accretion and subduction tectonics in the chugach mountains and copper river basin alaska initial results of the trans alaska crustal transect geologic seismic gravity and magnetic data from the northern chugach mountains and southern copper river basin alaska indicate that the chugach terrane cgt and the composite peninsular wrangellia terrane pet wrt are thin 10 km rootless sheets bounded on the south by north dipping thrust faults that sole into a shallow horizontal low velocity zone the cgt has been thrust at least 40 km beneath the pet wrt along the border ranges fault system brfs adjacent to the brfs uplift and erosion of 30 40 km since jurassic time have exposed blueschist facies rocks in the cgt and mafic and ultramafic cumulate rocks in the pet wrt four paired north dipping layers of low and high seismic velocities extend beneath the northern cgt and southern pet wrt and may be slices of subducted oceanic crust and upper mantle the upper two pairs may now be joined to the continental plate 15 references 5 figures

active continental subduction and crustal exhumation the taiwan orogeny a tectonic model of active continental subduction followed by crustal exhumation is proposed to explain the orogeny in taiwan the subducted crust is represented by a low velocity zone dipping eastwards beneath the major part of taiwan while the exhumed crust is marked by a high velocity bulge high heat flow and absence of seismicity beneath the eastern central range the boundary between the subducted and exhumed crust has been identified from surface geology and analyses of thermal history across the central range the dynamic force that has been driving the exhumed crust is identified by results from focal mechanisms structural geology and geodetic survey in the eastern central range such a tectonic model may provide a good explanation for the evolution of the taiwan orogeny as well as an active case for studying other long extinct systems of continental subduction and exhumation

identification of mantle reflections from a dense linear seismic array tectonic implications to the taiwan orogeny 1 a dense linear seismic array consisting of 18 short period seismic stations is deployed in the western foothills to study deep crustal structures across central taiwan where significant mountain building processes are considered as a result of the oblique convergence between the eurasian and philippine sea plates since a few million years ago the identification of mantle reflections pmp sms and smp from later phases generated by one earthquake ml 4 3 in eastern taiwan and recorded by the linear seismic array shows that crustal thickness gradually increases eastward from 40 km beneath the western central range to 50 km beneath the eastern central range the significant horizontal offset 30 km between the highest mountains and the deepest crust suggests that crustal deformation has not yet reached isostatic equilibrium it is concluded that continental subduction might be acceptable for explaining the dynamic processes in the taiwan orogeny

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

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

large effects of moho reflections sms on peak ground motion in northwestern taiwan abstract a total of 336 three component strong motion recordings from the m w 6 35 niu dou earthquake of 25 june 1995 at a focal depth of 39 9 km in northern taiwan are used to study the effects on strong ground motion due to moho reflection of s waves the residuals of both horizontal peak ground acceleration pga and peak ground velocity pgv recorded from the earthquake are analyzed the results confirm that many class e soft soil stations in the taipei basin and the ilan plain had the expected large amplification of about 1 7 and 1 5 times respectively the predicted median pga values surprisingly a large group of class c or d dense and stiff soil sites in taoyuan tcu007 lungtan tcu013 guanshi tcu021 hsinchu tcu095 and miaoli tcu047 areas in northwestern taiwan had unusually large amplification of about 3 4 8 1 and 1 7 3 3 times the predicted median pga and pgv values respectively they are interpreted in terms of focusing and interference between sms waves reflected from the horizontal and inclined portions of an east dipping moho discontinuity in this area this interpretation is supported by the close agreement between the expected amplitudes and arrival times of the largest shear waves with the observed data our results suggest that when a damaging earthquake occurs near an inclined moho boundary the reflected sms waves can result in significantly amplified ground motions at distances beginning about 50 km the exact distance range will depend on the thickness of the crust and the dip angle of the moho boundary

development of three dimensional basement structure in taiwan deduced from past plate motion consistency with the present seismicity 1 using colored clay we examined geometrical evolution of the three dimensional basement structure in taiwan due to relative plate motion under the following assumptions the motion of the philippine sea plate relative to the eurasian plate has been constant during the last 15 myr the ryukyu and manila trenches had been connected by a transform fault before the collision between the luzon arc and eurasian continental margin and the collision started at 5 ma the basement structure obtained from the clay model was fully consistent with observed deep seismicity which led us to the following conclusions there are no oceanic slabs beneath central taiwan instead the eurasian continental margin is underthrusting beneath it the philippine sea slab extends northwestward from the ryukyu trench while the south china sea slab extends southeastward from the manila trench the downdip length of the south china sea slab becomes shorter from south to north

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

three dimensional vp and vs structural models associated with the active subduction and collision tectonics in the taiwan region summary 3 d vp and vs models for the crust and upper mantle beneath the taiwan area have been determined using selected high resolution earthquake data from an island wide seismic network and two local seismic arrays lateral structural variations in the upper crust as also evident from surface geology are responsible for the observed large traveltime residuals or station corrections prior shallow velocity information inferred from traveltime residuals and joint hypocentral determination jhd station corrections for the uppermost crust is essential to facilitate a reliable tomographic inversion a finite difference method that is efficient and accurate for a highly heterogeneous velocity structure is applied to calculate p and s wave traveltimes from the source to receiving stations all earthquakes in the taiwan central weather bureau s catalogue are then relocated using the resultant 3 d vp and vs models the depth of the moho varies significantly especially along the east west direction in the western coastal plain and western foothills the depth of the moho is around 35 km which deepens gradually eastward reaches a maximum depth of 55 km beneath the eastern central mountain range shallows up rapidly beneath the longitudinal valley and coastal range and merges with the thin philippine sea plate offshore of eastern taiwan in central taiwan the central mountain range is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km therefore the uplifted and thickened central mountain range serves as a backstop for the converging eurasian and philippine sea plates the crust beneath the central mountain range is characterized by a brittle high velocity and seismically active upper crust 15 km and a ductile low velocity and aseismic mid to lower crust below 15 km most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east from the surrounding hotter upper mantle beneath the thickened continental crust and from shear heating during active collision the collision zone in eastern taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low vp and high vp vs ratio near the taitung region in southeastern taiwan it transforms into an active westward steeply dipping seismic zone along a transition zone between the high vp and vs oceanic crust and the low vp and vs continental crust near hualien region in central eastern taiwan there is no apparent seismicity within many sedimentary basins imaged from the tomographic inversion however af ew basins are either bounded on one side by an active fault or are characterized by blind faults beneath the geometry of the subduction zone in northeastern taiwan can be clearly imaged from the relocated earthquake locations behind the subduction a region of low vp and high vp vs ratio at depths of 5 to 10 km can be identified beneath the tatun chilung volcano group indicating a potential magma reservoir two steeply dipping linear seismic zones in the volcano region may mark the upward escape paths of the magmatic materials in the region

