observation and scaling of microearthquakes from the taiwan chelungpu fault borehole seismometers summary microearthquakes with magnitude down to 0 3 were detected by the taiwan chelungpu fault drillingproject borehole seismometers tcdpbhs despitethelargecoseismic slipof12m atthedrillsiteduringthe1999chi chiearthquake ourstudiesshowverylittleseismicitynear the tcdpbhs drill site 6 yr after the chi chi main shock the microearthquakes clustered at a depth of 9 12 km where the chelungpu thrust fault turns from a 30 dipping into the horizontal decollement of the taiwan fold and thrust tectonic structure continuous gps surveys did not observe post slip deformation at the larger slip region and no seismicity was observed near the drill site therefore we suggest that the thrust belt above the decollement is locked during this interseismic period we further investigated source parameters of 242 microearthquakes by fitting 2 shaped brune source spectra to our observation data using a frequency independent q model we find that the static stress drop increases significantly with increasing seismic moment however due to the intense debate on this topic of scalingrelations and the related self similarity of earthquakes we further improve the data analysis and correct for path and site effects using the projected landweber deconvolution pld method for events within some clusters the pld method analyses the source time functions of the larger and the smaller event by an iterative technique as a result we received source dimensions and stress drops of larger events including path and site effect corrections the results from the pld method are less scattered and also show a positive relation between static stress drop and seismic moment we find a similar positive trend for the apparent stress scaling with seismic moment

vp vs distribution in the northern taiwan area implications for the tectonic structures and rock property variations abstract the ratio of compressional wave velocity vp to shear wave velocity vs is often indicative of lithology and rock conditions in this study we determine the vp vs characteristics for the different tectonic features in northern taiwan and quantify the crack density and saturation by using the ob74 method the result shows that the highest vp vs values are located beneath the western foothills at depths between 4 and 21 km these vp vs highs are simultaneously characterized by low vp and low vs suggesting the presence of fluids the average crack density and saturation for each depth can reach as high as 25 and 91 at depths of 7 and 11 km respectively this distribution could be caused by the presence of less compacted sediments within the foreland basin and the progressive convergence of crustal faults with depth as the shallow portion of the foreland basin is highly fractured fluids could infiltrate into deeper parts to create a highly saturated environment however when the depth is deeper than 11 km the high crack density and saturation area migrate from the west to the east of the shanchiao fault this observation may indicate the presence of a preexisting syn rift basin buried at depth during the collision in the ilan plain the highest crack density and saturation are observed in its northeastern portion at a depth of 4 km with values of 17 1 and 60 3 respectively however slightly high saturation values spread throughout the plain at a depth of 15 km which may indicate a possible fluid conduit from a deep hot rock body to the surface and could be applied toward studies on geothermal fluid activity

evidence for non self similarity of microearthquakes recorded at a taiwan borehole seismometer array we investigate the relationship between seismic moment m 0 and source duration t w of microearthquakes by using high quality seismic data recorded with a vertical borehole array installed in central taiwan we apply a waveform cross correlation method to the three component records and identify several event clusters with high waveform similarity with event magnitudes ranging from 0 3 to 2 0 three clusters clusters a b and c contain 11 8 and 6 events with similar waveforms respectively to determine how m 0 scales with t w we remove path effects by using a path averaged q the results indicate a nearly constant t w for events within each cluster regardless of m 0 with mean values of t w being 0 058 0 056 and 0 034 s for clusters a b and c respectively constant t w independent of m 0 violates the commonly used scaling relation t w m 1 3 0 this constant duration may arise either because all events in a cluster are hosted on the same isolated seismogenic patch or because the events are driven by external factors of constant duration such as fluid injections into the fault zone it may also be related to the earthquake nucleation size

an interpretation of the active deformation of southern taiwan based on numerical simulation and gps studies abstract the fold and thrust belt of taiwan results from oblique convergence between the eurasia and the philippine sea plate and its front structures are strongly influenced by the presence of large horsts and basins in the foreland using a 2 d numerical modelling finite element and distinct element methods we evaluate the influences on the active deformation of southern taiwan of 1 the foreland structural highs 2 the major fault zones in the belt and 3 the presence of a subduction zone to the south to constrain our models we take into account for the first time the present day velocity field of southern taiwan estimated based on new geodetic data obtained through the global positioning system gps particular attention is paid to the role of geological discontinuities through an evaluation of the presence and role of mechanical decoupling along major faults which plays an important role in the distribution of the regional and local velocity and stress patterns this particular analysis of the behaviour and influence of weak shear zones in taiwan is carried out by using for the first time the distinct element method additional 3 d distinct element modelling allows better consideration of oblique shearing such as for the longitudinal valley fault of eastern taiwan we conclude that the active velocity field and tectonic stress pattern in southwestern taiwan strongly depend on 1 the presence and shape of the peikang high 2 the presence of the major active regional discontinuities the longitudinal valley fault and the major thrusts of western taiwan and 3 the neighbouring weakness zone of the accretionary prism of the northern manila subduction zone and cannot be explained by any of these factors taken solely

lithospheric stress states predicted from long term tectonic models influence of rheology and possible application to taiwan abstract the long term stress state of the lithosphere results from a combination of boundary forces surface loads and tractions from mantle flow here we have employed a numerical technique which allows for the self consistent treatment of the interaction between the lithosphere and mantle and takes into account the deformation of the free surface we have used this to study lithospheric dynamics in order to gain insights on the present day conditions of stress and deformation of the lithosphere our numerical approach takes the observed topography and far field plate velocities as input and computes the resultant deformation as a function of the rheological stratification our model application is inspired by taiwan tectonics stress data and earthquake focal mechanisms indicate that the upper crust in taiwan may be predominantly under compression the recent pingtung earthquake in 12 26 2006 however suggests that the sub moho mantle lithosphere under southern taiwan may be under extension if this earthquake indeed represents the local state of stress this raises the question as to why extension and compression can occur simultaneously in this region for addressing this issue we have performed a number of numerical simulations in which we studied the effects of rheological stratification boundary conditions and geometry on the stress state and lithospheric dynamics we have employed both layered viscosity models and models in which we use laboratory based non newtonian and temperature dependent creep laws results show that the rheological stratification plays an important role in lithospheric deformation the relatively hot lithospheric thermal state in taiwan makes it likely that part of the lower crust is relatively weak which results in decoupling from surface near deformation with deformation at depth bending of a subducting slab underneath taiwan causes an extensional stress state in the sub moho mantle lithosphere whereas the collision of the philippine sea plate with the eurasian margin is responsible for a compressional state of stress in the crust it is demonstrated that models with a weak lower crust are also in reasonable agreement with available gps data as well as with constraints on uplift rates from long term geodynamic modelling studies

