tectonic data abstract this paper focuses on the stress and strain rate field of the taiwan area the strain rate field in the taiwan region is studied qualitatively and quantitatively based on the gps observation in 1990 1995 it reflects the accommodation of the ongoing lithospheric deformation within the seismogenic portion of lithosphere and exhibits zones with contrasting deformation modes and amounts we then compare the obtained strain rate field with the tectonic information provided by studies of borehole breakouts and earthquake focal mechanisms for the present and by fault slip data analyses for the quaternary period in the first approximation the stress and strain rate fields show spatial similarity the orientation of principal shortening is generally consistent with the compressive stress orientation that reflects the oblique indentation of the luzon arc into the eurasian continental margin in more detail significant anomalies in the deformation pattern deserve consideration in that they may reveal ongoing stress accumulation despite the short term variations related to the earthquake cycle some major features of the strain rate field including the distribution of extension and compression highlight the long term tectonic behavior of the mountain belt at the lithospheric scale the time and space variations of strain should be a function of local heterogeneity and be transferred between interseismic and coseismic periods

geodynamic processes of taiwan arc continent collision and comparison with analogs in timor papua new guinea urals and corsica abstract the taiwan arc continent collision involves four geodynamic processes intra oceanic subduction initial arc continent collision advanced arc continent collision and arc collapse subduction these processes now occur simultaneously in 19 24 30 n but have operated sequentially southward since the late miocene although the geological and geophysical features appear to change progressively from north to south across the island they are distinct within individual tectonic regimes using the present scenario as a basis for comparison it is suggested that timor is at the initial arc continent collision stage while papua new guinea and urals proceeded to the advanced arc continent collision stage however corsica represents an even more advanced stage than in taiwan where a previously accreted arc has collapsed and been totally removed

enigmatic highly active left lateral shear zone in southwest japan explained by aseismic ridge collision global positioning system gps site velocities and earthquake focal mechanisms reveal an active left lateral shear zone cutting across kyushu in southwest japan surprisingly no active faults have been identified in association with this zone of rapid contemporary deformation to explain the existence of this shear zone we propose a model comprising subduction of an aseismic ridge kyushu palau ridge at the southwest end of the nankai trough because of rapid 40 mm yr along strike migration of the ridge we suggest that the ridge subduction point and resulting left lateral shear zone is never in one place long enough to enable the development of a through going fault zone that can be identified at the ground surface reconciling the mismatch between the gps seismological and geological data in this region our conceptual model is supported by numerical modeling results we also suggest that the along strike change in subducting plate buoyancy explains the recent counterclockwise rotation of the kyushu forearc documented in paleomagnetic studies as is found in many other western pacific subduction margins

transition from oblique subduction to collision in the northern luzon arc taiwan region constraints from bathymetry and seismic observations we systematically study the detailed bathymetry seismicity and source parameters of large and moderate sized earthquakes that occurred in the northern luzon arc taiwan region between 1964 and 1996 the purpose is to characterize the transition from oblique subduction to regional collision in terms of the distribution of morphological features the characteristics of seismogenic structures and the corresponding state of strain to the south of 21 5 n the existence of a subduction zone is clearly shown by both bathymetry and seismicity the subducted eurasia slab reaches a maximum depth of 200 km and the depth decreases sharply as it approaches taiwan focal mechanisms indicate that the subducted slab is in downdip extension for depths 150 km but switches to downdip compression at greater depths to the north of 23 n collision is clearly the predominant process with broad deformation on both sides of the suture in between the transition is accommodated by a distributed thrust deformation zone in the frontal portion of the accretionary prism closure of the forearc basin and the right lateral ne sw striking taitung canyon fault zone tcfz these structures resolve the differential movements between the subduction and collision zones and will repeatedly change their positions as the collision propagates southwestward the present day plate boundary follows the axis of the manila trench in the south broadens significantly in the zone of transition and finally becomes a zone of several tens of kilometers in width across eastern taiwan the deformation front on the other hand is located between the western foothills and coastal plain in taiwan then connects to the manila trench to the south seismic strain tensors indicate that the region to the north of tcfz has a compression rate 15 times larger than that to the south the northern luzon arc shows characteristics of oblique subduction but the effects of collision are not as significant within the subducted eurasian lithosphere as comparing to the subducted philippine sea plate beneath the southernmost ryukyu arc