joint vp and vs tomography of taiwan implications for subduction collision orogeny abstract the taiwan orogen sitting at the pre rifted eurasian margin and bracketed by two sub orthogonal subduction systems is subject to complicated geotectonics motivating numerous seismic tomographic studies in local and regional scale most of them obtained p wave velocities vp or with vp vs ratio jointly but few were for s wave velocities vs with unprecedented amount of s wave data in this study a new set of well tuned vp vs and vp vs models were determined by an elaborate joint inversion scheme integrating the data of p and s wave travel times s p times and the borehole logging data for near surface correction into one system this allowed us to revisit the subduction collision process in detail by comparing vp vs and vp vs ratio simultaneously with enhanced slab imaging of philippine sea plate psp under northeast taiwan we are able to discriminate the plausible missing luzon forearc basement along the plate boundary and propose a skateboarding edge model in which the westernmost psp rides on the east subducted forearc basement and subducts northward underneath the eurasian lithosphere the underlain forearc basement along with the psp subduction then results in the anomalous shallow double seismic zone observed only near taiwan island meanwhile at around 121 e the north subducting psp seems dipping west to collide with the deep eurasian lithosphere shaping an irregular corner wedge structure in addition we evaluate our model with previous ones by waveform modeling approach and show the current performance of travel time tomography in taiwan region

velocity structure of the northern baikal rift siberia from local and regional earthquake travel times the velocity structure of the upper lithosphere in the northern baikal rift nbr siberia is analyzed in order to bring some constraints on the early stages of continental rifting a simultaneous hypocenter velocity inversion is performed using a set of local and regional seismological data recorded in the nbr first arrival times of 3 492 direct and refracted p and s waves allow to investigate the crustal velocity structure and the moho geometry results of inversion reveal an unbalanced crust made of two adjacent parts of different thickness delimited by a rather sharp discontinuity following approximately the northern edge of the rift zone this structure is interpreted as the result of ante rift deformation phases creating a weakened zone where rifting deformation may easily occur oblique rifting is invoked to explain the formation of sedimentary basins without significant crustal thinning at depth

joint hypocenter velocity inversion for the eastern tennessee seismic zone a joint hypocenter velocity inversion for the eastern tennessee seismic zone etsz has resolved velocity features in basement rock below detached appalachian thrust sheets p and s wave arrival times from 492 earthquakes have been inverted for one 1 d and three dimensional 3 d velocity models to midcrustal depths the 3 d p and s wave velocity solutions are computed independly and are very similar in relation to the 1 d model velocity anomalies range from 8 to 16 in the first layer upper 5 km and between 7 in deeper layers prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer the most persistent anomaly is a low velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity the new york alabama ny al magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the p and s wave velocity images the spatial coincidence between velocity gravity and magnetic gradients suggests that major discontinuities are present in the basement relocation in the 3 d velocity model reduced the number of very deep earthquakes below 20 km and further accentuated differences in seismogenic properties on either side of the ny al lineament after relocation most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth most earthquakes occur in regions characterized by either average velocity or small velocity anomalies

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

3d topography of the moho discontinuity in the taiwan area as extracted from travel time inversion of pmp phases abstract the reflective signals are often used to find a particular reflective interface therefore we used the travel time of the pmp phase to investigate the moho depth beneath taiwan in this study we used the directionality and rectilinearity functions to increase the accuracy and the convenience of the pmp phase identification in all we selected 1054 earthquakes with 13 715 travel time data of the pmp phase to invert the attitude of the moho discontinuity from the results we offer these conclusions 1 the distribution of the moho depth in taiwan trends nne ssw which is consistent with the topography 2 the deepest part of the moho discontinuity is beneath the central mountain range 38 km in depth and the depth of the moho discontinuity beneath the peikang basement high is only 26 km 3 the western coastal plain is divided into an active continental margin and a passive continental margin by the peikang basement high 4 the moho depth beneath the northern part of the coastal range is deeper than the southern part because of more vigorous collision activity

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

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

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

the significance of the crustal velocity model in local earthquake locations from a case example of a panda experiment in the central united states reliable local earthquake locations depend on many factors including the quality of earthquake data the spatial distribution of network stations the location algorithms and the local and regional seismic velocity models the uncertainties of p and s wave arrival times at various seismic stations are in large part responsible for the uncertainty in the determination of crustal velocity models and earthquake locations we took the advantage of the high quality and high dynamic range panda data from a 33 month seismic array experiment in the central united states to investigate the importance of crustal velocity models especially the s wave model on earthquake locations several crustal models are used to relocate a group of selected earthquakes from the panda database it is clear that earthquake depths are determined less reliably than the epicenters when the shear wave velocity model is not properly determined the mislocations in some examples are significant that the importance of determining reliable p and s velocity models independently cannot be overlooked if the crustal p and s wave velocity structures in the region can be well determined reliable earthquake locations are possible even with only a few accurate p and s readings

simultaneous inversion for 3 d crustal structure and hypocentres including direct refracted and reflected phases iii application to the southern rhine graben seismic region germany summary the new technique of simultaneous inversion for hypocentres and crustal structure ssh is applied to the southern rhine graben p and s wave first and later arrival times of direct refracted and reflected phases of 350 seismic events are used in the study substantial improvements of over 100 per cent in the non linear residual fit are obtained for the 3 d and less for the 1 d models many of the structural results of the 3 d ssh models are consistent with major findings from other seismic refraction experiments and fit into the general geological petrological and geodynamica1 frame of this area thus the thick sedimentary layer in the rhine graben proper manifests itself by large negative seismic velocity anomalies for the middle crust the ssh models show a strong seismic differentiation between sections of the graben proper and the areas under its western and eastern shoulders of the vosges and the black forest respectively using a new a priori bayesian inversion technique it has been attempted to substantiate the refraction seismic low velocity zone of gajewski prodehl 1987 in the middle crust under the black forest the results demonstrate that such a velocity low is compatible with the observed traveltimes only if one assumes it as a more local crustal phenomenon in this area in the lower crust the ssh models are able to clarify some controversies from earlier refraction experiments in the region the models reveal many seismic features which are typical for continental rift zones such as high transitional velocity anomalies 7 4 km s l under parts of the graben proper rift cushion in the upper mantle the models show a ring like high velocity structure of 8 30 8 40 km s l centred around a low velocity area 7 9 km s l beneath the kaiserstuhl i conjecture this to be a thermal fingerprint of a partially molten upwelling asthenospheric diapir bulging under the moho the simultaneously relocated hypocentres for the 1 d models show some systematic downward movements for the events in the lower crust of the southern black forest area but otherwise do not differ too much from the initial ones to alter the present seismotectonic picture of the southern rhine graben thus the hypothesized phenomenon of the seismic gap along the southern section of the graben black forest wedge can both structurally and seismically be substantiated the relocated hypocentres suggest optically even a widening of this seismic gap the later has thus to be accepted as a possibly more general characteristic of a graben formation

seismic velocities in the shallow crust of western new england and northern new york the ontario new york new england refraction study o nynex provides a set of high quality data for a tomographic study of an intraplate continental region with low seismic activity this study focuses on the region crossed by the united states part of the o nynex profile tomographic inversion of the travel time data is used to construct a three dimensional model of the p and s wave seismic velocity distribution within the shallow crust of this region velocity structure is resolved in the top 15 km of the crust with the most detailed picture obtained for the depth range between 1 and 5 km the upper crust of the proterozoic grenville province is found to have a p wave velocity 0 2 to 0 3 km sec higher than the paleozoic appalachians the area of anorthositic intrusions within the adirondack mountains is characterized by p wave velocities 0 3 km sec higher and s wave velocities 0 1 km sec higher than the average values for the grenville province this high velocity anomaly extends to the depth over 5 km possibly as deep as 16 km the velocity structure of central vermont is found to be closer to that of the grenville province than to the rest of the appalachians central new hampshire shows relatively small lateral variation of seismic velocity no strong correlation is seen between velocity anomalies and intrusive bodies of the white mountains magma series

