identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

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

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

2003 mw6 8 chengkung earthquake and its related seismogenic structures abstract the mw 6 8 chengkung earthquake on december 10 2003 in eastern taiwan was associated with abundant aftershocks to explore the seismogenic mechanism and its causative fault system we relocated the aftershocks determined the focal mechanisms from p wave polarities and used the finite dimension source model method to identify the fault plane based on our results the earthquake can be divided into three seismic clusters one is located in the east of the southern longitudinal valley one on the chihshang fault and the other on the western side of the luyeh fault to the south both the seismic distribution and finite dimension source model method show that east dipping fault planes with thrust mechanisms coincide with previous studies based on background seismicity the third seismic cluster suggests thrust mechanism along west dipping fault plane that was not recognized by background seismicity this seismic cluster occurred along the central range fault previously proposed as active by geomorphic and geologic evidence

seismogenic structure in a tectonic suture zone with new constraints from 2006 mw6 1 taitung earthquake 1 a mw 6 1 earthquake occurred on april 1st 2006 near taitung eastern taiwan it produced significant coseismic ground displacements and a large number of aftershocks in the ensuing month this event provides an opportunity to diagnose the seismogenic structure in the southern longitudinal valley lv of eastern taiwan long viewed as one of the collision sutures between the philippine sea and the eurasian plates with precisely relocated main and aftershock hypocenters focal mechanisms for m 3 8 events and coseismic ground displacements from strong motion records we determine a main shock dislocation model our results indicate that the main shock occurred on a high angle fault azimuth 198 dip 77 the model comprises a fault with two segments the main shock and a large number of aftershocks are associated with the northern segment that exhibited predominantly left lateral strike slip motion in agreement with p wave first motions and waveform usgs solutions the southern segment exhibits a slightly larger thrust component in agreement with cmt solutions tectonically this event highlights a nne trending fault on the west side of the lv which is predominantly strike slip the aftershocks clustered to the east of the main shock which exhibit mainly thrust mechanisms indicate that shortening is still acting on the sedimentary materials deposited between the coastal and central ranges prior to collision as a result the southern lv is undergoing slip partitioning along different faults which has never been specified before

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

from passive continental margin to mountain belt insights from analytical and numerical models and application to taiwan abstract the physics of deformation at plate boundaries is insufficiently understood taiwan is particularly interesting in this context since geological and geophysical arguments indicate that passive margin sediments were exhumed into an active mountain belt over a relatively short period of time classically taiwan has been described as a critical wedge orogeny in which deformation is entirely restricted to the crust more recent data however indicate that deeper parts of the lithosphere also participate in deformation in order to study the physical feasibility of various endmember hypothesis we have performed two dimensional 2 d thermo mechanical numerical experiments in which both mantle lithospheric and surface processes are taken into account and in which crustal deformation evolves as a function of mantle flow our setup consists of ocean ocean subduction followed by passive continental margin deformation results indicate that in most cases the subducting slab becomes close to vertical upon arrival of continental material to the trench whereas crustal rheology deformation and surface processes do not modify mantle flow significantly they are important for crustal scale processes relatively little crustal exhumation occurs if the crust is homogeneous and erosion rates are small significant exhumation in a dome like structure is obtained if a weak lower crust and large erosion rates are present the presence of an oceanic arc in the overriding oceanic lithosphere modifies but does not control crustal deformation synthetic metamorphic facies maps and thermochronological data computed from the numerical models are in reasonable agreement with observations in southern taiwan if exhumation occurs in a dome like structure further physical insight in the doming mechanisms is obtained by a combination of analytical techniques and crustal scale numerical models this analysis indicates that the dome like exhumation mechanism can be attributed to a compressional mechanical instability facilitated by both a weak lower crust and large rates of erosion both conditions are likely to be satisfied in taiwan

geomorphology of the southernmost longitudinal valley fault implications for evolution of the active suture of eastern taiwan in order to understand fully the deformational patterns of the longitudinal valley fault system a major structure along the eastern suture of taiwan we mapped geomorphic features near the southern end of the longitudinal valley where many well developed fluvial landforms record deformation along multiple strands of the fault our analysis shows that the longitudinal valley fault there comprises two major strands the luyeh strand on the west has predominantly reverse motion the peinan strand on the east has a significant left lateral component between the two strands late quaternary fluvial sediments and surfaces exhibit progressive deformation the luyeh strand dies out to the north where it steps to the east and joins the peinan strand to become the main strand of the reverse sinistral longitudinal valley fault to the south the luyeh strand becomes an e w striking monocline this suggests that the reverse motion on the longitudinal valley system decreases drastically at that point the longitudinal valley fault system is therefore likely to terminate abruptly there and does not seem to connect to any existing structure further to the south this abrupt structural change suggests that the development of the longitudinal valley suture occurs through discrete structural jumps rather than by a continuous northward maturation