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

unblocking of the nares strait by greenland and ellesmere ice sheet retreat 10 000 years ago the extent of glaciation at the northern margin of the canadian greenland high latitude arctic region over the past 30 000 years is uncertain geological arguments have been made for greenland and ellesmere island ice sheets that coalesced to block the nares strait1 and for restricted ice sheets on the two islands2 leaving the strait open as it is today3 distinguishing between these two possibilities would provide significant constraints on present understanding of the past circulation between the arctic and atlantic oceans4 5 on estimates of past ice volume6 and on the response of the greenland ice sheet to climate change7 radiocarbon analyses provide dates for the deglaciation of the islands coasts but do not yield information on whether ice filled the strait here we present measurements of cosmogenic 36cl that has accumulated in situ in erratics and glacially polished bedrock on islands within the nares strait these data allow us to determine the time for which the rocks have been recently exposed to the atmosphere and thus the age of the final deglaciation of the strait we show that greenland and ellesmere ice sheets retreated from the nares strait about 10 000 years ago the strait was filled with ice during the last glaciation blocking this connection between the arctic and atlantic oceans and supporting the model of extensive and long lasting ice on land and sea in this region8 9 10 11

stress tensor inversion for the chi chi earthquake sequence and its implications on regional collision we study the stress regime associated with the 1999 chi chi taiwan earthquake sequence broadband waveforms of 115 events recorded by the broadband array in taiwan for seismology bats are analyzed to determine their focal mechanisms depths and seismic moments these solutions are then used to invert for the reduced stress tensor using the algorithm of angelier 1998 we perform the stress inversion in three different ways first we aim at defining in the first approximation a uniform stress field by taking the entire dataset as a whole the inversion result shows a stress regime with the maximum compression along the direction of n119 e remarkably consistent with the direction of the relative motion between the philippine sea plate and eurasia this solution is stable and varies little in terms of the orientations of stress axes within 8 and the ratio of stress differences between 0 26 and 0 44 in the second approximation we aim at explaining more subtle features of our observations by considering differently built subsets we divide the dataset according to different seismogenic structures as identified by kao and chen 2000 our results yield rather similar results axes of maximum compressive stress fall in the range of n99 138 e in terms of trends and in the range of 1 17 in plunges for all subsets except a small group in the footwall furthermore we observe a systematic clockwise change in the direction of compression from south to north representing the more general fan shaped pattern of stress trajectories that characterizes the whole collision zone finally the inversion is performed for the three main types of faulting that is reverse strike slip and normal types the axis of maximum compression for the reverse mechanisms has the same direction as that for the strike slip whereas the normal type subset yields a n54 e extension that trends obliquely to the compression the fact that the stress regime associated with the chi chi aftershock sequence is similar to that derived from geological and seismic data in the region implies that the stress regime corresponding to the regional collision in taiwan is responsible for the occurrence of the chi chi earthquake moreover systematic secondary variation in the state of stress is observed for different types of focal mechanisms and seismogenic structures such variation may play an important role in producing local stress concentration that leads to the nucleation of disastrous earthquakes in central taiwan manuscript received 12 january 2001