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

velocity structure in northern chile evidence of subducted oceanic crust in the nazca plate summary 2 d p wave velocity models were determined for the subduction zones near iquique and antofagasta in northern chile simultaneously inverting p and s wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations a 2 d parametrization was used because of the paucity of data but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3 d inversions the crust and upper mantle are parameterized by constant velocity regions of irregular shape with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data to resolve p wave velocities beneath the antofagasta region there is evidencc of an approximately 10 km thick layer of oceanic crust attached to the top of the subducting nazca plate this crust has a p wave velocity of 7 3 f 0 1 km s and is observed down to a depth of 60 f 10 km this depth also corresponds to the maximum depth of seismogenic coupling in the chilean subduction zone the subducted oceanic crust overlies an oceanic upper mantle with a p wave velocity of 8 0 f 0 1 km s apparently oceanic crust is being subducted beneath iquique as well however this feature is less constrained by the data available from this region

intraplate stresses and plate driving forces in the philippine sea plate we use a spherical shell elastic finite element analysis to characterize intraplate stresses and plate driving forces in the philippine sea plate our finite element mesh is comprised of 489 nodes and 914 elements and provide a spatial resolution of about 1 the philippine sea plate is unusual in that it does not have large numbers of seismic or tectonic stress indicators and there are no midplate borehole stress data which can be used as constraints in this finite element study however it does possess a unique boundary configuration which potentially can lend more insight to plate forces and stresses than can similar studies on plates with more seismically active interiors because the plate s boundary consists primarily of subduction zones with roughly equal proportions of boundary on the overlying and downgoing sides of trench the resultant plate stress field is very sensitive to subduction related forces as a result borehole stress measurements in certain parts of the plate would be useful in further constraining these forces we find that trench pull forces can explain a large component of the velocity of the philippine sea plate however trench pull has somewhat less overall effect on the stresses across the plate than do the lateral density and topographic variations within the plate i e gravitational or potential energy force and trench suction forces are probably about an order of magnitude smaller than the net trench pull forces collisional forces may dominate the stress fields near taiwan and the izu peninsula but they are of only secondary importance in determining the motion and deformation of the philippine sea plate the forces that generate stress orientations most closely matching the observed stress indicators produce a lithospheric stress field across the plate that averages about 15 mpa over a 70 km thick lithosphere scaling inversely with lithosphere thickness values up to four times as great are produced locally

characterization of the hydrothermal system of the tinguiririca volcanic complex central chile using structural geology and passive seismic tomography abstract a structural characterization of the hydrothermal volcanic field associated with the tinguiririca volcanic complex had been performed by combining passive seismic tomography and structural geology this complex corresponds to a 20 km long succession of n25 e oriented of eruptive centers currently showing several thermal manifestations distributed throughout the area the structural behavior of this zone is controlled by the el fierro el diablo fault system corresponding to a high angle reverse faults of oligocene miocene age in this area a temporary seismic network with 16 short period stations was setup from january to april of 2010 in the context of the msc thesis of lira 2010 covering an area of 200 km 2 that corresponds with the hydrothermal field of tinguiririca volcanic complex tvc central chile southern central andes using p and s wave arrival times a 3d seismic velocity tomography was performed the preliminary locations of 2270 earthquakes have first been determined using an a priori 1d velocity model afterwards a joint inversion of both the 3d velocity model and final locations have been obtained high vp vs ratios are interpreted as zones with high hot fluid content and high fracturing meanwhile low vp vs anomalies could represent the magmatic reservoir and the conduit network associated to the fluid mobility based on structural information and thermal manifestations these anomalies have been interpreted in order to visualize the relation between local geology and the velocity model the volume associated with the magma reservoir and the fluid circulation network has been delimited using an iso value contour of vp vs equal to 1 70 the most prominent observed feature in the obtained model is a large v shaped low velocity anomaly extending along the entire study region and having the same vergency and orientation as the existing high angle inverse faults which corroborates that el fierro el diablo fault system represents the local control for fluid mobility this geometry coincides with surface hydrothermal manifestations and with available geochemical information of the area which allowed us to generate a conceptual model of fluid circulation in the volcanic hydrothermal system as well as define the location of the magmatic reservoir

slab breakoff as a mechanism for flipping of subduction polarity in taiwan we propose a model that involves breakoff of the east dipping eurasian slab beneath the taiwan orogen to account for the flipping of subduction polarity in northern taiwan the breakoff was initiated in southern ryukyu in the early pliocene and then propagated southwestward into taiwan like an opening zipper detachment of the eurasian slab has created a mantle window for the north dipping philippine sea plate to move in laterally causing a switch in the subduction polarity slab breakoff not only provides a testable model for the lithospheric structure of taiwan but also accounts for the high heat flow rapid uplift synorogenic extension deep focus earthquakes and north diminishing crustal contraction in the taiwan mountain belt

elastic wave velocities in isochemical granulite and amphibolite origin of a low velocity layer at the slab mantle wedge interface simultaneous measurements of compressional and shear wave velocities vp and vs in granulite and amphibolite with identical chemistry were carried out from room temperature to 850 c at 1 gpa both vp and vs in granulite and amphibolite decreased linearly with increasing temperature the values of vp and vs at 700 c are 7 75 km s and 4 37 km s for granulite and 7 40 km s and 4 21 km s for amphibolite respectively 3 4 lowering of elastic wave velocities in hydrous amphibolite compared with anhydrous granulites may be consistent with seismological observations indicating the presence of a low velocity layer at the slab mantle wedge interface in subduction zones