modeling the effects of secular variation of geomagnetic field orientation on the ionospheric long term trend over the past century 1 a middle and low latitude ionospheric theoretical model is used for the first time to assess the effects of the secular variations of geomagnetic field orientation on ionospheric long term trends over the past century it is found that the varied geomagnetic field can produce ionospheric long term trends in both fof2 and hmf2 since the amplitudes of geomagnetic field change depend on location the modeled trends by secular variations of geomagnetic field orientation differ from place to place the modeled ionospheric trends have obvious seasonal and diurnal variations these variations show typical regional features by comparison with existing results we suggest that the changes of the global geomagnetic field may partly contribute to the inconsistent seasonal and local time variation patterns in ionospheric trends from different observations the average trends of fof2 and hmf2 are 0 0047 mhz a and 0 0107 km a during 1900 2005 which is near or even comparable with those of other trend origins in comparison with the trend results from worldwide observations we conclude that the effects of geomagnetic field orientation on the ionospheric long term trend cannot be ruled out especially in areas with large geomagnetic field variations

shear wave splitting at the edge of the ryukyu subduction zone abstract intraslab events recorded by ocean bottom seismometers in the okinawa trough provide an extended depiction of shear wave splitting in the southwest section of the ryukyu subduction zone at 100 200 km from the western edge of the subduction system we observed trench normal fast polarization direction in the back arc compatible with 2d slab or rifting driven corner flow towards the edge the fast directions are sub parallel to the trench in the arc back arc region and rotate to trench normal within 50 km of the edge splitting constrained by land stations with paths mostly in the mantle wedge exhibits similar trench normal fast directions in the subduction edge zone further inland the dominant component of fast directions becomes roughly parallel to the taiwan orogenic fabric splitting of p to s phases converted at the moho of the okinawa trough and of s phases from shallow events suggest that crustal anisotropy may affect the measured splitting but the observed pattern reflects predominantly mantle anisotropy the variation in splitting along the okinawa trough cannot be explained by a b type a type olivine fabric transition in the mantle wedge it may indicate the presence of an along arc flow in the mantle wedge towards the edge where it is blocked and deflected by the eurasian lithosphere this scenario bolsters previous studies suggesting a significant impact of eurasian lithosphere on the dynamics of the ryukyu subduction system

conjugate thrust faulting associated with the 1999 chi chi taiwan earthquake sequence 1 the geometrical structure of the responsible faults of the 20 september 1999 chi chi taiwan earthquake ml 7 3 mw 7 6 and its aftershocks can be clearly depicted by well located hypocenters and focal mechanisms of large aftershocks the mainshock and two large aftershocks with ml 6 8 were characterized by thrust faulting along a n s striking fault plane dipping to the east the underground structure of the chelungpu fault which is probably merging with the decollement beneath the western foothills can be clearly associated with the seismicity pattern and the focal mechanisms of the three largest events a group of deeper aftershocks including two moderate events ml 6 3 and 6 0 respectively were located to the southeast of the mainshock along a fault plane dipping steeply to the west down to a depth of about 30 km our results suggest that the spatial pattern of the aftershocks in the southern part of the source area can be interpreted by a conjugate fault system this conjugate fault system is comprised of the gently east dipping chelungpu fault and a steeply west dipping deeper fault zone

sks skks splitting and taiwan orogeny 1 we determine 58 sets of splitting parameters fast direction and delay time t from broadband sks skks waves recorded at temporary and permanent stations in taiwan these and earlier results from s and scs as well as sks show an island wide pattern of alignment with n nne structural trends in southern and central taiwan and then turn smoothly to ew following the structural bight in northern taiwan remarkable differences in t for eastern and southwestern events are found resulting probably from a more anisotropic upper mantle in the east under the central range with t an average of 1 3 seconds our observations are consistent with the hypothesis that the orogeny involves shear or flow in the upper mantle in central and southern taiwan in northern taiwan near the ends of the collision and subduction zones the extrusion and the hydration of olivine may contribute to the orientation of

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

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

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

structural geodetic and seismological evidence for tectonic escape in sw taiwan abstract recent structural geodetic and seismological data in sw taiwan are analysed and discussed in terms of present day tectonic escape occurring in response to the active n100 collisional shortening the escaping area corresponds to the onland extension of the manila accretionary wedge this region comprises a rheologically weak thick muddy cover which is decoupled from the underlying basement by a decollement and which deforms mainly by aseismic creep it is separated from the northern actual collisional area by a major wnw to nw trending structural and kinematic transition zone oblique to the structural grain of the belt the chishan transfer fault zone geodetic data are further used to define several poorly deforming blocks undergoing nearly uniform displacement velocities and bounded by kinematic discontinuities that fit the major faults and to determine the present day across strike and along strike motions on these major faults although direct onland structural evidence of tectonic escape is poor reconstruction of quaternary paleostress patterns demonstrate that this escape probably began during the late pleistocene later than in northeastern taiwan as a result of the southward migration of the collision through time offshore structural data help to constrain the geometry and the southern extension of the escaping blocks finally a tentative model of lateral extrusion in sw taiwan is proposed

taiwan orogeny thin skinned or lithospheric collision abstract the taiwan orogeny is young and presently very active it provides an excellent environment for studying ongoing orogenic processes especially since the region is monitored intensively with dense seismological and geodetic networks and new studies aiming at deciphering shallow and deep structures in and around taiwan have been recently conducted or are being planned the available data can be used continually to test critically hypotheses of the taiwan orogeny hypotheses dealing with the mechanics of mountain building are basic to the understanding of taiwan orogeny and are particularly amenable to testing the widely cited thin skinned tectonics hypothesis was formulated to explain mainly the geologic and relatively shallow