okinawa trough backarc basin early tectonic and magmatic evolution the okinawa trough lying between japan and taiwan is a backarc basin formed by extension within the continental lithosphere behind the ryukyu trench arc system stress directions associated with the two last extensional phases in the southwestern okinawa trough have been deduced from a comparison with analog modeling the direction of extension is n 150 o for the pleistocene phase of extension 2 0 1 ma and n 170 o for the late pleistocene holocene phase of extension 0 1 0 ma the present day ryukyu volcanic arc a narrow continuous feature extending from japan to taiwan is located on the eastern side of the okinawa trough 80 100 km above the wadati benioff zone the minimum depth for emplacement of arc magmatism scarce present day backarc volcanism appears in the middle and southern okinawa trough within linear en echelon bathymetric depressions a n045 o oriented seamount volcanic chain cuts across obliquely the southwestern okinawa trough and lies in the direct line of extension of the gagua ridge a n s linear volcanic feature of the philippine sea plate associated with this extension of the gagua ridge a large reentrant located at the base of the ryukyu prism the uplift of part of the nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross backarc volcanism could be tied to the subduction of the gagua ridge located there at a depth of 80 100 km beneath the backarc basin a second area of anomalous volcanism has been identified in the middle okinawa trough in the ene extension of the daito ridge a wnw ese 400 km long volcanic feature of the philippine sea plate we suggest that the gagua and daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere allowing magmas with arc affinities to erupt at the seafloor this excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts the okinawa trough is consequently still in an early stage of evolving from arc type to backarc activity

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

okinawa trough genesis structure and evolution of a backarc basin developed in a continent abstract the okinawa marginal basin was opened by crustal extension into the asian continent north of the taiwan collision zone it is located behind the ryukyu trench subduction zone and the ryukyu active volcanic arc if we except the andaman sea the okinawa trough is the only example of marginal backarc basin type opened into a continent at an early stage of evolution active rifting and spreading can be observed synthesis of siesmic reflection seismic refraction drilling dredging and geological field data has resulted in interpretative geological cross sections and a structural map of the ryukyu okinawa area the main conclusions of the reconstruction of this backarc basin volcanic arc evolution are 1 backarc rifting was initiated in the volcanic arc and propagated along it during the neogene it is still active at both ends of the basin remnants of volcanic arc are found on the continental side of the basin 2 there was synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the forearc terrace the late miocene erosional surface is now 4000 m below sea level in the forearc terrace above the trench slope retreat and subsidence of the ryukyu trench line relative to the asian continental plate could be one of the causes of tilting of the forearc and extension in the backarc area 3 a major phase of crustal spreading occurred in pliocene times 1 9 my ago in the south and central okinawa trough 4 en echelon rifting and spreading structures of the central axes of the okinawa trough are oblique to the general trend of the arc and trench the ryukyu arc sub plate cannot be considered as a rigid plate rotation of 45 to 50 of the southern ryukyu arc since the late miocene is inferred the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events the indentation process due to the collision of the north luzon arc with the china margin could have provoked lateral extrusion clockwise rotation 45 to 50 according to palaeomagnetic data and buckling of the south ryukyu non volcanic arc tension in the weak crustal zone constituted by the south ryukyu volcanic arc and opening of the south okinawa trough similar lateral extrusions rotations buckling and tensional gaps have been observed in indentation experiments additional phenomena such as thermal convection retreating trench model or anchored slab model could maintain extension in the backarc basin such a hypothetical collision lateral backarc opening model could explain the initiation of opening of backarc basins such as the mariana trough bonin trough parece vela shikoku basin and sea of japan a new late cenozoic palaeogeographic evolution model of the philippine sea plate and surrounding areas is proposed

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

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

a tear fault boundary between the taiwan orogen and the ryukyu subduction zone more than twenty seismic lines were analysed across the intersection of the taiwan orogen and the ryukyu arc forearc system the main results of this study concern the recognition of two superposed sedimentary basins that were previously unknown we have named the recent one the hoping basin and the older tectonized one the suao basin the suao basin containing sediments more than 3 km thick records subsidence of about 3 km on its southwestern portion after being shortened in the direction of plate convergence on the basis of seismicity gravity present day plate kinematics as well as seismic reflection data we relate the dramatic subsidence of the suao basin to the initiation of a wnw trending tear fault within the philippine sea plate this tear fault decouples the subducting philippine sea plate to the northeast and the apparent overthrusting philippine sea plate in the coastal range to the southwest back arc rifting may thus occur in the westernmost section of the okinawa trough above the sinking philippine sea slab while lithospheric shortening prevails south of hualien as a consequence the triangular area located southwest of the tear fault between the taiwan coastline and the toe of the yaeyama ridge no longer belongs to the ryukyu forearc it is now passively transported with the non subducting portion of the philippine sea plate we propose an evolutionary model for the relative chronology of tectono sedimentary events in this region over the last 5 m y