mountain building in taiwan a thermokinematic model the taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening however a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge we propose here a new thermokinematic model of the taiwan mountain belt reconciling existing kinematic thermometric and thermochronological constraints in this model shortening across the orogen is absorbed by slip on the most frontal faults of the foothills crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge tananao complex tc where the uplift rate is estimated to 6 3 mm a 1 and beneath the westernmost internal region of the orogen hsueshan range units hr where the uplift rate is estimated to 4 2 mm a 1 our model suggests that the tc units experienced a synchronous evolution along strike despite the southward propagation of the collision it also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures exhumation of the hr units increases northward but has not yet reached an exhumational steady state presently frontal accretion accounts for less than 10 of the incoming flux of material into the orogen although there is indication that it was contributing substantially more 80 before 4 ma the incoming flux of material accreted beneath the tc significantly increased 1 5 ma ago our results also suggest that the flux of material accreted to the orogen corresponds to the top 7 km of the upper crust of the underthrust chinese margin this indicates that a significant amount 76 of the underthrust material has been subducted into the mantle probably because of the increase in density associated with metamorphism we also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field by combining available geological data on the thermal and kinematic evolution of the wedge our study sheds new light onto mountain building processes in taiwan and allows for reappraising the initial structural architecture of the passive margin

a seismic structure study in the kaoping area southwestern taiwan the difference between s wave and s to p wave conversion s p phase arrival times is enhanced with rectilinear motion detector filtering to describe alluvial sediment thickness in the kaohsiung pingtung kaoping plains area a more complete understanding of the underground structures of the kaoping area is provided in this paper and explains why the surrounding regions in taiwan experience more earthquakes than the kaoping area data are based on seismic activity recorded by the portable array for numerical data acquisition panda for the period from 1995 to 1997 the difference between s wave and s p phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts p wave travel time residuals high frequency attenuation parameters kappa and quality factor q p q s and coda waves confirm this result we also determined the orientation of the chaochou fault using the first motion of p wave arrivals to the east of the chaochou fault stress trends southeast northwest while to the west it trends northeast southwest the change in stress trends east and west of chaochou fault suggests the presence of a highly fluid accretionary wedge in the kaoping area the chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the kaoping plains we demonstrate that these features are the reason there are relatively few earthquakes in the kaoping area the presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest kaoping coasts the peikung high acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region online material figures illustrating calculations of kappa q c p and s wave spectra q p and q s from ground motion data

three dimensional seismic velocity structure of the san francisco bay area seismic travel times from the northern california earthquake catalogue and from the 1991 bay area seismic imaging experiment basix refraction survey were used to obtain a three dimensional model of the seismic velocity structure of the san francisco bay area nonlinear tomography was used to simultaneously invert for both velocity and hypocenters the new hypocenter inversion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm numerous inversions were performed with different parameters to test the reliability of the resulting velocity model most hypocenters were relocated 12 km under the sacramento river delta 6 km beneath livermore valley 5 km beneath the santa clara valley and 4 km beneath eastern san pablo bay the great valley sequence east of san francisco bay is 4 6 km thick a relatively high velocity body exists in the upper 10 km beneath the sonoma volcanic field but no evidence for a large intrusion or magma chamber exists in the crust under the geysers or the clear lake volcanic center lateral velocity contrasts indicate that the major strike slip faults extend sub vertically beneath their surface locations through most of the crust strong lateral velocity contrasts of 0 3 0 6 km s are observed across the san andreas fault in the middle crust and across the hayward rogers creek calaveras and greenville faults at shallow depth weaker velocity contrasts 0 1 0 3 km s exist across the san andreas hayward and rogers creek faults at all other depths low spatial resolution evidence in the lower crust suggests that the top of high velocity mafic rocks gets deeper from west to east and may be offset under the major faults the data suggest that the major strike slip faults extend sub vertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike slip motion the extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity strong ground motion and regional stress

basin inversion in central taiwan and its importance for seismic hazard proyectos intramurales 2006 301010 ministerio de ciencia e innovacion cgl2009 11843 bte and the csic predoctoral program junta para la ampliacion de estudios jae predoc

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

a study of the seismogenic process beneath the south western foothills in taiwan in relation with the 2010 jiasian earthquake abstract a moderate earthquake occurred near the town of jiasian in kaohsiung county taiwan at 8 18 am local time on 4 march 2010 the epicenter was located about 60 km northeast of kaohsiung city about 70 aftershocks occurred during the same day in this study we invert a 3 d velocity structure beneath the southwestern part of taiwan furthermore we accurately determine the locations the focal mechanisms and the regional stress tensor from the inverted 3 d velocity model according to the results we can conclude 1 a low vp anomaly zone with a high vp vs ratio in the southwestern part of the research area may have been caused by the subducting oceanic lithosphere 2 a high vp anomaly zone with low vp vs ratio in the northeastern part of the research area demonstrates that there is an intervening continental sliver 3 based upon the inverted 3 d velocity model and the relocated seismicity we suppose that there might be a nw se trending blind fault which dips to the north 4 according to the results of the focal mechanisms of the jiasian earthquake sequence and the regional stress tensor inversion we can surmise that the possible blind fault may be a thrust fault with left lateral characteristic and 5 based upon the different tectonic process in this area we can conclude that the chishan transfer fault zone leads to a possible oblique slip fault locks and exhibit high seismic potential which caused the jiasian earthquake sequence