mechanical properties of quartz at the α β phase transition implications for tectonic and seismic anomalies 1 the anelastic response of single crystal and polycrystalline quartz and quartz rich sandstones to applied stress at low frequencies has been investigated by dynamic mechanical analysis and force torsion pendulum measurements the dynamic modulus was measured on heating and cooling through the phase transition under shear and bending stress applied at low frequency 1 hz single crystal quartz shows highly anisotropic properties in both the real and imaginary part of the modulus polycrystalline novaculite quartz displays the phase transition temperatures tc around 8 c higher than seen in single crystal quartz due to the pinning stress effect of grain boundaries the properties of the sandstone vary with heating and cooling cycles as the cementation and microfracturing of the grain structure changes at high temperature the phase transition in quartz is ferrobielastic and as such energy differences between domains in the transformation microstructure arise under conditions of applied shear stress pre transition softening and an increase in anelastic loss above tc are attributed to ferrobielastic switching of incipient domain microstructures that arise in the silica microstructure the influence of temperature on this microstructure is to induce increased switching of domains as a response to thermal stress inherent in the anisotropic thermal expansion of the polycrystalline structure these results indicate that ferrobielastic switching in quartz is an important mechanism in controlling changes in mechanical properties as such we anticipate that it will induce measurable anomalies in seismic signatures of quartz rich portions of the earth s crust in regions of high thermal flux the transition may also be responsible for observed seismic velocity reductions and tectonic weakening in western north america and in certain parts of tibetan plateau

constraints on average taiwan reference moho discontinuity model receiver function analysis using bats data summary by analysing teleseismic waveform data we propose a large scale taiwan reference moho discontinuity model trmdm through coherent stacking and h analysis of receiver functions rfs tectonic structural features are determined from accurate crustal thickness h and reliable vp vs ratio consequently poisson s ratio estimations for taiwan the best estimated regional average moho depth is 33 5 2 9 km noted is a westward crustal thinning from 39 3 0 km in eastern taiwan to 30 1 5 km in southeastern china the averaged moho depth is 23 0 2 5 km in northern taiwan and increases to 33 5 2 0 km in the south the markedly thin crust and high vp vs ratio in northern taiwan are mainly associated with backarc opening and magmatic activities based on h estimations and associated seismicity patterns beneath suang long sslb and adjacent four stations we propose two moho reference models that exhibit distinctive crustal thickening in central taiwan the spatial variations of crustal thickness models show strong lateral change in the moho topography the thick crust in the south thin crust in the north and transition in central taiwan mark the different tectonic units between the subduction wedge and the collision prism onshore in view of the following list of constraints thicker crust in the south than in the north eastward crust thickening significant moho depth changes clear 9 11 s subcrustal boundaries from five stations the compelling thickening of crust and upper mantle beneath the central mountain belt seismicity patterns with the absence of mantle earthquake in the wadatibenioff zone regional geological features the occurrence of the inland chi chi earthquake the compelling low at yulb 1 58 0 13 that may extend to central taiwan at sslb 1 59 0 23 relatively thick crust at yulb 39 5 1 6 km and sslb 40 5 1 3 km or 52 5 1 5 km and finally the apparent horizontal offset between highest yu shan mountain and the deepest moho at sslb we are able to suggest that the initial subduction of the eurasian plate occurred in southern taiwan and most likely extended to the transition zone in central taiwan although the current rf study is limited by large interstation distance our analysis provides more notable constraints than previous studies to explicitly determine the characteristic features of moho discontinuity this detailed approach means that rather than treating the tectonic framework as a whole heterogeneous lithospheric feature clear moho boundary geometry variations indicate that active orogenic processes and differential tectonic stress loading create separate and complex crustal and subcrustal to upper mantle structural features caused by the arc continent collision

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

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

identifying the rupture plane of the 2001 nisqually washington earthquake the 2001 nisqually earthquake occurred within the subducting juan de fuca plate previous seismic and geodetic studies could not confidently identify its actual fault plane from the two nodal planes in this study we apply the recently developed source scanning algorithm to local seismic waveforms and show unambiguously that the steeply east dipping plane is the rupture plane the rupture began near the bottom of the subducting crust and propagated downward into the subducting uppermost mantle if intraslab earthquakes are assumed to be due to dehydration embrittlement the source dimension is unlikely to grow any larger because the warm thermal state of the subducting juan de fuca plate limits dehydration to a shallow depth below the slab surface numerical modeling of the thermal structure indicates that dehydration embrittlement can only take place in the top 10 km of the subducting mantle implying that the maximum size of an intraslab earthquake in northern cascadia would be m w 7 or less

seismic evidence for the α β quartz transition beneath taiwan from vp vs tomography 1 knowledge of the rock types and pressure temperature conditions at crustal depths in an active orogeny is key to understanding the mechanism of mountain building and its associated modern deformation erosion and earthquakes seismic wave velocities by themselves generally do not have the sensitivity to discriminate one rock type from another or to decipher the p t conditions at which they exist but laboratory measured ratios of velocities of p to s waves vp vs have been shown to be effective results of 3 d vp and vp vs tomographic imaging based on dense seismic arrays in the highly seismic environment of taiwan provides the first detailed vp vs structures of the orogen the sharp reduction in the observed vp vs ratio in the felsic core of the mountain belts implies that the quartz transition temperature is reached at a mean depth of 24 3 km the transition temperature is estimated to be 750 25 c at this depth yielding an average thermal gradient of 30 3 c km