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

origin of vein structures in the slope deposits of modern accretionary prisms vein structures that have been described from the slope apron sediments of modern accretionary prisms represent brittle failure caused by an effective tensile stress the effective stress arises from abnormal pore water pressure acting in opposition to the compressional stresses on the sediment thus even though 3 the minimum stress may be compressional if the pore water pressure is of greater magnitude than 3 the resultant effective stress will be tensional and failure opening of the veins will occur in the 1 2 plane as the veins open the pore water can escape into the veins and eventually out of the sediment along more permeable coarser beds the initial movement of pore water into the veins may entrain finer grains from the host rock near the vein walls resulting in the observed finer grained vein filling then as the veins close as pore water escapes the elongate and platy grains may be rotated into near parallelism with the vein walls the tributaries that are observed at the ends of some veins may be a result of continued dewatering after the veins have closed as the pore water is forced to diffuse back into the sediment after being concentrated into an efficient flow by the parallel preferred orientation of the platy mineral grains in the vein

three dimensional elastic wave velocity structure of the western and central tien shan arrival times of compressional p and shear s waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central tien shan are used to determine one and three dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt the best fit one dimensional structures suggest that the average depth of the mohorovicic discontinuity in this area is 50 km the three dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region a 7 km thick wedge of sediment beneath the chu depression is outlined at depth by a south dipping plane of seismic activity suggesting the presence of an active decollemont these low velocities extend continuously to the southeast toward issyk kul suggesting a structural relationship between the two however rather than being consumed it appears that issyk kul is overthrusting the surrounding ranges the low velocity sediments in the fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity velocities at midcrustal depths generally are lower beneath the central tien shan than beneath the western tien shan this pattern becomes more evident in the uppermost mantle with p velocity contrasts of as much as 10 across a boundary that corresponds roughly to the geographical position of the talasso fergana fault the low velocities beneath the central tien shan exceed 150 km depth but do not appear to be deeper than 300 km depth there is no evidence for a lithospheric root beneath this part of the range rather the low velocities imply the presence of a positive buoyancy force uplifting the mountains evidence that this low velocity region existed before the collision suggests that the tien shan may not owe its rejuvenation simply to its location at the northern edge of a strong tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere

ten second love wave propagation and strong ground motions in taiwan applying the surface wave gaussian beam method to a three dimensional 3 d structure of taiwan we have studied the generation and propagation of short period 10 s surface waves in a region of pronounced crustal heterogeneity especially in a region defined to be at an epicentral distance larger than a few focal depths away where high frequency near field motions are largely attenuated and surface waves are adequately developed by perturbing the source model as well as the crustal model in an iterative inversion process we have achieved an excellent fit to the recent strong motion observations recorded during the tung ao earthquake june 5 1994 ml 6 0 in taiwan this allows us to 1 obtain a refined 3 d crustal model of taiwan from an initial model that was derived from recent tomographic results and more important 2 explain the distribution patterns of the strong shaking reported in terms of intensity maps for historical large earthquakes and predict the long period strong motion distribution patterns for future large earthquakes in taiwan with an improved 3 d structure we have gained better insight into short period surface wave propagations as they are modified by crustal lateral heterogeneity focusing and defocusing are clearly demonstrated of particular interest is the central mountain range a massive tertiary metamorphic body forming the backbone of taiwan that seems to serve as a divergent lens for the propagating surface waves whereas the large sedimentary basin in southwestern taiwan seems to focus the wavefronts instead the computed distortions of the propagating surface wave field due to lateral heterogeneity are correctly reflected by the observations

crustal structures from the wuyi yunkai orogen to the taiwan orogen the onshore offshore wide angle seismic experiments of the taiger and atsee projects abstract knowledge of the crustal structure is important for understanding the tectonic framework and geological evolution of southeastern china and adjacent areas in this study we integrated the datasets from the taiger taiwan integrated geodynamic research and atsee across taiwan strait explosion experiment projects to resolve onshore offshore deep crustal seismic profiles from the wuyi yunkai orogen to the taiwan orogen in southeastern china three seismic profiles were resolved and the longest profile was 850 km unlike 2d and 3d first arrival travel time tomography from previous studies we used both refracted and reflected phases pg pn pcp and pmp to model the crustal structures and the crustal reflectors in total data from 40 shots 2 earthquakes and approximately 1 950 stations were used 15 612 arrivals were selected among three transects using these data we determined the complex crustal evolution since the paleozoic era involving the closed paleozoic rift basin in central fujian the cenozoic extension due to the south china sea opening beneath the coastline of southern fujian and the on going collision of the taiwan orogen the shape of the moho which also reflects the crustal evolution can be summarized as follows 30 km deep to the west of fujian deepening toward central fujian 35 km becoming shallower toward the taiwan strait 28 km deepening again toward the mountain belt of taiwan 42 km and becoming shallower toward the pacific ocean 10 km

deterministic frequency wavenumber methods and direct measurements of rupture propagation during earthquakes using a dense array data analysis data recorded by the smart 1 array in taiwan are used to make the first direct measurements of two dimensional earthquake rupture propagation using frequency wavenumber techniques and ray theory we obtain estimates of the spatial extent duration and directions of rupture propagation during two earthquakes located offshore taiwan our estimates include uncertainties due to errors in fault plane location and orientation velocity structure and slowness ray parameter measured at the array we find that the january 29 1981 ml 6 3 taiwan earthquake ruptured unilaterally updip and toward the west on a 60 dipping 109 striking reverse fault using p waves we find its fault length and duration to be 25 18 km and 7 4 3 4 s respectively s waves indicate a fault length and duration of 27 15 km and 9 4 3 6 s respectively we suggest that the november 14 1986 ml 7 0 hualien taiwan earthquake was triggered by a shallow h 14 km foreshock ml 5 4 which occurred on a subparallel or splay fault approximately 5 s before the mainshock based on p waves rupture propagation during the foreshock is unilateral towards the northeast and slightly downdip on a 58 dipping 38 striking reverse fault its spatial extent and duration are approximately 23 km and 5 4 s the mainshock initiated at greater depth h 34 km than the foreshock with its primary direction of rupture unilateral toward the northeast and slightly updip on a 57 dipping 43 striking reverse fault using p waves we found a spatial extent and duration of 68 15 km and 12 4 2 2 s s waves indicate a similar extent duration and direction of propagation for the mainshock however we found it difficult to distinguish the foreshock and mainshock 5 waves we show by example that these results represent an improvement over what can be obtained using long period t 5 s teleseismic methods comparison of our results with those obtained using teleseismic data suggests either a short duration slip time function or that most slip occurs near the rupture front our results indicate that much of the complexity in the seismograms recorded during these events by the smart 1 array is due to spatial and temporal variations of the source this suggests that at least in some cases predictions of both the phase and amplitude of strong ground motion as a function of time would require a priori knowledge of source properties finally we recommend siting small 10 station dense arrays on land above offshore subduction zones which are considered seismic gaps because they can provide more detailed measurements of rupture propagation than is presently possible