stress field along the ryukyu arc and the okinawa trough inferred from moment tensors of shallow earthquakes abstract we investigated the stress field along the ryukyu arc and the okinawa trough using regional nied mt and global harvard cmt moment tensor catalogs using the combined dataset of 4 years of data from the nied mt catalog and 24 years of data from the harvard cmt catalog we found an arc parallel extensional stress province along the forearc of the ryukyu arc and observed a detailed extensional stress field in the okinawa trough this arc parallel extension is observed in the entire region of the ryukyu arc except at its northeastern end this stress field is clearly separated from the backarc extensional stress field in the okinawa trough by the volcanic chain since it exists in a normal subduction region as well as in a region of weakly coupled subduction the effects of oblique subduction might not be enough to explain the formation of the arc parallel extension thus we consider that the backarc opening process should play an important role in this arc parallel extension along the okinawa trough an arc perpendicular extensional stress field is observed in the southeastern and central ryukyu arc while in the northeastern okinawa trough the direction of the extensional axis is oblique to both the arc normal direction and the direction of plate motion two dimensional mechanical interaction models which consider the interactions between the subducting plate forearc and backarc are inadequate because an arc parallel extensional stress field is observed in the entire ryukyu arc and the extension axis in the northeastern okinawa trough is oblique to the arc trend the oblique backarc stress may originate beneath the backarc or on the continental side of the backarc one of the simplest ways to explain this observation might be active rifting in the northeastern okinawa trough

display and quantitative assessment of distributions of earthquake focal mechanisms summary this paper describes quantitative methods for evaluating the distribution of focal mechanism orientations of groups of earthquakes and shows how to display this distribution on a triangle diagram we present a 2 based statistical test for determining whether two sets of focal mechanisms are drawn from distinct populations we apply these methods to 3625 better determined mechanisms for shallow earthquakes in the harvard centroid moment tensor cmt catalogue we describe the distributions of mechanisms in the whole catalogue and for catalogue subsets in some specific tectonic environments in addition we explore the geographical locations of mechanisms with orientations that occur relatively infrequently that is mechanisms that are unlike thrust normal or strike slip mechanisms such mechanisms are relatively rare along mid ocean ridges and in oceanic subduction zones the majority of these unusual mechanisms occur along plate boundaries where crustal thickness is highly variable and in regions where the plate convergence direction becomes oblique and thus relative motion changes from convergence to transform motion

direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern california in inverting populations of focal mechanisms to estimate stress misfits may arise owing to 1 errors in determining the fault plane solutions and 2 real variations in the state of stress among a number of techniques that invert for stress most effectively regard only the latter and only indirectly describe the former through statistical means so make it difficult to differentiate real stress variations from noisy data ambiguity arises because realistic methods for determining focal mechanisms for small earthquakes depend on p wave first motions in a highly nonlinear manner which is difficult to propagate into stress inversions to address these issues we present a new method for constraining stress tensors in which first motion observations are directly inverted for stress without assuming that focal mechanisms are known the technique produces estimates of four stress parameters a suite of focal mechanisms consistent with both the stress tensor and the first motions and estimates of the uncertainty in stress it also provides a natural test of the stress homogeneity hypothesis and a means to identify those earthquakes whose first motions are not consistent with a homogeneous stress tensor when tested against artificial data the method correctly recovers input stresses and yields uncertainties that depend upon both data distribution and reliability of first motion picks as expected utilizing first motions from the southern california catalog the method is applied to several clusters and aftershock sets from 1981 to 1992 typical uncertainties in stress orientation exceed 20 30 at the 95 confidence level resulting from 5 mispick rates typical of even the cleanest data these uncertainties exceed some previous estimates for similar populations presumably because they account for focal mechanism un certainties and perhaps indicating that some previously reported stress variations are not statistically significant most populations are found to be internally consistent with a homogeneous stress field the most significant exceptions being aftershock sequences of some major earthquakes in at least one case the 1992 landers sequence aftershocks at the north end of the rupture zone appear to reflect exceedingly heterogeneous stresses overall the method provides a useful and robust approach for evaluating stress constraints provided by p wave first motions