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

shear enhanced nonlinear flow in rough walled rock fractures abstract nonlinear flow in 3d rough walled rock fractures is simulated by solving the navier stokes equations the emphasis is on the impact of shear caused aperture changes variable apertures and asperity contacts and flow conditions inertial term upon nonlinear flow behavior in order to compare shear effects two 3d fracture models with and without shear were established with identical initial rough walled surfaces topographies of a realistic rock sample five groups of simulations with different inflow boundary conditions of flowrates reynolds numbers re were conducted to demonstrate shear enhanced nonlinearity of flow fields and limitations of local cubic law lcl approach the flow results clearly show channeling flow along the preferential paths transverse flow around the contact spots and eddy flows behind contact spots with increasing re which cannot be observed in 2d models the effective transmissivity of the 3d fracture model was calculated from the modeling results of velocity and pressure fields the results showed that the effective transmissivity is a function of local apertures with important uncertainties even when re is small i e re 0 4 in this study thus the validity of the transmissivity evaluation using lcl approach for nonlinear flow in 3d rough walled rock fractures is questionable

nature of the upper crust beneath central tibet abstract a wide angle reflection refraction seismic line obtained through sino french joint research near the bangung nujiang suture in tibet 1 yielded crustal velocity estimates for both p and s waves avp vs ratio of 1 65 was derived fo waves propagating in the upper 50 c km near the surface a shallow low velocity zone is expected to form in a granitic upper crust for both p and s waves a two dimensional ray tracing study indicates that the observed first p and s waves 1 can be interpreted as diving waves above a low velocity zone where a positive gradient exists the low velocity zone commences at a depth of 10 km and extends possibly to 20 km or deeper

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

magnetotelluric imaging beneath the taiwan orogen an arc continent collision 1 the taiwan orogen has formed since the late miocene by oblique collision between the luzon volcanic arc on the philippine sea plate and the eurasian continental margin this oblique collision has produced an orogen that decreases in age from north to south and permits study of the temporal evolution of an arc continent collision these factors make taiwan a favorable location to study the process of arc continent collision the first long period magnetotelluric mt measurements were recorded in taiwan as part of the taiwan integrated geodynamics research taiger project in 2006 7 measurements were made at 82 sites on three transects across south central and north taiwan that span the breadth of the orogen and cross all major tectonic boundaries robust remote reference processing of the mt time series data resulted in high quality soundings that were modeled in both 2 and 3 dimensions these mt models support predictions of lithospheric deformation i e thick skinned tectonics beneath the central ranges in south and central taiwan but are inconsistent with predictions of orogen scale thin skinned models the mt resistivity model for northern taiwan is consistent with dewatering of the subducting philippine slab and with deformation described by the subducting indenter tectonic model modeling the taiger mt data has definitively shown that conductive and seismically active crustal structures exist to 30 km beneath the orogen these conductive regions interpreted as interconnected fluid map pervasive zones of collisional deformation that are lithospheric in scale

upper mantle anisotropy in the new zealand region shear wave splitting parameters of fast polarization direction and delay time t are determined using data from the southern alps passive seismic experiment sapse on the south island of new zealand and in the surrounding region our results clearly show that are subparallel to trends of the alpine and marlborough faults and to the pacific australian plate boundary the t values range from 0 6 2 2 s with an average value of 1 6 s the largest values are from the central south island the main source of the observed shear wave splitting is an anisotropic region between 40 400 km the width of the zone is approximately 200 km we attribute the coincidence of surface structural trends with the measured and the large t values to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle

active detachment of taiwan illuminated by small earthquakes and its control of first order topography we use 110 000 small earthquakes to locate and map active faults in three dimensions within the taiwan arc continent collision the structure is dominated by a nearly horizontal band of small earthquakes at 10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt with other illuminated fault zones abutting the detachment zone the zone steepens below eastern taiwan to 30 90 and reaches depths of 30 60 km the three dimensional shape of the detachment zone in relation to topography allows a new test of critical taper wedge mechanics and suggests that the reversal of topographic slope across taiwan is controlled by the shape of the detachment

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

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

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

from accretion to collision motion and evolution of the chaochou fault southern taiwan 1 the chaochou fault ccf is an important tectonic boundary of the taiwan orogenic belt between the metamorphosed slate belt to the east and the western foothills foreland fold and thrust belt to the west although the fault is known to be a high angle oblique sinistral thrust fault in places both its kinematic history and its current role in the development of the orogen are poorly understood field fabric data suggest that hanging wall structural orientations vary along strike particularly in the on land extension of the intersection of the eurasian continent ocean boundary and the luzon arc slip lineations also reveal a change in fault motion from dip parallel in the north to a more variable pattern in the south this correlates somewhat with recent gps results that indicate that the direction of current horizontal surface motion changes along strike from nearly perpendicular to the fault in the northern to oblique and nearly parallel to the fault in the south mountain front sinuosity and the valley floor width valley height ratio indicate higher activity and uplift in the north geodetic and geomorphic data together indicate that along the northern segment of the ccf the slate belt hanging wall is currently undergoing rapid uplift related to oblique arc continent collision between the eurasian continent and the luzon arc the southern segment of the fault is significantly less active perhaps because 1 the fault is blind or 2 this portion of the fault is south of the point where the eurasian continental platform intersects the orogenic front