high resolution 3d p wave velocity model for the east ventura san fernando basin california and relocation of events in the northridge and san fernando aftershock sequences we determined a high resolution 3d p wave velocity model for an 80 km 80 km area around the san fernando valley extending 22 km below sea level we used events recorded from 1981 to 2000 as well as 799 aftershocks of the 1971 san fernando earthquake recorded by a portable network the total number of events and stations used were 13 455 and 101 respectively most of the events are aftershocks of the 1994 northridge earthquake the inversion software includes a raytracing subroutine that can handle the sharp lateral velocity variations that exist in the area the resulting model shows an overall good agreement with the southern california earthquake center scec 3d velocity model but images the deeper structure of the san fernando valley in more detail in addition our results are supported by the inversion of synthetic data and by the remarkable agreement with a density model derived from gravity data along a 55 km long profile an approximately 10 km wide and sharply defined zone of northridge aftershocks occurred in basement rocks between about 8 and 20 km depth and identifies the fault that slipped during the mainshock horizontal slices at deep 8 18 km levels show that the northridge earthquake occurred within a high velocity basement block that may have controlled the size of the earthquake the fault plane inferred from the seismicity is in good agreement with the geodetic fault plane determined by hudnut et al 1996 assuming uniform slip most of the aftershocks of the northridge earthquake occurred within the sedimentary rocks of the san fernando basin with a large concentration of events located to the east of the eastern boundary of the mainshock fault plane the latter events form a zone that dips about 45 to the southwest at depths less than about 14 km these may represent a lateral ramp this zone terminates at the steeply inclined eastern edge of the san fernando basin these events do not lie on the mainshock fault plane and because the san fernando aftershocks also lie east of the northridge fault plane the san fernando and the northridge mainshocks must have occurred on conjugate en echelon faults i e they do not abut each other in addition an 8 km wide cross section along the density model referred to previously suggests the northridge aftershocks east of the mainshock and most of the aftershocks of the san fernando earthquake illuminate two thrust faults that have a common high velocity basement block in their footwalls the velocity model produced by our analysis suggests that thrust faults formed by modern contraction in this region are strongly influenced by prior basin and basement block geometry online material 3d p wave velocity models without seismicity

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

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

fault geometries illuminated from seismicity in central taiwan implications for crustal scale structural boundaries in the northern central range abstract data sets of collapsed earthquake locations earthquake focal mechanisms gps velocities and geologic data are integrated to constrain the geometry and kinematics of a crustal block within the accreted continental margin rocks of taiwan s northeastern central range this block is laterally extruding and exhuming towards the north northeast the block is bound on the west southwest by the previously recognized sanyi puli seismic zone and on the east by a vertical seismic structure that projects to the eastern mountain front of the central range focal mechanisms from the broadband array of taiwan seismicity bats catalog consistently show west side up reverse displacements for this fault zone a second vertical structure is recognized beneath the slate belt metamorphic belt boundary as a post chi chi relaxation oblique normal fault bats focal mechanisms show east side up normal displacements with a minor left lateral component the vertical and lateral extrusion of this crustal block may be driven by the current collision between the philippine sea plate and the puli basement high indenter on the eurasian plate and or trench rollback along the ryukyu subduction zone in addition the vertical extent of the two shear zones suggests that a basal decollement below the eastern central range is deeper than previously proposed and may extend below the brittle ductile transition

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

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

seismic cycle stress change in western taiwan over the last 270 years 1 the island of taiwan is affected by intense seismic activity which includes large events as the disastrous 1999 chi chi earthquake to improve seismic hazard assessment in this area we estimate the effect of both interseismic loading and major events since 1736 on the state of stress of major active faults we focus our approach on western taiwan which is the most densely populated part of taiwan we pay a specific attention to faults geometry and to both interseismic and coseismic slip distributions our results suggest that both earthquakes and interseismic loading before 1999 increase the coulomb stress in the north western part of the chelungpu fault a region which experienced the highest coseismic slip during the chi chi earthquake more importantly our results reveal a coulomb stress increase in the southern part of the changhua thrust fault below a densely populated area

3 d plate interactions in central taiwan insight from flexure and sandbox modeling abstract we have conducted flexure and sandbox experiments to investigate the 3 d plate interactions in central taiwan the abnormally low surface slope mountain belt in central taiwan fails to account for the large deflection of the underlying lithosphere we propose that the northward subduction of the philippine sea plate applies additional bending moments and shear forces to the eurasian plate at the plate suture the interactions between the eurasian plate and the philippine sea plate in central taiwan could have enhanced the deflection of the continental margin and created a deep tear fault identified in an early study we have conducted flexure and gravity modeling to confirm that additional downward shear forces are exerted on the eurasian plate along the suture zone of the two plates these forces increase northwardly with the subduction of the philippine sea plate the fact that the eurasian plate deflects with the northward bending of the philippine sea plate implies an eastward subduction of the eurasian plate in central taiwan in addition our model suggests that the bending moment changes from positive bending upward to negative bending downward southwardly along the longitudinal valley this finding implies a transition from a continental crust to an oceanic one or to a transitional crust southward along the plate suture or and delamination of a light crust during subduction of the southern part of the eurasian plate the spatial variation in bending moment along the plate suture could also result from a break off of the east leading edge of the eurasian plate near the northern end of the longitudinal valley we have also investigated surface structure by incorporating oblique deflection of the continental margin into our sandbox model unlike previous extrusion models our results show that a salient structure with low surface slope mountain ranges can develop even without the presence of a rigid basement high

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

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

lithospheric structure of taiwan from gravity modelling and sequential inversion of seismological and gravity data abstract using a bouguer anomaly map and a dense seismic data set we have performed two studies in order to improve our knowledge of the deep structure of taiwan first we model the bouguer anomaly along a profile crossing the island using simple forward modelling the modelling is 2d with the hypothesis of cylindrical symmetry second we present a joint analysis of gravity anomaly and seismic arrival time data recorded in taiwan an initial velocity model has been obtained by local earthquake tomography let of the seismological data the let velocity model was used to construct an initial 3d gravity model using a linear velocity density relationship birch s law the synthetic bouguer anomaly calculated for this model has the same shape and wavelength as the observed anomaly however some characteristics of the anomaly map are not retrieved to derive a crustal velocity density model which accounts for both types of observations we performed a sequential inversion of seismological and gravity data the variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple let moreover the sequential model explained about 80 of the observed gravity anomaly new 3d model of taiwan lithosphere is presented