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

implications of paleomagnetic data on the collision related bending of island arcs paleomagnetic studies from the marianas central philippines ryukyus sulawesi fiji new hebrides all show major differences in the declination values observed from individual islands within the same arc comparable differences are not observed in the inclination values a simple plate tectonic model would predict that each island arc should experience the same rotation along its entire length as the plate moves around its rotation axis bathymetry from each of these arcs show that the differentially rotated segments of the upper plate occur in areas where the downgoing plate carries a feature such as a seamount chain rifted continental fragment or an island arc the coincidence of the bathymetric feature and the discordant paleomagnetic declination data support an earlier hypothesis of vogt 1973 that these features are buoyant zones on the downgoing plate which interact with the margin of the upper plate as a result of this interaction the upper plate boundary is locally deformed in addition to the paleomagnetic evidence other geologic and geophysical data from these areas suggest such interactions for example there appears to be a good correlation between the deflections in major structures around the collision zone and the paleomagnetic directions that are reported as this deformation proceeds eventually the stresses resulting from the collision may result in any of the following tectonic processes 1 slip line style strike slip faults allowing for the sideward extrusion of portions of the upper plate tapponnier et al 1983 2 changes in the polarity of the subduction zone 3 development of strike slip faults around the margin of the indenter or 4 reorganization of the entire plate margin in addition data from the marianas southern end of the ryukyu arc and the lau basin suggest that back arc basin development may result from the collision related deformation

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

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

plate collision and paleostress trajectories in a fold thrust belt the foothills of taiwan abstract field analyses of compressional faulting and folding in the foothills of western taiwan enable us to reconstruct paleostress trajectories over a large area and to establish the relative chronology of tectonic events two main compressional events have contributed to the present structure of the fold thrust belt stratigraphic data show that these events are plio pliocene in age older normal faulting indicates nnw sse extension across the chinesse passive continental margin during the neogene the two main compressional events of the taiwan collision correspond to similar fan shaped distributions of maximum compressive stress trajectories with a counterclockwise shift of 30 50 between the two events using the relationship between recent stress trajectories and the direction of recent plate motion as a guide we reconstruct the direction of plate convergence for the older event we suspect that the relative motion philippine sea plate eurasia has rotated counterclockwise of at least 35 45 in taiwan during collision this conclusion is in agreement with independent plate tectonic reconstructions several problems provide objectives to further tectonic and paleomagnetic studies including the duration and diachronism of compressional events as well as possible clockwise rotation of northernmost taiwan

a bayesian approach to estimating tectonic stress from seismological data summary earthquakes are conspicuous manifestations of tectonic stress but the non linear relationships between the stresses acting on a fault plane its frictional slip and the ensuing seismic radiation are such that a single earthquake by itself provides little information about the ambient state of stress moreover observational uncertainties and inherent ambiguities in the nodal planes of earthquake focal mechanisms preclude straightforward inferences about stress being drawn on the basis of individual focal mechanism observations however by assuming that each earthquake in a small volume of the crust represents a single uniform state of stress the combined constraints imposed on that stress by a suite of focal mechanism observations can be estimated here we outline a probabilistic bayesian technique for estimating tectonic stress directions from primary seismological observations the bayesian formulation combines a geologically motivated prior model of the state of stress with an observation model that implements the physical relationship between the stresses acting on a fault and the resultant seismological observation we show our bayesian formulation to be equivalent to a well known analytical solution for a single errorless focal mechanism observation the new approach has the distinct advantage however of including 1 multiple earthquakes 2 fault plane ambiguities 3 observational errors and 4 any prior knowledge of the stress field our approach while computationally demanding in some cases is intended to yield reliable tectonic stress estimates that can be confidently compared with other tectonic parameters such as seismic anisotropy and geodetic strain rate observations and used to investigate spatial and temporal variations in stress associated with major faults and coseismic stress perturbations