crustal structure across the brunswick magnetic anomaly offshore georgia from coincident ocean bottom and multi channel seismic data we present results from a coincident wide angle and deep penetrating multichannel reflection seismic experiment conducted on the continental shelf of the southeast georgia embayment over 5000 air gun shots were recorded by a 6 km long towed streamer and six ocean bottom seismic instruments along a 250 km north south profile crossing the brunswick magnetic anomaly bma these data indicate a transition in seismic properties across the bma including higher seismic velocities south of the bma particularly in the upper crust 7 15 km depth and a transition from reflective to transparent crust from north to south we interpret this transition to indicate an increased mafic content in crust south of the bma magnetic modeling based on our seismic results indicates that the bma may be explained as an edge effect anomaly of the more mafic upper crust we suggest that the increased mafic content toward the south is due to rift related mafic intrusion an alternative interpretation is discussed in which the seismic transition across the bma is related to an alleghanian suture athough there is no direct indication in the data for the association of the bma and an alleghanian suture along our profile such an association cannot be ruled out the southern portion of the profile crosses the margin hinge zone where we observe seaward dipping reflectors similar to those observed along the northern blake plateau basin and carolina trough hinge zones which have been interpreted as volcanic sequences our seismic data include no evidence for very high velocity 7 0 km s mid crust to lower crust inboard of the hinge zone such as is observed in the carolina trough outboard of the hinge zone this indicates that the volume of emplaced mafic material changes dramatically across the hinge zone and suggests that rift related magmatic processes along the margin were highly focused

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

does extrusion occur at both tips of the taiwan collision belt insights from active deformation studies in the ilan plain and pingtung plain regions abstract we analyse the present day deformation in two key areas of the taiwan collision belt the ilan plain to the nw and the pingtung plain to the sw our approach is mainly based on consideration of horizontal displacement revealed by recent geodetic gps surveys derived strain rate tensors indicating horizontal deformation and three dimensional seismotectonic stress regimes issued from inversion of focal mechanisms of earthquakes we reconstruct a consistent albeit complex tectonic pattern involving non rigid rotations clockwise in ne taiwan anticlockwise in sw taiwan transitions from pure compression near the mountains to transtension near the sea and simple shear affecting the deforming domain nw se left lateral in ne taiwan nne ssw right lateral in sw taiwan these tectonic patterns reflect lateral extrusion processes towards mechanically weak domains with respect to collision zone i e adjacent subduction zones ryukyu to the ne manila to the sw the extrusion of the ilan plain area occurs towards the se whereas that of the pingtung plain area occurs towards the sw the extrusion in ne taiwan is facilitated by the opening of the okinawa trough so that velocity and deformation rates are higher than in sw taiwan despite less active collision in this northern area the symmetry of the extrusion patterns is altered by differences in relative positions and orientations of collision belt and adjacent subduction zones which depend on the plate tectonic configuration and tectonic history of the taiwan region

modern vertical deformation rates and mountain building in taiwan from precise leveling and continuous gps observations 2000 2008 1 to characterize the present day vertical displacement field in the active taiwan orogenic belt 1843 precise leveling and 199 continuous gps measurements from 2000 to 2008 are collected and analyzed in this study vertical velocities derived from the leveling data are placed in a reference frame of the chinese continental margin using continuous gps observations at nearby sites the leveling and gps vertical velocities generally reveal a dome shaped pattern with uplift of 0 2 18 5 mm yr in the interior of the mountain range and subsidence on the flanks of the mountains and coastal plains modern uplift rates in the active fold and thrust belt are generally consistent with geologic uplift rates however present day uplift rates in the central range are faster than the million years averaged exhumation rates the modern subsidence rates are generally consistent with geologic rates except for the rates in western coastal areas due to groundwater pumping present day subsidence in the southern central range and northern coastal range is however inconsistent with long term uplift which may reflect interseismic elastic strain accumulation across faults present day subsidence in northern taiwan occurs in a region of postcollisional orogenic collapse we model the present day and geologic vertical velocities and published gps horizontal velocity data across southern taiwan using a 2 d lithospheric model the model suggests a combined slip rate of 40 mm yr on the frontal thrusts and 45 mm yr on the longitudinal valley fault the model requires an additional source of crustal thickening under the central range to match the observed present day uplift rates

anomalous pn waves observed in eastern taiwan implications of a thin crust and elevated oceanic upper mantle beneath the active collision zone suture normal pn waves are commonly observed in taiwan from shallow regional earthquakes at epicentral distances larger than 120 km similar to the observations in many other continental regions however the critical distances to observe pn waves for shallow eastern taiwan earthquakes vary with azimuth corresponding to a significant variation of crustal thickness in particular anomalous pn waves are commonly observed for shallow eastern taiwan earthquakes recorded on seismic stations at epicentral distances as small as 60 km along the collision zone suture the longitudinal valley for the same event normal pn waves are observed at other seismic stations elsewhere on the island the apparent velocity of the anomalous and normal pn waves from the same event is 7 8 0 15 km sec which is consistent with the average pn velocity in the taiwan area thus the unusually short critical distance for pn waves in eastern taiwan suggests that the crust beneath the collision zone suture must be very thin and the upper mantle beneath the longitudinal valley must be relatively elevated compared with that beneath the other parts of taiwan assuming a simple 1d layered velocity model the moho depth beneath the suture zone can thus be estimated at 23 2 km this observation is consistent with the recent report from a high resolution 3d tomographic inversion that a narrowly confined anomalously elevated and north northeast south southwest elongated oceanic upper mantle was imaged beneath the longitudinal valley from hualien in the north to taitung in the south kim et al 2005 2006 furthermore the preceding observations may also support the interpretation that the conduction of excess heat supply from the elevated hot oceanic upper mantle into the adjacent mid to lower continental crust over a long period of geological time may play an important role in the crustal deformation beneath the continent including metamorphism thickening and uplifting