three dimensional velocity structure and hypocenters of earthquakes beneath the hazara arc pakistan geometry of the underthrusting indian plate the three dimensional p and s wave velocity structures and hypocenters of 420 events beneath the western hazara arc are obtained simultaneously by inverting travel time data observed at fifteen tarbela seismic stations in general the p and s wave velocity distribution of the top layer 0 6 km depth correlates well with surface geology within this layer we find a low velocity region beneath the hazara thrust zone htz corresponding to the underthrusted murree formation and there are high velocity regions south of the main mantle thrust mmt which are associated with the exposed cambrian late paleozoic and tertiary granites a low velocity zone immediately to the west of the hazara kashmir syntaxis hks indicates the existence of a miocene foreland basin which is covered by late stage southeasterly directed thrusts along the hazara arc and is consistent with the idea that the hks is detached from the lower crust from the salt range to the htz the indian plate dips at a shallow angle about 2 3 to the northeast north of the htz the underthrusting indian plate dips gently to the northeast with an increased slope of 5 to 8 until it reaches the indus kohistan seismic zone iksz along the nw trending iksz the indian plate bends more steeply to the northeast beneath a seismically active midcrustal wedge directed to the southwest the larger events in the iksz are interpreted as occurring on a major thrust zone that can be followed to a depth of 24 km the iksz appears to consist of an upper seismic zone from the surface to about 8 km and a lower seismic zone 12 km to 24 km separated by an aseismic region about 4 km thick the lower iksz may represent the leading edge of a southwestward directed slab which has not yet ruptured the surface hypocenters of relocated earthquakes indicate that the htz is about 30 km wide with most of the larger microearthquakes occurring at 12 14 km seismicity along the htz suggests that the panjal and murree thrusts are active

tomographic imaging of the alaska subduction zone the alaska subduction zone is characterized by the pacific plate descending beneath the north american plate causing abundant seismic activity in the crust and along the wadati benioff zone down to a depth of approximately 200 km we have used 142 908 p wave arrival times from 12 237 shallow and intermediate depth earthquakes recorded by the alaska earthquake information center jointly run by the geophysical institute university of alaska fairbanks and u s geological survey in the period from january 1977 to november 1991 to investigate the three dimensional 3 d p wave velocity structure beneath central and southern alaska travel times and ray paths are accurately calculated by using an efficient 3 d ray tracing technique the nonlinear tomographic problem is solved by iteratively conducting linear inversions and the velocity structure and hypocentral locations are simultaneously determined we conducted two types of inversions one is an inversion with a laterally homogeneous starting model the others are what we call slab inversions in which we introduce into the initial model the high velocity subducting pacific plate as a priori information we found that the slab inversions gave a seismologically more plausible result and a final root mean square travel time residual significantly smaller than that of the inversion with the homogeneous starting model detailed p wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30 60 km the tomographic image of the upper crust correlates well with the major surface geological features such as slow sedimentary basins and fast ultramafic bodies prominent low velocity anomalies exist in the crust and upper mantle beneath active volcanoes in the mantle wedge the low velocity anomalies dip toward the continental side and extend to a depth of about 150 km which are considered to be associated with the active volcanism in the alaska subduction zone the results suggest that the subducting pacific plate has a thickness of 45 55 km and a p wave velocity 3 6 higher than that of the surrounding mantle

seismic evidence for central taiwan magnetic low and deep crustal deformation caused by plate collision abstract crustal seismic velocity structure was determined for the northern taiwan using seismic travel time data to investigate the northeastern extension of the northern south china sea s high magnetic belt in order to increase the model resolution a joint analysis of gravity anomaly and seismic travel time data have been conducted a total of 3385 events had been used in the inversion that was collected by the central weather bureau seismological network from 1990 to 2015 the main features of the obtained three dimensional velocity model are 1 a relatively high vp zone with velocity greater than 6 5 km s is observed in the middle to lower crust 2 the high vp zone generally parallels to the north south structural trending of the chuchih fault and hsuehshan range 3 at 25 km depth slice the high vp zone shows structural trends change from northeastward to northward in central taiwan where the values of high magnetic anomalies are rapidly decreasing to low values a combination of seismic gps and structural interpretations suggests that the entire crust has been deformed and demagnetized in consequence of the collision between the philippine sea plate and the asian continental margin we suggest that the feature of sharp bending of the high vp zone would migrate southwestward and cause further crustal deformation of the peikang high in the future

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

relocation of the 2006 pingtung earthquake sequence and seismotectonics in southern taiwan article i nfo we combine the arrival time data from eleven ocean bottom seismometers obs and from permanent stations of the central weather bureau seismic network cwbsn and the taiwan strong motion instrumentation program tsmip to improve the three dimensional vp and vp vs structures in the taiwan region relocate hypocenters of earthquakes in southern taiwan including the 2006 pingtung earthquake sequence and determine the focal mechanisms of some of the larger events our tomography result suggests that in the region south of taiwan the eurasian continental lithosphere is being subducted beneath the oceanic philippine sea plate leading to a region of low p wave speed with seismicity on top of a relatively high p wave speed zone the hypocentral distribution of the pingtung earthquake sequence indicates that the first event in the initial two largest shocks was a normal faulting quake caused by the bending of the subducting lithosphere the second event perhaps triggered by the first one was a strike slip earthquake

vertical crustal movements in eastern taiwan and their tectonic implications abstract leveling surveys along several routes and tidal observations at various points in eastern taiwan demonstrate rapid vertical crustal movements and they provide an effective means for monitoring the effects of collision between the philippine sea plate and the eurasian plate rapid elevation changes were detected in the southern section of the longitudinal valley repeated releveling along three routes since 1984 reveals that the coastal range was uplifted with respect to the longitudinal valley floor at a rate of about 3 0 cm yr this deformation is found to be confined within a very narrow zone along the east coast of taiwan two surveys 1984 and 1987 revealed that the coast is also undergoing uplift but at different rates from 0 cm yr to 3 5 cm yr the point of the most rapid uplift is located near 23 n and the rate generally diminishes towards the north the changes in rates may be controlled by the movements across faults tidal observations while providing a local datum show that uplift of the tidal station has occurred significant horizontal creep has been observed across many faults but rapid vertical creep as observed in the longitudinal valley is not very common

crustal scale seismic profiles across taiwan and the western philippine sea abstract we have used combined onshore and offshore wide angle seismic data sets to model the velocity structure of the taiwan arc continent collision along three cross island transects although taiwan is well known as a collisional orogen relatively few data have been collected that reveal the deeper structure resulting from this lithospheric scale process our southern transect crosses the hengchun peninsula of southernmost taiwan and demonstrates characteristics of incipient collision here 11 km thick transitional crust of the eurasian plate eup subducts beneath a large rapidly growing accretionary prism this prism also overrides the n luzon forearc to the east as it grows just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure because this break is accompanied by intense seismicity we interpret that the forearc block is being detached from the n luzon arc and philippine sea plate psp at this point our middle transect illustrates the structure of the developing collision steep and overturned velocity contours indicate probable large scale thrust boundaries across the orogen the leading edge of the coherent psp appears to extend to beneath the east coast of taiwan deformation of the psp is largely limited to the remnant n luzon arc with no evidence of crustal thickening to the east in the huatung basin our northern transect illustrates slab continent collision the continuing collision of the psp and eup as the psp subducts the collisional contact is below 20 km depths along this transect ne of hualien this transect shows elements of the transition from arc continent collision to ryukyu arc subduction both of our models across the central range suggest that the paleozoic to mesozoic basement rocks there may have been emplaced as thick coherent thrust sheets this suggests a process of partial continental subduction followed by intra crustal detachment and buoyancy aided exhumation although our models provide previously unknown structural information about the taiwan orogen our data do not define the deepest orogenic structure nor the structure of western taiwan additional seismic active and passive geologic and geodynamic modeling work must be done to fully define the structure the active deformation zones and the key geodynamic process of the taiwan arc continent collision