analysis of crustal deformation in luzon philippines using geodetic observations and earthquake focal mechanisms we utilize regional gps velocities from luzon philippines with focal mechanism data from the harvard centroid moment tensor cmt catalog to constrain tectonic deformation in the complex plate boundary zone between the philippine sea plate and eurasia the sundaland block processed satellite imagery and digital elevation models are used with existing gravity anomaly seismicity and geologic maps to define a suite of six elastic blocks geodetic and focal mechanism data are inverted simultaneously to estimate plate rotations and fault locking parameters for each of the tectonic blocks and faults comprising luzon major tectonic structures that were found to absorb the plate convergence include the manila trench 20 100 mm yr 1 and east luzon trough 9 15 mm yr 1 philippine trench 29 34 mm yr 1 which accommodate eastward and westward subduction beneath luzon respectively the left lateral strike slip philippine fault 20 40 mm yr 1 and its northward extensions the northern cordillera fault 17 37 mm yr 1 transtension and the digdig fault 17 27 mm yr 1 transpression the macolod corridor a zone of active volcanism crustal thinning extension and extensive normal and strike slip faulting in southwestern luzon is associated with left lateral transtensional slip of 5 10 mm yr 1 the marikina fault which separates the central luzon block from the southwestern luzon block reveals 10 12 mm yr 1 of left lateral transpression our analysis suggests that much of the philippine fault and associated splays are locked to partly coupled while the manila and philippine trenches appear to be poorly coupled luzon is best characterized as a tectonically active plate boundary zone comprising six mobile elastic tectonic blocks between two active subduction zones the philippine fault and associated intra arc faults accommodate much of the trenchparallel component of relative plate motion 2006 elsevier b v all rights reserved

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

active deformation of taiwan from gps measurements and numerical simulations using a two dimensional distinct element model we evaluate the relationships between plate kinematics and present day deformation in taiwan where active collision occurs in particular the distribution of velocity fields calculated in our models is compared with the actual velocity field revealed by the most recent geodetic data gps obtained in taiwan and the surrounding islands of the philippine sea plate and the eurasian shelf the main aim of this paper is to produce a mechanically consistent 2 d model that accounts for the observed velocity field taken as whole within the limits of acceptable rheological parameters and reasonable boundary displacement conditions we evaluate how the active deformation of taiwan is influenced by the presence of strong and weak zones such as the structural highs in the foreland and subduction zones with accretionary prisms respectively major mechanical discontinuities such as the main fault zones in the mountain belt and the opening of the okinawa trough particular attention is paid to the role of preexisting discontinuities since the presence of mechanical decoupling along major faults strongly affects the distribution of the velocity and stress patterns we show that despite parameter uncertainties several tectonic factors the presence of the strong kuanyin and peikang highs in contrast to the weak subduction zone to the south the weak active regional shear zones and the opening of the back arc okinawa trough concur to provide an acceptable mechanical model for this regional deformation these sources are related not only to the geometry of the plate boundary the direction of plate convergence and the shape of the chinese margin but also to the presence of major zones of relative weakness and mechanical decoupling such as the longitudinal valley fault zone and the western thrust belt of taiwan

trench parallel stretching and folding of forearc basins and lateral migration of the accretionary wedge in the southern ryukyus a case of strain partition caused by oblique convergence detailed seafloor mapping in the area east of taiwan revealed trench parallel stretching and folding of the ryukyu forearc and lateral motion of the accretionary wedge under oblique convergence east of 122 40 e a steep accretionary wedge is elongated in an e w direction a major transcurrent right lateral strike slip fault accommodates the strain partitioning caused by an oblique convergence of 40 a spectacular out of sequence thrust may be related to the subduction of a structural high lying in the axis of the n s trending gagua ridge this asperity is likely responsible for the uplift of the accretionary wedge and forearc basement and may have augmented strain partitioning by increasing the coupling between the two plates west of 122 40 e the low taper accretionary wedge is sheared in a direction subparallel to the convergence vector with respect to the ryukyu arc the bayonet shape of the southern ryukyu arc slope partly results from the recent re opening of the southern okinawa trough at a rate of about 2 to 4 cm yr right lateral shearing of the sedimentary forearc with respect to the nonlinear ryukyu backstop generates trench parallel extension in the forearc sediment sequence at dilational jogs and trench parallel folding at compressive jogs the hoping basin lies above a diffuse trench trench fault ttf or tff unstable triple junction moving toward the south along a n s transform zone which accommodates the southward drift of the ryukyu arc with respect to eurasia