toward real time regional earthquake simulation ii real time online earthquake simulation ros of taiwan earthquakes abstract we developed a real time online earthquake simulation system ros to simulate regional earthquakes in taiwan the ros uses a centroid moment tensor solution of seismic events from a real time moment tensor monitoring system rmt which provides all the point source parameters including the event origin time hypocentral location moment magnitude and focal mechanism within 2 min after the occurrence of an earthquake then all of the source parameters are automatically forwarded to the ros to perform an earthquake simulation which is based on a spectral element method sem a new island wide high resolution sem mesh model is developed for the whole taiwan in this study we have improved sem mesh quality by introducing a thin high resolution mesh layer near the surface to accommodate steep and rapidly varying topography the mesh for the shallow sedimentary basin is adjusted to reflect its complex geometry and sharp lateral velocity contrasts the grid resolution at the surface is about 545 m which is sufficient to resolve topography and tomography data for simulations accurate up to 1 0 hz the ros is also an infrastructural service making online earthquake simulation feasible users can conduct their own earthquake simulation by providing a set of source parameters through the ros webpage for visualization a shakemovie and shakemap are produced during the simulation the time needed for one event is roughly 3 min for a 70 s ground motion simulation the ros is operated online at the institute of earth sciences academia sinica http ros earth sinica edu tw our long term goal for the ros system is to contribute to public earth science outreach and to realize seismic ground motion prediction in real time

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

effects of realistic surface topography on seismic ground motion in the yangminshan region of taiwan based upon the spectral element method and lidar dtm we combine light detection and ranging lidar digital terrain model dtm data and an improved mesh implementation to investigate the effects of high resolution surface topography on seismic ground motion based upon the spectral element method in general topography increases the amplitude of shaking at mountain tops and ridges whereas valleys usually have reduced ground motion as has been observed in both records from past earthquakes and numerical simulations however the effects of realistic topography on ground motion have not often been clearly characterized in numerical simulations especially the seismic response of the true ground surface here we use lidar dtm data which provide two meter reso lution at the free surface and a spectral element method to simulate three dimensional 3d seismic wave propagation in the yangminshan region in taiwan incorporating the effects of realistic topography a smoothed topographic map is employed beneath the model surface in order to decrease mesh distortions due to steep ground surfaces numerical simulations show that seismic shaking in mountainous areas is strongly affected by topography and source frequency content the amplification of ground motion mainly occurs at the tops of hills and ridges whilst the valleys and flat topped hills experience lower levels of ground shaking interaction between small scale to pographic features and high frequency surface waves can produce unusually strong shaking we demonstrate that topographic variations can change peak ground accel eration pga values by 50 in mountainous areas and the relative change in pga between a valley and a ridge can be as high as a factor of 2 compared to a flat surface response this suggests that high resolution realistic topographic features should be taken into account in seismic hazard analysis especially for densely populated moun tainous areas

effects of topography on seismic wave propagation an example from northern taiwan topography influences ground motion and in general increases the amplitude of shaking at mountain tops and ridges whereas valleys have reduced ground motions as is observed from data recorded during and after real earthquakes and from numerical simulations however recent publications have focused mainly on the implications for ground motion in the mountainous regions themselves whereas the impact on surrounding low lying areas has received less attention here we develop a new spectral element mesh implementation to accommodate realistic topography as well as the complex shape of the taipei sedimentary basin which is located close to the central mountain range in northern taiwan spectral element numerical simulations indicate that high resolution topography can change peak ground velocity pgv values in mountainous areas by 50 compared to a half space response we further demonstrate that large scale topography can affect the propagation of seismic waves in nearby areas for example if a shallow earthquake occurs in the i lan region of taiwan the central mountain range will significantly scatter the surface waves and will in turn reduce the amplitude of ground motion in the taipei basin however as the hypocenter moves deeper topography scatters body waves which subsequently propagate as surface waves into the basin these waves continue to interact with the basin and the surrounding mountains finally resulting in complex amplification patterns in taipei city with an overall pgv increase of more than 50 for realistic subduction zone earthquake scenarios off the northeast coast of taiwan the effects of topography on ground motion in both the mountains and the taipei basin vary and depend on the rupture process the complex interactions that can occur between mountains and surrounding areas especially sedimentary basins illustrate the fact that topography should be taken into account when assessing seismic hazard

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

the deep structure of south central taiwan illuminated by seismic tomography and earthquake hypocenter data abstract the taiwan mountain belt is generally thought to develop above a through going basal thrust confined to within the sedimentary cover of the eurasian continental margin surface geology magnetotelluric earthquake hypocenter and seismic tomography data suggest however that crustal levels below this basal thrust are also currently being involved in the deformation here we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south central taiwan in this paper we define the basement as any pre eocene rifting rocks and use a p wave velocity of 5 2 km s as a reference for the interface between these rocks and their sedimentary cover we found that beneath the coastal plain and the western foothills clustering of hypocenters near the basement cover interface suggests that this interface is acting as a detachment this detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt inherited basement faults appear to determine the geometry of this detachment and their inversion in the alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them however across the shuilikeng and the chaochou faults earthquake hypocenters define steeply dipping clusters that extend to greater than 20 km depth above which higher velocity basement rocks are uplifted beneath the hsuehshan and central ranges we interpret these clusters to form a deeply penetrating east dipping ramp that joins westward with the detachment at the basement cover interface it is not possible to define a basal thrust to the east beneath the central range

three dimensional p wave velocity structure of mt etna italy the three dimensional p wave velocity structure of mt etna is determined to depths of 15 km by tomographic inversion of first arrival times from local earthquakes recorded by a network of 29 permanent and temporary seismographs results show a near vertical low velocity zone that extends from beneath the central craters to a depth of 10 km this low velocity region is coincident with a band of steeply dipping seismicity suggesting a magmatic conduit that feeds the summit eruptions the most prominent structure is an approximately 8 km diameter high velocity body located between 2 and 12 km depth below the southeast flank of the volcano this high velocity body is interpreted as a remnant mafic intrusion that is an important structural feature influencing both volcanism and east flank slope stability and faulting