seismicity and tectonics of a continent island arc collision zone at the island of taiwan a collision between the eurasian plate continental margin and a volcanic island arc on the philippine sea plate about 4 m y ago uplifted the island of taiwan many lines of geological and geophysical evidence suggest that the collision is still active today and is currently propagating southward due to plate interaction geological and tectonic settings have become more complicated since the onset of the collision the collision suture is along the longitudinal valley lv in eastern taiwan seismic studies including earthquake monitoring and seismic refraction lines conducted in 1985 in eastern taiwan and its offshore area using land seismic stations and ocean bottom seismographs have generated results that better constrain the present day plate interaction both the crustal velocity structure and earthquake hypocenter distribution suggest that the subduction of the philippine sea plate along the ryukyu trench extends to the east coast of taiwan near about 23 5 n very high seismicity with dominant right lateral strike slip faulting is found in the back arc region behind the ryukyu trench this seismic activity is interpreted as a result of plate convergence and back arc wrenching due to the relative motion between the okinawa platelet and the eurasian plate the region close to the lv in eastern taiwan showed only shallow and a relatively low level of seismicity during this experiment with higher seismicity to the east of the lv the lack of intermediate depth events suggests that subduction is no longer active along the collision zone and the difference in seismicity between the regions east and west of the lv reflects the difference in strength of the adjacent continental and island arc crusts to the south of the lv seismicity continued southward with intermediate depth events distributed mainly to the east of taiwan along a nearly vertical benioff zone this supports the argument that convergence and collision are still active south of taiwan

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

crustal structure beneath taiwan using frequency band inversion of receiver function waveforms receiver function analysis is used to determine local crustal structure beneath taiwan we have performed preliminary data processing and polarization analysis for the selection of stations and events and to increase overall data quality receiver function analysis is then applied to data from the taiwan seismic network to obtain radial and transverse receiver functions due to the limited azimuthal coverage only the radial receiver functions are analyzed in terms of horizontally layered crustal structure for each station in order to improve convergence of the receiver function inversion frequency band inversion fbi is implemented in which an iterative inversion procedure with sequentially higher low pass corner frequencies is used to stabilize the waveform inversion frequency band inversion is applied to receiver functions at six stations of the taiwan seismic network initial 20 layer crustal models are inverted for using prior tomographic results for the initial models the resulting 20 1ayer models are then simplified to 4 to 5 layer models and input into an alternating depth and velocity frequency band inversion for the six stations investigated the resulting simplified models provide an average estimate of 38 km for the moho thickness surrounding the central range of taiwan also the individual station estimates compare well with the recent tomographic model of and the refraction results of rau and wu 1995 and the refraction results of ma and song 1997

landslide databases for climate change detection and attribution abstract a systematic inventory of landslide events over regional spatial scales and through time is required for investigating changes in landslide frequency along side changes in landslide triggers this paper describes the methods used to compile a european wide landslide inventory and some of the methodological and practical obstacles that inhibit better use and development of such inventories we argue that these methods can be used more widely to provide a comprehensive picture of landslide populations and to further enrich our understanding of the impact of climate change and other drivers on landslide frequency and magnitude

distribution of magma beneath the toba caldera complex north sumatra indonesia constrained by three dimensional p wave velocities seismicity and gravity data 1 we estimate the one and three dimensional p wave velocity structure beneath the toba caldera complex a 30 100 km topographic depression in north sumatra using arrival time data of local earthquakes recorded by a 40 station seismic network that operated for 4 months inversions reveal the presence of p velocities up to 37 below normal that likely map the distribution of magma within this subduction related volcanic system considered the world s largest in the upper 10 km of crust the largest low velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome a smaller volume of low velocities is observed in the upper crust under the north end of the depression separating the two regions is a zone of locally high velocities indicating that the shallow subcaldera magma system is composed of two separate reservoirs not a single one that extends the entire length of the caldera complex above each low velocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse 74 kyr ago a low velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low frequency earthquakes in the 20 to 40 km depth range the low frequency earthquakes apparently record the migration of melt in the mafic roots of this large volume crustal magma system

three dimensional inversion of regional p and s arrival times in the east aleutians and sources of subduction zone gravity highs free air gravity highs over forearcs represent a large fraction of the power in the earth s anomalous field yet their origin remains uncertain seismic velocities as indicators of density are estimated here as a means to compare the relative importance of upper plate sources for the gravity high with sources in the downgoing plate p and s arrival times for local earthquakes recorded by a seismic network in the eastern aleutians are inverted for three dimensional velocity structure between the volcanic arc and the downgoing plate a three dimensional ray tracing scheme is used to invert the 7974 p and 6764 s arrivals for seismic velocities and hypocenters of 635 events one dimensional inversions show that station p residuals are systematically 0 25 0 5 s positive at stations 0 30 km north of the aleutian volcanic arc indicating slow material while residuals at stations 10 30 km south of the arc are 0 1 0 25 s negative both features are explained in three dimensional inversions by velocity variations at depths less than 25 35 km tests using a one dimensional or a two dimensional slab starting model show that below 100 km depth velocities are poorly determined and trade off almost completely with hypocenters for earthquakes at these depths the locations of forearc velocity highs in the crust of the upper plate correspond to the location of the gravity high between the trench and volcanic arc free air anomalies calculated from the three dimensional velocity inversion result match observed gravity for a linear density velocity relationship between 0 1 and 0 3 mg m 3 km s 1 when a 50 km thick slab is included with a density of 0 055 0 005 mg m 3 values outside these ranges do not match the observed gravity the slab alone contributes one third to one half of the total 75 150 mgal amplitude of the gravity high but predicts a high that is much broader than is observed the inclusion of upper plate velocity anomalies predicts the correct width of the anomaly 100 150 km where the anomaly is most positive because the forearc gravity high is continuous along the entire aleutian arc and is found in most arcs globally high upper plate forearc velocities are suspected to be a common feature of the upper plate of most subduction zones the forearc mass excesses appear to be sustained by upward regional flexure of the upper plate that partly balances the depression of the lower plate at the trench thus elevating high density and high velocity material thus a part of the downward flexure of the subducting plate is regionally compensated by shallow positive mass anomalies in the upper plate and the strength of the upper plate helps generate the forearc gravity highs