refining the image of the san andreas fault near parkfield california using a finite difference travel time computation technique abstract the parkfield area seismic observatory paso was a dense telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the san andreas fault observatory at depth safod drill site the main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for safod drilling and in the process develop a high for passive seismological techniques resolution image of the fault zone structure a challenging aspect of the analysis of this data set was the known existence of large 20 25 contrasts in seismic wavespeed across the san andreas fault the resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques in order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer thurber c roecker s roberts k gold m powell m l and rittger k 2003 earthquake locations and three dimensional fault zone structure along the creeping section of the san andreas fault near parkfield ca preparing for safod geophys res lett 30 3 10 1029 2002gl016004 with one derived from a relatively fine grid and an application of a finite difference algorithm hole j a and zelt b c 1995 3 d finite difference reflection traveltimes geophys j int 121 2 427 434 in both cases we inverted arrival time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three dimensional vp and vp vs structure included are data from an active source seismic experiment around the safod site as well as from a vertical array of geophones installed in the 2 km deep safod pilot hole drilled in summer 2002 our results show that the main features of the original analysis are robust hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same at the same time we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated and our estimate of the depth of the main part of the seismogenic zone decreases in places by 100 m tests using virtual earthquakes borehole receiver gathers of picks for surface shots indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

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

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

rates of cooling and denudation of the early penglai orogeny taiwan as assessed by fission track constraints abstract as an attempt to define the timing of the peak temperature of the penglai orogeny and estimate the early cooling and denudation rates of the taiwan mountain belt fission track dating of zircon and apatite was conducted on several key metamorphic rock samples the zircon fission track ages for the metasandstone clasts collected from the central and northern parts of the coastal range were determined as 4 0 0 5 and 3 6 0 3 ma respectively both of the clasts were identified as from the ancient central range which was metamorphosed at temperatures high enough to cause a complete reset of the zircon fission track system during 1 2 ma they were exhumed and deposited in the coastal range basin due to the later penglai orogeny of taiwan obviously they have not further been annealed since their deposition in the coastal range the difference between the above fission track ages and the stratigraphical age of the host sedimentary formation represent the ancient cooling ages when they were exposed on the early central range these ancient cooling ages are comparable with the zircon fission track age of a present day outcrop of the tananao schist 1 8 0 2 ma this accordance implies that at ca 5 ma the northern and central parts of the central range achieved the peak temperature of the penglai orogeny and then they began to emerge above sea level accepting this scenario we calculate the rates of denudation and cooling of the central range to be ca 2 5 4 6 mm yr 1 and 120 c m y 1 respectively for the last 4 ma

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

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

rheology and strength of the eurasian continental lithosphere in the foreland of the taiwan collision belt constraints from seismicity flexure and structural styles 1 deformation in western taiwan is characterized by variable depth frequency distribution of crustal earthquakes which are closely connected with along strike variations of tectonic styles thin or thick skinned around the peikang high a major inherited feature of the chinese margin to fit the calculated high crustal geotherm and the observed distribution of the crustal seismic activity a qz diorite and granulite composition for the upper and the lower crust is proposed we then model the plate flexure through te estimates using brittle elastic ductile plate rheology flexure modeling shows that the best fit combination of te boundary condition is for thrust loads acting at the belt front the calculated te vary in the range of 15 20 km these values are primarily a reflection of the thermal state of the rifted chinese margin inherited from the oligocene spreading in the south china sea however other mechanical properties such as the degree of crust mantle coupling and the thickness of the mechanically competent crust and mantle are considered south of the peikang high flexure modeling reveals lower te associated with thinner mechanically strong layers variable stress strain distribution associated with a higher degree of crust mantle decoupling is examined to explain plate weakening we first show that plate curvature cannot easily explain strength reduction and observed seismic activity additional plate boundary forces arising from the strong coupling induced by more frontal subduction of a buoyant crustal asperity i e the peikang high with the overriding plate are required favorably oriented inherited features in the adjacent tainan basin produce acceleration of strain rates in the upper crust and hence facilitate the crust mantle decoupling as attested by high seismic activity and thick skinned deformation the relative weakening of the lower crust and mantle then leads to weaken the lithosphere by contrast to the north more oblique collision and the lack of inherited features keep the lithosphere stronger this study suggests that when the eurasian plate enters the taiwan collision tectonic inheritance of the continental margin exerts a strong control on the plate deformation by modifying its strength