three dimensional vp and vp vs structure of the upper crust in the erzincan basin eastern turkey 1 using aftershocks of the 13 march 1992 erzincan earthquake ms 6 8 three dimensional vp and vp vs velocity structure of the erzincan basin and its surroundings were modeled by local earthquake tomography travel times of body waves belonging to 1025 selected high quality events recorded by 58 temporary stations were inverted iteratively and simultaneously by using the simul2000 algorithm synthetic and resolution tests were performed to analyze the sensitivity of tomographic results and model parameterization the resulting 3 d vp and vp vs tomographic images show velocity anomalies related to geologic and tectonic structures beneath the erzincan basin while low vp anomalies down to 2 3 km depths are associated to thick neogene sediments deposited in the erzincan basin high vp anomalies indicate bed rocks and intrusive magmatic rocks beneath the mountain ranges vp vs anomalies are an important indicator of local tectonic structures and the physical mechanical and compositional variations in the rocks the low vp vs anomalies 1 7 observed at shallow depths 0 4 km track the main fault systems and thus the weakness zones in the basin the high vp vs anomalies 1 9 located between 4 and 6 km depth in the southeast of the erzincan basin may correspond to fluid saturation high pore pressure and carbonate content in the rock matrix the high vp vs ratios play an important role for increasing seismicity in this area because of the fluid saturation and the high pore pressure finally on the basis of the 3 d tomographic images basement of the erzincan basin deepens down to about 10 km depth and possesses an irregular and asymmetric shape the thick and unconsolidated sedimentary layer increases seismic risk to erzincan and other cities located on the basin as it magnifies ground motion caused by large earthquakes

shear concentration in a collision zone kinematics of the chihshang fault as revealed by outcrop scale quantification of active faulting longitudinal valley eastern taiwan abstract repeated measurements of active deformation were carried out at three sites along the active chihshang fault a segment of the longitudinal valley fault zone of eastern taiwan the present day plate boundary between the philippine sea plate and eurasia reliable annual records of displacement along an active fault were obtained based on detailed surveys of faulted concrete structures along the active chihshang fault striking n18 e we determined average motion vectors trending n37 w with an average shortening of 2 2 cm yr thus the transverse component of motion related to westward thrusting is 1 8 cm yr whereas the left lateral strike slip component of motion is 1 3 cm yr the fault dips 39 45 to the east so that the vertical displacement is 1 5 3 cm yr and the actual oblique offset of the fault increases at a rate of 2 7 3 7 cm yr this is in good agreement with the results of regional geodetic and tectonic analyses in taiwan and consistent with the n54 w trend of convergence between the northernmost luzon arc and south china revealed by gps studies our study provides an example of extreme shear concentration in an oblique collision zone at chihshang the whole horizontal shortening of the longitudinal valley fault 2 2 cm yr on average occurs across a single narrow fault zone so that the whole reverse slip about 2 7 3 7 cm yr depending on fault dip was entirely recorded by walls 20 200 m long where faults are tightly localized this active faulting accounts for more than one fourth 27 of the total shortening between the luzon arc and south china recorded through gps analyses further surveys should indicate whether the decreasing shortening velocity across the fault is significant revealing increasing earthquake risk due to stress accumulation or not revealing continuing fault creep and weak behaviour of the chihshang fault

a simple algorithm for local earthquake location using 3d vp and vs models test examples in the central united states and in central eastern taiwan traditional local earthquake location using a horizontally layered homogeneous velocity model is limited in its resolution and reliability due to the existence of frequently overlooked 3d complexity of the real earth during traditional 3d seismic tomography simultaneous earthquake relocation using the resultant 3d velocity model has produced reliable earthquake locations however only a small subset of events are typically used and thus relocated in the inversion the rest of the events in a catalog must then be relocated using the 3d models the repeated calculation of travel times across 3d v p and v s models is also not efficient and not practical for a routine network earthquake location when the very time consuming exact 3d raytracing is used because high resolution earthquake data are now available from many modern seismic networks representative high resolution 3d v p and v s models for a region can be better determined by taking advantage of recently available high speed computer technology and large disk space we implemented a simple algorithm to efficiently locate every local earthquake using the best available regional 3d v p and v s models once the v p and v s information for all cubic cells in a 3d grid model are determined p and s travel times from each grid point to all seismic stations can be calculated and stored on disk files for later usage during the iteration process for earthquake location travel times from a trial hypocenter to all recording stations can be determined simply by a linear interpolation from those of the adjacent eight grid points available in the previously stored disk files without the need for raytracing the iterations continue until the hypocenter adjustments at the end of the last iteration are below the given criteria and the travel time residual or the difference between the observed and the calculated travel times is a minimum therefore any local earthquake can be efficiently and reliably located using the available 3d velocity models this simple location program has been applied to relocate earthquakes in the new madrid seismic zone nmsz of the central united states and in the central eastern taiwan region preliminary results in both regions reveal that earthquake hypocenters can be efficiently relocated in spite of the very significant lateral structural variations tests with data from taiwan further demonstrate that the resolution of seismic tomography and the relocated seismicity is sensitive to relative distribution of seismic network stations and background seismicity thus this single event location program can be applied to relocate all earthquakes in a seismic network catalog and more importantly to allow routine earthquake location for any seismic network using the available 3d velocity models

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

three dimensional simulations of seismic wave propagation in the taipei basin with realistic topography based upon the spectral element method we use the spectral element method to simulate strong ground motion throughout the taipei metropolitan area mesh generation for the taipei basin poses two main challenges 1 the basin is surrounded by steep mountains and 2 the city is located on top of a shallow low wave speed sedimentary basin to accommodate the steep and rapidly varying topography we introduce a thin high resolution mesh layer near the surface the mesh for the shallow sedimentary basin is adjusted to honor its complex geometry and sharp lateral wave speed contrasts variations in moho thickness beneath northern taiwan are also incorporated in the mesh spectral element simulations show that ground motion in the taipei metropolitan region is strongly affected by the geometry of the basin and the surrounding mountains the amplification of ground motion is mainly controlled by basin depth and shallow shear wave speeds although surface topography also serves to amplify and prolong seismic shaking

gravity monitoring of tatun volcanic group activities and inference for underground fluid circulations abstract the tatun volcano group tvg located on the northern coast of taiwan adjacent to the city of taipei experiences active hydrothermalism but has no historical record of volcanic eruption yet recent studies suggest that tvg is dormant active rather than extinct to monitor mass transfers and to gain further understanding of this volcanic area gravity variations have been recorded continuously since 2012 using a superconducting gravimeter and once every few months since 2005 using absolute gravimeters we analyze the continuous gravity time series and propose a model that best explains the gravity variations due to local groundwater redistribution by correcting these variations we identify gravity changes as large as 35 gal that occurred concomitantly to fluid pressure induced earthquakes and changes in the gas composition at dayoukeng one of tvg s fumaroles over 2005 2007 we examine several fluid movements that can match the gravity observations yet too few additional constraints exist to favor any of them in particular no significant ground displacements are observed when these gravity variations occurred on the other hand the model of gravity changes due to local groundwater redistribution can be routinely computed and removed from the ongoing time gravity measurements in order to quickly identify any unusual mass transfer occurring beneath tvg