a new insight on the geometry of subducting slabs in northern luzon philippines abstract we used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern luzon and propose a model for describing the geometry of the subducted slab of the eurasian plate beneath the northern segment of the manila trench we took into consideration some of the observed bathymetric features i e the bend in the trench line of the manila trench at 20 n lat and the collision and subduction of an extinct mid oceanic ridge at 16 17 n lat and the intraplate deformation pattern in the north luzon ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area an earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released while earthquakes covering the instrumental years 1963 1997 were used to analyze vertical distributions of earthquakes in sections we refined the location of some of the historical earthquakes using macroseismic data the inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods a focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab the focal mechanism database consists of first motion solutions from previous authors and cmt solutions from harvard university seismicity strain energy release and focal mechanism maps and cross sections were drawn in order to have a three dimensional visualization of the geometry and stress regimes of seismogenic zones a new model of the subducted slab of the eurasian plate beneath the manila trench is proposed the model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20 n lat to explain the sharp bend in the trench line the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone a tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18 n lat the gap in seismicity and strain energy release 65 300 km depth at around 17 n lat may be used to infer the trajectory and location of the subducted extinct mid oceanic ridge mor the aseismic behavior is probably caused by the subducted ridge which is still hot and is deforming plastically this is supported by the heatflow data which shows high values along the extinct mor the subducted part of the mor may serve as the weakest zone where this tear could be localized the tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18 n lat the above model which is a refinement of the model introduced by yang et al tectonophysics 258 1996 85 is consistent with the observed seismicity and deformation pattern observed bathymetric features spatial distribution and geochemical character of volcanism in northern luzon

new geomorphic data on the active taiwan orogen a multisource approach a multisource and multiscale approach of taiwan morphotectonics combines different complementary geomorphic analyses based on a new digital elevation model dem side looking airborne radar slar and satellite spot imagery aerial photographs and control from independent field data this analysis enables us not only to present an integrated geomorphic description of the taiwan orogen but also to highlight some new geodynamic aspects well known major geological structures such as the longitudinal valley lishan pingtung and the foothills fault zones are of course clearly recognized but numerous previously unrecognized structures appear distributed within different regions of taiwan for instance transfer fault zones within the western foothills and the central range are identified based on analyses of lineaments and general morphology in many cases the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently in turn the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic features in other areas examples are the conjugate pattern of strike slip faults within the central range and the oblique fold and thrust pattern of the coastal range furthermore neotectonic and morphologic analyses drainage and erosional surfaces have been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in taiwan such as for the longitudinal valley fault next at a more general scale numerical processing of digital elevation models resulting in average topography summit level or base level maps allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance finally a preliminary morphotectonic sketch map of taiwan combining information from all the sources listed above is presented

seismic tomography of taiwan improved constraints from a dense network of strong motion stations in this study a large collection of 41 141 s p times from the untapped records of the taiwan strong motion instrumentation program tsmip network is combined with the p and s wave arrival times from the taiwan central weather bureau seismic network cwbsn to image the v p and v p v s structures beneath taiwan the records from the 680 tsmip stations throughout taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously our result for the v p structure largely confirms previous studies but brings better constraint on the v p v s structure the colliding luzon volcanic arc is characterized by a belt of high v p and high v p v s with high seismicity that includes the offshore islands of lutao and lanyu and the coastal ranges in eastern taiwan at the depth between about 13 and 25 km this high v p v s belt can be traced to the subduction zone in the region between hualian and ilan in the deeper portion the shallow portions of the southwestern coastal plain and the pingtung region are also characterized by a belt of high v p v s with lower seismicity most of the events occurred at the base of the high v p v s zones we suggest that material strength in those regions may be too low to accumulate stress which may indicate water saturated young sediments finally the central range region is characterized by a low v p v s belt