two station measurement of rayleigh wave phase velocities for the huatung basin the westernmost philippine sea with obs implications for regional tectonics summary a broad band ocean bottom seismometer obs deployed 180 km east of taiwan provides a first glimpse into the upper mantle beneath the westernmost section of the philippine sea or the huatung basin hb we measured interstation phase velocities of rayleigh waves between the obs and stations on the eastern coast of taiwan the phase velocities show smooth variations from 3 8 to 3 9 km s 1 for periods of 25 40 s in this short period range phase velocities are comparable to those characterizing the 15 30 ma parece vela basin of the philippine sea modelling of the finite frequency effect proves the validity of the measurement for the average hb the shear wave velocity models inverted from the 25 to 40 s dispersion show a velocity at lithospheric depths about 0 1 km s 1 lower than that of the west philippine sea which agrees with the age effect derived from the pacific pure path model inversions incorporating the less reliable data above 40 s yield a shear velocity 4 0 km s 1 below 150 km an unrealistic value even for a hotspot plume environment the seismological evidence together with the correlation in seafloor depth suggests that the hb and the parece vela basin may have a similar age this is at odds with the previous geochronological study suggesting an early cretaceous age for the hb thermal rejuvenation of the lithosphere was examined as a potential solution to reconciling the two age models

crustal and upper mantle s wave velocity structures across the taiwan strait from ambient seismic noise and teleseismic rayleigh wave analyses abstract although orogeny tapers off in western taiwan large and small earthquakes do occur in the taiwan strait a region largely untouched in previous studies owing mostly to logistical reasons but the overall crustal structure of this region is of particular interest as it may provide a hint of the proto taiwan before the orogeny by combining time domain empirical green s function tdegf from ambient seismic noise using station pairs and traditional surface wave two station method ts we are able to construct rayleigh wave phase velocity dispersion curves between 5 and 120 s using broadband array in taiwan for seismology bats stations in taiwan and in and across the strait we are able to derive average 1 d vs structures in different parts of this region the results show significant shear velocity differences in the upper 15 km crust as expected in general the highest vs in the upper crust observed in the coastal area of mainland china and the lowest vs appears along the southwest offshore of the taiwan island they differ by about 0 6 1 1 km s for different parts of the strait the upper crust vs structures are lower in the middle by about 0 1 0 2 km s relative to those in the northern and southern parts the upper mantle vs structure moho 150 km beneath the taiwan strait is about 0 1 0 3 km s lower than the ak135 model the overall crustal thickness is approximately 30 km much thinner and less variable than under the taiwan island the inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000 m should take water layer into consideration except for the continental shelves

source complexity of the 4 march 2010 jiashian taiwan earthquake determined by joint inversion of teleseismic and near field data abstract the jiashian earthquake m l 6 4 occurred on 4 march 2010 it was the largest inland event in southern taiwan of 2010 the mainshock location was unexpected since it occurred in an area with relatively low background seismicity in addition reports of earthquake focal mechanisms do not fit with any known active fault geometry in order to understand the origin of this earthquake especially its rupture process we perform a joint source inversion by using teleseismic body wave gps coseismic displacements and near field ground motion data in this study we considered a northwest southeast trending fault with a northeast dip retrieved from gps coseismic data and aftershocks distribution to analyze the detailed slip distribution in space and time we used near field 3d green s functions provided by spectral element method and a full time space inversion technique we find a complex rupture process with several slip patches distributed inside two main asperities the slip map reveals a mean slip of 12 9 cm for a maximum slip of 27 3 cm leading to a m w 6 47 for this event the rupture initiates in the deepest portion of the fault at 20 km depth and propagated upward up to 2 km depth to form the two asperities the source time function of this event revealed two pulses corresponding to the two asperities for a total duration time of about 16 s most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one we infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake

extensional collapse of the northern taiwan mountain belt as an active collision zone between the luzon arc and the china continental margin the taiwan mountain belt particularly its south central part is undergoing crustal shortening and rapid uplift the northern part of the orogen is however subjected to crustal stretching and rifting as a result of flipping of subduction from the northwest facing luzon arc system to the south facing ryukyu arc system taiwan and its neighboring southern ryukyu used to be part of the rifted china continental margin before the collision started in late miocene time engineered by the northwest movement of the philippine sea plate in the past 5 m y the north south trending luzon arc obliquely overrode the northeast southwest trending continental margin and tectonized it into a collision orogen that progressively enlarged and migrated from northeast to southwest following the southwest propagating collision the north dipping philippine sea plate which was subducting beneath the ryukyu arc also extended westward and caused flipping of subduction in the northern part of the collision orogen in consequence the orogen lost the compressive support by the colliding plates and became subjected to lithospheric stretching induced by the trench suction the collision orogen collapsed as a result of crustal thinning and its central part foundered into a deep submerged basin a whole spectrum of active tectonic scenarios ranging from orogenic buildup by collision to orogenic collapse by rifting and subsidence can be readily observed in the taiwan ryukyu area and provides a vivid example for the process of orogenic collapse

split s waveforms observed in northern taiwan implications for crustal anisotropy short period stations ent and nsk in northern taiwan have frequently recorded split s waveforms from earthquakes in the underlying subduction zone to determine splitting parameters together with objective straightforward error estimations we employ a waveform cross correlation method the error for each measurement is estimated by translating the lower limit of the 95 confidence interval of the cross correlation coefficient into azimuth and time only measurements with 95 confidence regions excluding zero delay time are accepted as splitting data we also use an energy minimization method to constrain the solutions with minor splitting we interpret the splitting as caused by stress aligned cracks in the upper crust the average delay times are 0 03 0 04 s for ent and 0 07 s for nsk the fast directions at the two stations are predominantly n s which roughly match the stress trajectories predicted by a model simulating the arc continent collision in the taiwan area however at ent there is a subset of data with fast directions oriented in roughly e w the nearly 90 rotation of the inferred crack orientation occurs in a horizontal distance of 10 15 km in the upper crust both the predicted heterogeneous stress regime and the local structural setting in northeastern taiwan favor a lateral variation with a similar length scale