insights into active tectonics of eastern taiwan from analyses of geodetic and geologic data 1 about 50 mm yr of convergence between the philippine sea and eurasian plates is absorbed in eastern taiwan and it remains unclear how the convergence is partitioned among active faults the longitudinal valley fault lvf the most seismically active fault in eastern taiwan creeps at the surface in the south and not in the north however it is unclear how much of the fault is locked or creeping at depth to address these problems we model holocene and interseismic deformation of elastic lithospheric blocks moving over a viscoelastic asthenosphere in eastern taiwan through a fully probabilistic scheme we invert gps interferometric synthetic aperture radar creepmeter and holocene marine terrace data for block motions fault slip rates and distribution of interseismic creep the data are explained with four blocks separated by three faults central range fault lvf and an offshore fault the model explains the essential features of interseismic and holocene deformation we find that 35 55 mm yr of slip on the offshore fault is necessary to fit marine terrace uplift rates which is a larger fraction of the plate convergence than previously recognized the lvf has a holocene slip rate of 20 30 mm yr with approximately equal magnitudes of reverse slip and left lateral strike slip components only about half of the surface area of the longitudinal valley fault appears to be locked the southern segment of the lvf creeps at a rate of 5 28 mm yr down to a depth of 15 20 km while the northern segment is locked from the surface to a depth of 20 km

elevated mixed layers and associated severe thunderstorm environments in south and north america abstractthis study presents a climatological and composite analysis of elevated mixed layers emls in south and north america derived from the ncep climate forecast system reanalysis the emls are identified based on objective criteria applied to the reanalysis data composite analyses of synoptic scale conditions and severe weather parameters associated with spring eml cases are presented emls are more frequent immediately to the east of the andes and the rockies the north american emls form by surface heating over the higher terrain of the rockies with peak frequency occurring in spring and summer emls in south america are generated by differential temperature advection due to ageostrophic circulations east of the andes as indicated by the temperature lapse rate tendency equation which relates to the higher frequency of emls during the cold season in south america emls over north america are about 100 hpa lower than over south america due to the lower height of the rockies in comparison to the a

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

postseismic deformation following the 1999 chi chi earthquake taiwan implication for lower crust rheology on 1999 september 21 the mw 7 6 chi chi earthquake ruptured a segment of the chelungpu fault a frontal thrust fault of the western foothills of taiwan the stress perturbation induced by the rupture triggered a transient deformation across the island which was well recorded by a wide network of continuously operating gps stations the analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements with relaxation times varying by a factor of more than ten and large cumulative displacements at great distances in particular along the longitudinal valley in eastern taiwan where relaxation times are also longer we show that while afterslip is the dominant relaxation process in the epicentral area viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale we model the spatiotemporal behavior of the transient deformation as the result of afterslip on the decollement that extends downdip of the chelungpu thrust and viscoelastic flow in the lower crust and in the mid crust below the central range we construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled the model is compatible with the shorter relaxation times observed in the near field which are due to continued fault slip and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the central range our preferred model shows a viscosity of 0 5 1 x 10 19 pa s at lower crustal depths and 5 x 10 17 pa s in the mid crust below the central range between 10 and 30 km depth the low viscosity zone at mid crustal depth below the central range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures estimated to be between 300 c and 600 c from the modeling of thermo chronology and surface heat flow data

structural complexities in a foreland thrust belt inherited from the shelf slope transition insights from the alishan area of taiwan the alishan area of taiwan spans the transition from the platform with full thickness of the eurasian continental margin in the north to the thinning crust of its slope in the south this part of the foreland thrust and fold belt includes important along strike changes in structure stratigraphy and seismic velocities in this paper we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend fault contour stratigraphic cutoff and branch line maps provide 3 d consistency between the cross sections minimum shortening is estimated to be 15 km with displacement overall to the northwest a p wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there p wave velocities of 5 2 km s point toward the presence of basement rocks in the shallow subsurface throughout much of the southeastern part of the area forming a basement culmination the changes in strike of thrusts and fold axial traces the changing elevation of thrusts and stratigraphic contacts and the growing importance of middle miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination these transverse structures appear to be associated with the inversion of eocene and miocene age extensional faults along what was the shelf slope transition in the early oligocene uplifting the margin sediments and their higher p wave velocity basement during pliocene pleistocene thrusting

imaging the crustal magma sources beneath mauna loa and kilauea volcanoes hawaii three dimensional seismic p wave traveltime tomography is used to image the magma sources beneath mauna loa and kilauea volcanoes hawaii high velocity bodies 6 4 km s in the upper 9 km of the crust beneath the summits and rift zones of the volcanoes correlate with zones of high magnetic intensities and are interpreted as solidified gabbro ultramafic cumulates from which the surface volcanism is derived the proximity of these high velocity features to the rift zones is consistent with a ridge spreading model of the volcanic flank southeast of the hilina fault zone along the south flank of kilauea low velocity material

destruction of luzon forearc basin from subduction to taiwan arc continent collision abstract along offshore to the east of southern taiwan different stages of subduction and collision occur simultaneously along strike of the convergent boundary as a result the evolution of the luzon arc and its forearc basin can be studied from the younger subduction zone to the south to the collision zone to the north examining more than 8000 km of seismic lines we analyzed the seismic stratigraphy of strata in a forearc basin and its successive basins in the collision zone to study the processes related to arc collapse and forearc basin closure the study area presents three evolutional stages intra oceanic subduction initial arc continent collision and arc continent collision we divided 9 seismic sequences in the forearc basin and found older sub parallel basin fill sequences 4 9 and younger divergent sequences 1 3 isochron maps of the sequences were used to interpret different deformation modes and their areal extends on the arc side of the basin of the subduction and initial collision zones we found relatively undisturbed strata showing little arc deformation on the trench side the growth strata in sequences 1 through 3 are the result of recent tectonic wedging along the rear of the accretionary prism tectonic wedging and back thrusts incorporate the forearc strata into the rear of the accretionary prism until they close the forearc basin at a region with a 2200 m basement relief this relief is not caused by active deformation as young flat forearc strata lap onto it and mark the transition from initial collision to collision where many growth strata to the north suggest abrupt increase in active arc basement deformation the 1 deforming basement 2 back thrusts and 3 other sedimentary processes affect the architecture of the successive basins in the collision zone until the arc is juxtaposed to the rear of the fold and thrust belt on land