budget of crustal shortening and subduction of continental crust in the alps the stratigraphy surface structure and deep structure of the alps permit but do not require that a significant amount of european continental crust was subducted into the mantle beneath adria the fragment of continental lithosphere now comprising much of italy and the adriatic sea floor the present thin crust in much of europe suggests that the crust that was stretched in mesozoic time to form the southern margin of europe was initially thinner than normal the stratigraphic history of sediment deposited on that margin before its collision with adria suggests that the crustal basement on which much of this material was deposited had become particularly thin during the formation of the margin most of the crustal basement underlying the european margin and involved in the collision and the formation of the chain probably was thinner than 20 km and much of it may have been thinner than 10 km profiles of the present deep structure of the alps yield a range of possible values for the the average cross sectional area of crust involved in the collision that permit as much as 280 km of crustal shortening if this crust was as thin as implied by the stratigraphy thus although subduction of hundreds of kilometers of continental crust into the mantle beneath adria could have occurred the budget of crustal material involved in the formation of the alps does not require such crustal subduction

seismotectonics of taiwan abstract high quality seismicity data and focal mechanism solutions obtained during 1973 1983 by the permanent taiwan telemetered seismographic network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the taiwan area seismicity distribution in southern taiwan clearly reveals an east dipping benioff zone which has a thickness of about 30 km and begins to deepen along 121 e at a dip angle of 55 60 the leading edge of this benioff zone reaches a depth of about 180 km between 21 n and 22 n but tapers off to a shallower depth of about 100 km from 22 n to 23 n the presence of this seismic zone implies that subduction of the south china sea plate under the philippine sea plate extends from luzon northward to about 23 n the position of the northern boundary of the south china sea plate as tentatively defined according to the seismicity distribution passes through southern taiwan from the offshore area in the taiwan strait west of kaohsiung in an east northeast direction to the taitung area where a triple junction probably lies seismicity is found to disappear abruptly below a certain depth in many parts of taiwan this phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely developed faults and fractures however the converse is not necessarily true this may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas finally focal mechanism solutions for west central taiwan and the kuangfu fuli area in eastern taiwan predominantly show a maximum horizontal compression in the se nw direction which can be related to collision between the eurasian and philippine sea plates however focal mechanism solutions for both the hualien area in eastern taiwan and the tainan area in southwestern taiwan show remarkable irregularities which may result from local tectonic complexities

three dimensional p velocity structures of the lithosphere beneath taiwan from the analysis of taiger and related seismic data sets 1 the convergence of the philippine sea and the eurasian plates in the taiwan region led to the formation of a young collisional mountain between two subduction zones of nearly orthogonal polarities the geological processes underlying the collision and its relation to subduction are the primary targets of taiger taiwan integrated geodynamic research project newly acquired passive and active sources data on land supplemented by ocean bottom as well as permanent seismic network data are used to derive a new 3 d tomographic velocity model using the 7 5 km sec contour as a marker for crustal deformation two trend parallel roots one under the central range with a maximum depth of 55 km and the other under the coastal range at 40 km are found to extend from southern to central 24 n taiwan between the two roots and lying approximately beneath the longitudinal valley the 7 5 km sec contour rises to 25 km depth in the upper mantle a high velocity zone east dipping and its upper surface coinciding with a wadati benioff zone in the south 22 8 n becomes near vertical and ill defined in the north 23 9 n the crustal deformation as defined by the roots and the along trend variations of the upper mantle structures under taiwan provide key information for the orogeny with the thickening of crust to 55 km or more processes such as eclogitization and delamination may come into play

seismic imaging using microearthquakes induced by hydraulic fracturing abstractseismic imaging using microearthquakes induced by hydraulic fracturing produces a three dimensional 3 d s wave velocity model of the fractured zone improves the calculated locations of the microearthquakes and may lead to better estimates of fractureplane orientations fracture density and water flow paths such information is important for predicting the amount of heat energy that may be extracted from geothermal reservoir a fractured zone was created at the los alamos hot dry rock reservoir in north central new mexico within otherwise impermeable basement rock by injecting 21000 m3 of water into a borehole under high pressure at a depth of 3 5 km induced microearthquakes were observed using four borehole seismometers the p wave and s wave arrival times have been inverted to find the 3 d velocity structures and the microearthquake locations and origin times the inversion was implemented using the separation of parameters technique and constraints were incorporated to require smooth vel

thermal modeling of continental subduction and exhumation constrained by heat flow and seismicity in taiwan abstract the thermal evolution of crustal exhumation subsequent to the subduction of the continental margin is modeled through numerical solutions to the two dimensional heat conduction equation the boundary conditions used in the modeling are basically constrained by available geophysical and geological observations in the taiwan area temperature distributions are calculated at one million year intervals over a 5 ma duration although this study presents a tentative estimation based on some reasonable assumptions the preliminary results not only explain the sharp change in seismicity within the crust but also agree with the general heat flow pattern on the surface of taiwan more specifically both the high heat flow and the aseismic belt beneath the eastern central range which are probably attributed to higher temperatures within the crust are modeled by crustal exhumation after the subduction of the continental margin however the occurrence of deeper earthquakes east of the central range is a result of the lower thermal gradient which is due to the subduction of the cold continental crust into the uppermost mantle in summary preliminary thermal modeling supports the possibility of a tectonic model of active continental subduction and crustal exhumation for the taiwan orogen

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

seismicity characteristics of a potentially active quaternary volcano the tatun volcano group northern taiwan the tatun volcano group tvg is located at the northern tip of taiwan near the capital taipei and close to two nuclear power plants because of lack of any activity in historical times it has been classified as an extinct volcano even though more recent studies suggestthattvgmighthavebeenactiveduringthelast20ka inmay2003aseismicmonitoringprojectatthetvgareawasinitiated by deploying eight three component seismic stations some of them equipped with both short period and broadband sensors during the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events or as a continuous sequence in the form of spasmodic bursts mixed and low frequency events were also present during the same period eventhoughtheyoccurredonlyrarely arrivaltimesfromeventswithclearp s wavephaseswereinvertedinordertoobtaina minimum 1dvelocity modelwithstation corrections probabilistic nonlinear earthquakelocationswere calculated for allthese events using the newly derived velocity model most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity forming tight clusters at depths shallower than 4 km relative locations calculated using the double difference method and utilising catalogue and cross correlation differential traveltimes showed insignificant differences when compared to the nonlinear probabilistic locations in general seismicity in the tvg area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts mixed low frequency events and a b value 1 17 0 1 higher than in any other part of taiwan these observations that are similar to those reported in other dormant quaternary volcanoes indicate that a magma chamber may still exist beneath tvg and that a future eruption or period of unrest should not be considered unlikely