the basement versus the no basement hypotheses for folding within the appalachian plateau detachment sheet within the appalachian plateau detachment sheet of southwestern pennsylvania tectonic thickening is a central element to the growth of anticlines in a silurian devonian section containing a three tiered mechanical stratigraphy that is a basal detachment zone a lower imbrication zone and an upper wedge zone the detachment zone is predominantly within disturbed shale of the silurian vernon formation which sits above a disturbed surface on the lockport dolomite large scale anticline growth is in part a consequence of small scale blind imbrication of the more competent mechanical layers in the lower portion of the detachment sheet the imbrication zone here salt within the silurian syracuse formation hosts secondary detachment responsible for imbrication and the development of triangle zones in the core of the anticlines some fold amplification is also accomplished by extensive smaller scale thrust wedging and concomitant tectonic thickening of the less competent devonian section in the upper portion of the detachment sheet the wedge zone these spatially periodic anticlines are separated by horizontally bedded synclines characterized by the absence of thrust wedging and fault imbrication across all three zones tectonic thickening continued behind the foreland propagation of the detachment tip line within the vernon shale as indicated by a systematic hinterland increase in the cross sectional width and amplitude of the anticlines on the basis of seismic images each detachment sheet anticline is situated above prominent periodically spaced pre alleghanian structures in the footwall of the detachment sheet these footwall structures arise from a combination of thrust imbrication at the depth of the ordovician trenton group and high angle basement involved faulting at the depth of the cambrian gatesburg formation some of the high angle fault displacement is a consequence of alleghanian inversion on faults associated with the extensional rome trough and other basement structures developed during the late proterozoic rifting of the eastern margin of laurentia evidence for tectonic inversion is present in seismic reflection images that show buttress anticlines in the cambro ordovician carbonate section the superposition of detachment sheet anticlines above footwall structures strongly suggests that the foreland transport was disrupted by topographic bumps on the base of the detachment zone bending of the detachment sheet over these irregularities may have promoted the spatially periodic collapse and concomitant tectonic thickening in the over riding sheet as it was pushed laterally toward the foreland a strain hardening of the detachment zone part of the detachment sheet at the topographic irregularities permitted the growth of double sided tapered wedges that is each anticline the overall structural profile of the detachment sheet is consistent with the growth of a coulomb wedge whose low basal friction is interrupted by patches of strain hardening

shear wave anisotropy beneath the taiwan orogen abstract shear wave splitting in regional and teleseismic shear waves scs and s has been analyzed for fast polarization directions and delay times at 33 short period and 17 broad band permanent and temporary seismic stations in the taiwan region the orientations of anisotropy for the taiwan stations are generally parallel to the strike of the mountain belt with a clockwise rotation following the trend of regional geologic fabric in north taiwan the delay times on the other hand vary greatly from 0 5 to 2 1 s the largest values are from the southwestern taiwan foothills and an abrupt change in delay times is found in south taiwan the main source of shear wave splitting in taiwan is compatible with an anisotropic region between 25 and 230 km depths the observed splitting parameters are consistent with upper mantle anisotropy resulting from the collisional tectonics that built the taiwan orogen

precise temperature estimation in the tibetan crust from seismic detection of the α β quartz transition in the deep crust temperature which is among the key pa rameters controlling lithospheric dynamics is inferred by extrap olation from the surface using several assumptions that may fail in regions of active tectonics and fluid migration in the rare case that temperatures of 700 8c or higher are exceeded in the upper and middle continental crust composed of quartz rich felsic rocks the a b quartz transition abqt will occur generating a mea surable seismic signature and offering the possibility for precisely estimating temperature from the known abqt phase diagram here it is shown that all expected seismic features of the abqt are met by the boundary between the upper and middle crust be low the indepth iii profile in central tibet this finding implies that a temperature of 700 8c is achieved at a depth of 18 km under the southern qiangtang block which agrees with the depth to the top of a high electrical conductivity anomaly likely representing partially melted crust to the south in the northern lhasa block the abqt is at 32 km depth corresponding to a temperature of 800 8c it thus appears that this seismic boundary representing the abqt is the result of recent geologic processes rather than being a lithologic boundary

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

probing the structure of the deep continental crust old buried deformed crystalline rocks apparently make up most of the 40 kilometer thick continental crust this part of the earth is poorly explored and constitutes a major frontier of modern earth science two techniques seismic reflection profiling and drilling which were developed by industry for other purposes offer special potential for such exploration seismic profiling of the deep crust by cocorp the consortium for continental reflection profiling has already produced important information including evidence for extensive thin skinned thrusting of older rocks over a continental margin as the corresponding ocean basin closed deep drilling of crystalline rocks of the continents for scientific purposes is so far relatively unexploited in the united states but is already being carried on elsewhere in general big science is likely to become more important in basic geology as this frontier is explored

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

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