present day kinematics of active mountain building in taiwan from gps observations during 1995 2005 1 we characterize the kinematics of modern crustal deformation in taiwan and evaluate the potential for large earthquakes by computing tectonic block motions and fault slip rates from 531 gps horizontal velocities these new gps velocity field indicates that lateral extrusion in the southern transition from collision to subduction is primarily achieved by motion along several major reverse faults and internal distortion of blocks the northern transition is characterized by asymmetric opening of the okinawa trough and collision induced rotation between the ryukyu trench and okinawa trough we suggest that the differences in style of deformation in northern and southern taiwan are a result of differences in trenchward motions between the overriding plate and forearc sliver along strike variations in basin thickness and the presence of foreland basement obstacles in central taiwan result in clockwise rotation with sinistral motion on faults and counterclockwise rotation with dextral motion on faults north and south of the obstacle respectively in eastern taiwan high slip rate of 43 mm yr on the southern longitudinal valley fault lvf is responsible for the full collision of taiwan orogeny e w syn orogenic extension in the southern central range has been inferred by our model patches with high slip rate deficits on the lvf and the chelungpu fault from our model respectively mainly correspond to the source areas of the 1951 m 7 1 longitudinal valley earthquake sequence and of the 1999 mw 7 6 chi chi earthquake

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

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

two dimensional crustal structures of taiwan from gravity data to delineate the tectonic character of taiwan an island wide gravity survey of taiwan was conducted between 1980 and 1987 the bouguer anomaly map shows that in general isogals trend nne in consonance with the overall structural trend of the island with seismic and other geophysical data as constraints the subsurface density structures were modeled along three profiles across the major structural trends the gravity data are consistent with average continental moho depths of 26 km in the coastal plain and the western foothills 28 km underneath the coastal range in eastern taiwan and 33 km under the central range a lack of significant correlation of the bouguer anomalies with the topography implies dynamic rather than isostatic support of topography in the taiwan region

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

seismogenic structures of the 1999 mw 7 6 chi chi taiwan earthquake and its aftershocks abstract high resolution 3 d vp and vs velocity models around the source region of the mw7 6 chi chi taiwan earthquake show significant lateral and vertical variations the mainshock occurred within a narrow low velocity zone along the chelungpu fault a sudden increase of velocity and seismicity took place across the shuilikeng fault to the east most aftershocks were located in areas of high vp and vs a sharp east dipping zone extending from the shuilikeng fault on the surface to a depth of about 15 km separates the low velocity region in the west from the high velocity region in the east an apparent high angle west dipping seismic zone lies at depths of 15 30 km beneath the western central range numerous aftershocks with normal faulting were located west of the middle segment of the chelungpu fault

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

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

investigation of the temporal change in attenuation within the ruptured fault zone of the 1999 mw7 3 chi chi taiwan earthquake a decrease in q s values within the hanging wall of the ruptured chelungpu fault two years following the 1999 chi chi earthquake was revealed by q s tomography images and an analysis of single path q s the synthetic and sensitivity tests of the q s determination were carried out accordingly to justify the temporal variation a q s value within the hanging wall above the hypocenter was determined to be 157 18 two years following the chi chi earthquake which is significantly lower than the q s tomography values of 238 17 and 289 13 prior to and two years after the main shock respectively similar values using a signal path q s analysis from events within the ruptured fault zone to stations along the fault were obtained the corresponding q s values were 247 85 prior to the chi chi earthquake after the earthquake we obtained q s values of 158 75 and 318 80 for 2 years following and 2 years after the earthquake respectively considering the two independent methods in determination of q s the reduction in q s by 89 two years following the chi chi earthquake in both methods is significant along with 1 v s reduction revealed by the analysis of repeating earthquakes our studies suggested possible reduction both in v s and q s values within the fault zone after the chi chi earthquake temporal changes in q s after the chi chi earthquake imply variations of pore fluid saturation in the ruptured fault zone the reduction in q s two years following the chi chi earthquake indicates high pore fluid saturation within the fractured fault zone due to the postseismic fluid redistribution

three dimensional velocity structure in northern california coast ranges from inversion of local earthquake arrival times abstract the three dimensional p wave velocity structure in the upper crust in the northern california coast ranges from santa rosa to ukiah is obtained by simultaneous inversion of local earthquake and refraction travel time data for velocity and hypocentral parameters in this region where there are lateral heterogeneities the three dimensional inversion provides important results not found in two dimensional refraction studies the medium is parameterized with velocities at the intersections gridpoints of a nonuniform three dimensional grid travel times are calculated with approximate ray tracing along arcuate paths which are reasonable for lateral distances less than 45 km inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance the inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error anomalously low velocities to at least 6 km depth are found along the maacama and healdsburg rodgers creek hrc fault zones the low velocity region is widest at the en echelon junction of the maacama and hrc fault zones low velocities are associated with the clear lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits there is a high velocity body located southeast of the geysers between the maacama and collayomi fault zones which may be associated with igneous intrusives or high grade metamorphism related to an underlying inferred partial melt body modeled in previous teleseismic travel time studies there is some indication in the vapor dominated geothermal reservoir of lower velocities relative to surrounding water saturated rock as has been shown in some laboratory experiments hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one dimensional velocity model and station corrections for the inversion with 2 to 3 km gridspacing however some groups of events become more tightly clustered