3d velocity structure around the source area of the 1999 chi chi taiwan earthquake before and after the mainshock 3d v p and v p v s models for the source area before and after the chi chi earthquake are derived from a tomographic inversion of seismic travel time data from the taiwan central weather bureau seismic network cwbsn resulting 3d velocity models reveal a strong lateral variation in the crust particularly at a depth range of 5 to 25 km along the trend of the dipping chelungpu thrust in general low v p and high v p v s anomalies are found in and around the source volume of the chi chi earthquake a volume centered around the entire 100 km 40 km chelungpu rupture surface we pay special attention to the crustal properties and their changes due to the chi chi event and the association of the crustal anomalies with seismicity combining the results with those of other geological and geophysical surveys the crustal anomalies are interpreted as images of a highly fractured and fluid filled chi chi source region fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the chi chi earthquake the v p models obtained before and after the chi chi event show some interesting heterogeneities regions of low v p anomaly appear to have further expanded with the occurrence of the chi chi earthquake the v p v s model shows high anomalies near the source rupture especially near where the rupture is believed to have nucleated after the chi chi event the v p v s model shows a substantial change that may reflect an outward shift of the fluid filled fractured source region this shift is shown by a general expansion in v p v s anomalies to a broader region e g where two intensive aftershock clusters occurred and a significant v p v s reduction to more normal values near the chelungpu rupture we attribute these changes in v p and v p v s anomalies as well as the expansion and shift of the anomalous regions to be a response to the localized stress change caused by the slip of the chi chi rupture the location of the seismic events is correlated with that of the crustal v p and v p v s anomalies large events appear to occur in the transition zones of both v p and v p v s anomalies this finding may be of use in the identification of weakened crustal regions in a seismic zone thus suggesting where impending large earthquakes are likely to occur all input data and initial and final models of this article are given as electronic files in the cd attached to the end of this issue

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

seismicity and tectonics in jujuy province northwestern argentina the portable array for numerical digital analysis panda network a digitally recorded seismic array operated for nine months in jujuy province of northwestern argentina the network was deployed along the eastern edge of the altiplano puna plateau in a major n s thrust belt that is transitional in style between the thin skinned deformation of the bolivian foreland to the north and basement involved deformation of the pampean region to the south teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front no crustal seismicity was detected beneath the puna plateau to the west peak seismicity levels beneath the foreland occurred between 20 and 25 km depth a sharp decrease in seismicity was observed below 25 km an estimate of 42 km for the thickness of the jujuy foreland crust was inferred from wide angle moho reflections observed on the digital seismograms the highest concentration of crustal seismicity was located beneath sierra de zapla a broad anticline immediately east of san salvador de jujuy many of the earthquakes in the 20 25 km depth range have a shallow west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74 closely paralleling both the current nazca south america convergence direction and the shortening direction derived from regional quaternary fault slip data we interpret the earthquakes as occurring on planes of weakness first produced during cretaceous rifting and later reactivated by neogene compressive stresses crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment along which significant late cenozoic e w shortening has occurred

three dimensional qp and qs tomography beneath taiwan orogenic belt implications for tectonic and thermal structure summary we determined the 3 d qp and qs structure of the taiwan orogenic belt to enhance understanding of the related tectonic and thermal structure beneath the collision zone the inversion used t values measured from the spectra of p and s waves from the dense taiwan strong motion network for moderate size earthquakes ml 4 5 5 5 to avoid source complexity the time period 1991 2007 includes the aftershock sequence of the 1999 chi chi earthquake that provides good ray coverage in central taiwan over 18 000 velocity spectra from 883 earthquakes were analysed a non linear least square technique is applied to the spectra for t determination by assuming a 2 source model for the frequency band of 1 30 hz a frequency independent q was assumed in this study the corner frequency of a specific event was fixed for the corresponding stations and a quality index was defined to assure good quality data for the inversion the results reveal the sharp variation of qp and qs across the recently ruptured chelungpu fault and the kaoping and chaochou faults in pingtung plain the q values in the hangingwall are smaller by about 85 and 110 for qp and qs respectively relative to the footwall the fault geometry is distinctly delineated by the contour of qp qs of 1 2 that extends to the depth of the geologically identified decollement structure beneath the central range the low qp low qs and high qp qs features coincide well with the aseismic zone comparison to the recent thermomechanical numerical models of taiwan shows that the low q zone corresponds to the exhumation of the lower crust the low qs regime high attenuation beneath the central ranges at the depth of 5 22 km coincides with predicted temperatures of 400 600 c the qs comparison with the major tectonic and thermal mechanical models of taiwan reveals that the shear wave attenuation model contains comprehensive rheological and thermal information of relevance to understanding mountain building processes this technique appears particularly useful for distinguishing strong and weak crustal regions in the absence of other constraints

active thrust front of the greater caucasus the april 29 1991 racha earthquake sequence and its tectonic implications although fault bounded thrust sheest are common in the geological record seismic evidence for their motion is sparse the april 29 1991 racha earthquake m s 7 0 the largest instrumentally recorded earthquake in the greater caucasus is one of the largest recent earthquakes in continental thrust belts and provides evidence on mechanisms of thrust sheet motion using data from a deployment of program for array seismic studies of the continental lithosphere passcal digital seismographs and various other instrumental we locate 1952 aftershocks occurring between may 7 and june 30 1991 the aftershocks form a zone 70 km long and 10 25 km wide striking e w following the racha ridge at the southern boundary of the greater caucasus thrust system teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks the solutions show thrust faulting with centroid depths of 3 10 km comparable to depths of locally recorded aftershocks 2 12 km the shallow dipping nodal plane the aftershock distribution and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping nne at 20 o 31 o bounding the southern slope of the greater caucasus this fault system thrusts the greater caucasus structures south over the dzhirula basement massif the inferred fault geometry suggests that the active fault is either a detachment between sediments and dzhirula basement or cuts through the basement at shallow depths the 1500 m high racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes hence the 1991 earthquake sequence shows that the western greater caucasus is accommodating plate convergence at a rate possibly comparable to the eastern greater caucasus a few millimeters per year along strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area no strong evidence for transfer structures could be found along strike suggesting that differences in collisional style between the western and eastern greater caucasus may reflect differences in mechanical properties rather than differences in convergence rate a june 15 1991 event and its aftershocks southeast of the primary aftershock zone along strike show fault planes and slip vectors rotated 41 o clockwise from the mainshock this rotation is consistent with an along strike change in direction of the thrust front near 44 o e longitude and demonstrates strong local structural or topographic control on slip direction the rotation requires along strike shortening within the greater caucasus thrust system at a rate comparable to the rate of thrusting

tomographic imaging of lithospheric structures under taiwan abstract tomographic images of the crustal and mantle velocity structures under taiwan are obtained by simultaneous inversion of local earthquake p wave arrival times for hypocenters and p wave velocity structures in northern taiwan a high velocity zone coinciding with the wadati benioff zone can readily be identified as the subducted philippine sea plate the imaged zone dips toward the north at an angle of 40 from a depth range of 20 55 to 100 130 km an upper mantle low velocity wedge ranging from 40 to 80 km in depth exists above the subducted slab above this wedge is the ilan plain of northern taiwan which lies at the west end of the okinawa trough a well recognized back arc basin the crustal velocities under the plain are also relatively low the well defined high velocity zone and the low velocity wedge provide some constraints on the thermal structures of the subduction system under northern taiwan in tomographic images across the central section of taiwan thickening of the crust and up arching of the lower crustal materials under the central range are commonly observed the crust under the western foothills region is clearly thinner and the near surface low velocity layers are well developed the structures under the central range show that although the taiwan orogeny is quite young a root deeper in the north and shallowing to the south has formed the results of our tomography show that a significant portion of the lithosphere is involved in the taiwan orogeny

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