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

significant contribution of the shallow crust to seismic pkp travel time residuals and implications an example from taiwan and nearby islands abstract array observations indicate that the relative pkp travel time residuals in the taiwan region vary from 2 5 to 2 0 s a range greater than the largest known teleseismic travel time residuals from deep earth pkp waves are generally employed to probe structures of the deep earth however the contribution of shallow geological structures to pkp waves is generally undefined and the implications of such a contribution have rarely been discussed current travel time tomographic models of taiwan are able to predict less than 40 of the observed pkp residuals and the pkp residuals are consistent with the pattern of station corrections used in determining local earthquakes detailed island wide gravity measurements indicate that the distribution of bouguer anomalies which were previously proposed to be induced by thick sedimentary deposits in large tectonic basins is consistent with the pattern of pkp travel time residuals in conclusion the observed pkp residuals are strongly affected by shallow sediments therefore the contribution of residuals from shallow crustal structures should be carefully considered when employing pkp observations for imaging structures of the deep earth or the deep crust and upper mantle

crustal scale weak zone along a collisional suture revealed by spatial variations in velocity structures and seismicity abstract high quality first arrival data collected with a high density temporary seismic array and regional seismic network were used to construct a p wave velocity model and identify the precise location of earthquakes in the active collision zone of southeastern taiwan a crustal scale weak zone defined by high seismicity is characterized by a steeply east dipping lower velocity anomaly and represents a boundary between an uplifted upper mantle and the luzon volcanic arc the main features of the weak zone with its associated low velocity zone and seismicity pattern vary significantly along the collisional boundary of southeastern taiwan along the weak zone unlithified sediments or highly fractured materials are compacted as the philippine sea plate moves toward the eurasian plate in areas of more advanced collision soft material is more compacted and can therefore store significantly greater amounts of strain energy this energy is released through episodic earthquakes

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

application of the jhd technique to the loma prieta california mainshock aftershock sequence and implications for earthquake location relocation of the loma prieta mainshock aftershock sequence using the jhd technique places the events an average of 1 km to the southwest of the locations determined by other authors the new locations show a concentration of seismic activity in the wedge between the san andreas and sargent faults furthermore there seems to be little seismic activity to the east of the sargent fault these features of the seismicity are not observed in other published locations determined using similar data sets a depth cross section perpendicular to the sargent fault near its intersection with the san andreas fault shows two clusters of events along a line segment between 3 and 10 km that dips to the ne and that intersects the main sw dipping plane these clusters may correspond to secondary faults postulated by other authors the jhd station corrections vary between 0 6 and 0 6 sec and indicate the presence of large velocity variations across and along the san andreas fault these corrections are very close to the weighted average of the station residuals obtained when the events are located singly the locations obtained with a single event location program and using the average residuals as station corrections give locations almost coincident with the jhd locations this high similarity of locations is very unusual and is interpreted as the result of quasi random lateral velocity variations in the loma prieta area to estimate the effect of these variations on the location of events synthetic travel times were computed for a realistic although preliminary three dimensional 3d velocity model these synthetic data were then treated as the actual data the corresponding results show that the jhd technique is able to locate events with an average error of 0 5 km in epicenter depths on the other hand are consistently deeper by about 1 km in addition the station corrections are close to the observed corrections additional tests with synthetic data also show that the observed station corrections suggest velocity variations of up to 40 in the top 3 km of the crust although these variations may seem extremely large they are within the range of variation found by other authors and most importantly are required by the observations

spatial variation of seismogenic depths of crustal earthquakes in the taiwan region implications for seismic hazard assessment abstract this paper presents the first whole taiwan scale spatial variation of the seismogenic zone using a high quality crustal seismicity catalog the seismicity onset and cutoff depths i e seismogenic depths are determined by the earthquake depth moment distribution and used to define the upper and lower boundaries of the seismogenic zone respectively together with the published fault geometries and fault area moment magnitude relations the depth difference in the onset and cutoff depths i e seismogenic thickness is used as the fault width to determine the moment magnitudes of potential earthquakes for the major seismogenic faults results show that the largest mw7 9 8 0 potential earthquake may occur along the changhua fault in western taiwan where the seismic risk is relatively high and seismic hazard mitigation should be a matter of urgent concern in addition the first motion focal mechanism catalog is used to examine the relation between the seismogenic depths and earthquake source parameters for crustal earthquakes 50 km the shallowest onset and cutoff depths are observed for normal and strike slip events respectively this observation is different from the prediction of the conventional continental rheology model which states that thrust events have the shallowest cutoff depth thus a more sophisticated rheology model is necessary to explain our observed dependence of the seismogenic depths on faulting types meanwhile for intermediate to large crustal mw 4 depth 50 km earthquakes thrust events tend to occur at the bottom region of the seismogenic zone but normal and strike slip events distribute at a large depth range

sequential thrusting model for mountain building constraints from geology and heat flow of taiwan in this study we analyze the thermal evolution of the taiwan fold and thrust belt based on the available geological information a finite element program is used to simulate the complex kinematics of the observed sequential thrusting and the interplay of uplift erosion and sedimentation processes the discrete sequential thrusting model successfully reproduces the observed decrease in heat flow at the deformation front in northern taiwan this reveals that the heat flow drop may be due to the recent stacking of cold thrust sheets at the deformation front this inference is consistent with the high seismicity and dense quaternary fault distribution in western taiwan the model also successfully reproduces the observed high heat flow in the central ranges suggesting that the observed high heat flow may be a result of tectonic unroofing due to active underplating uplift and erosion these conclusions are insensitive to uncertainties in the frictional coefficient on the thrust plane

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

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

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

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

the shuilikeng fault in the central taiwan mountain belt for over 200 km along strike the shuilikeng fault of taiwan separates miocene rocks of the western foothills from the largely eocene and oligocene rocks of the hsuehshan range to the east despite its importance in the taiwan mountain belt the structure and kinematics of the shuilikeng fault are not well known here we present results from new geological mapping along 100 km of its strike length at the surface the shuilikeng fault is a steeply east dipping brittle fault with a series of splays and bifurcations along its southern part it cuts an earlier fold and fault system outcrop kinematic data vary widely from thrusting to strike slip the surface data are integrated with a relocated and collapsed seismicity database to interpret the fault location at depth these data indicate that the shuilikeng fault can be traced to greater than 20 km depth some 260 focal mechanisms from this dataset indicate that its kinematics is overall transpressive from a regional perspective we interpret the shuilikeng fault to reactivate a pre existing rift related basin bounding fault to the east of which rocks in the hsuehshan range are being exhumed

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

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

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

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