hypocentre determination offshore of eastern taiwan using the maximum intersection method summary the maximum intersection maxi method which derives from the master station method msm determines within a 3 d velocity model the absolute hypocentral location based on observed arrival times first the spatial node that better satisfies the arrival time differences computed at all station pairs plus or minus an error tolerance value in seconds is defined as the preliminary hypocentral solution pred second because pred depends neither on the estimate of origin time nor on the residual root mean square rms residual outliers are objectively detected and cleaned out from the original data set without any iterative process or weighting third a statistical minimization residual rms is conducted in a small domain around the pred node which results in a unique final solution the maxi method is applied to the determination of earthquake hypocentres with the proper station correction terms in the southernmost extremity of the ryukyu subduction zone where several dense seismic clusters occur near the seismogenic plate interface the location of earthquakes recorded at both the taiwanese and japanese networks is obtained for about a thousand events between 1992 and 1997 the process uses a detailed 3 d velocity model based on multiple geophysical data sources obtained in the junction area between subduction and collision east of taiwan the earthquake clustering and the significant drop in residual statistics 1 20 0 80 and 0 35 s for taiwanese catalogue msm and maxim solutions respectively indicate the accuracy of the method which can be used to routinely determine absolute hypocentre location based on observed arrival times

present day interseismic surface deformation along the longitudinal valley eastern taiwan from a ps insar analysis of the ers satellite archives 1 in taiwan about one third of the lithospheric plate convergence between eurasia and the philippine sea plate is accommodated on the eastern coast across the narrow longitudinal valley lv the longitudinal valley fault lvf is the main seismically active fault zone in this region however the spatial distribution of ground deformation due to interseismic loading on locked fault segments remains widely unknown to address this problem we use a permanent scatterer ps interferometric synthetic aperture radar insar approach for processing the ers satellite archives the ps mean velocity maps allow accurately mapping and quantifying the ground deformation all along the lv assuming that close to the surface the fault plane along the central lvf creeping segment dips 55 to the east we derive a slip vector at the surface of 25 mm yr with a mean rake of 70 in agreement with our ps observations shallow seismicity indicates a clear change in the fault mechanical behavior of the creeping segment around fuli town finally we propose a combination of the ps velocity fields for estimating a 3 d map of ground deformation in the southern part of the lv the vertical component reveals an uplift of the coastal range with respect to the lv of 10 mm yr moreover between taitung and luyeh the vertical deformation is essentially accommodated along the luyeh strand finally north of rueisuei the coastal range uniformly subsides at 15 mm yr relative to the western side of the lv however a local uplift pattern is detected around latitude 23 42

flux and fate of small mountainous rivers derived sediments into the taiwan strait article i nfo high resolution chirp sonar profiles across the taiwan strait reveal a large silt sand dominated deltaic clinoform up to 50 m thick overlying the postglacial transgressive sea floor across the southeastern central and northern strait delta like configuration and internal depositional sequences indicate a northwestward progradation from western taiwan primarily from the choshui zhuoshui river grain size and mineral data confirm the sediment s taiwanese derivation the chirp profiles together with existing radiocarbon and geomagnetic dates suggest that the clinoform has formed over the past 10 kyr the estimated volume of 375 km 3 of sediment mainly sand and silt suggests a mean annual accumulation of 60 10 6 t yr presumably much of fine mud delivered by taiwanese rivers has been washed away by the local currents and escaped either northeastward into the southern okinawa trough or southward into the south china sea numerous shallow borings onshore over the central western taiwan coastal plain reveal an additional 350 km 3

inversion of fluvial channels for paleorock uplift rates in taiwan the transient response of erosion to changes in rock uplift rate leads to the preservation of rock uplift history in the long profiles of rivers however extracting this information is nontrivial as changes in channel steepness are the result of both spatial and temporal changes in rock uplift rate as well as other factors such as climate and rock type we exploit an analytical linear solution for river channel profile evolution in response to erosion and tectonic uplift to investigate the rock uplift history of taiwan the analytical approach allows us to solve the linear inverse problem efficiently extracting rock uplift as a function of space and time from digital elevation data we assess the potential of fluvial topography to resolve rock uplift rates using three approaches 1 a synthetic resolution test 2 analysis of the forward model to demonstrate where in space and time the fluvial topography constrains rock uplift rate and 3 interpretation of the model resolution matrix furthermore the potential to analyze large data sets reduces the influence of stochastic processes such as landslides small scale river network reorganization and also local lithological variability in taiwan our analysis suggests that current rock uplift rates exceed erosion rates across much of the island and that there has been an increase in rock uplift rates since 0 5 ma across the central range

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 flexure under a critically tapered mountain belt a new technique to study the evolution of the tertiary taiwan orogeny abstract a joint analysis of both the lithospheric flexure of an underlying plate and mountain building mechanics is developed based on the critical wedge theory and the flexure response of an elastic plate over an inviscid asthenosphere according to the analysis the amount of deflection beneath a critically tapered mountain belt strongly depends upon the internal and basal friction coefficients of that belt and the flexural rigidity of the underlying plate rather than the horizontal stress induced by plate convergence an increase in the internal friction of a mountain belt or a decrease in the friction along its basal decollement tends to enlarge the deflection of the underlying plate the tendency toward an increased deflection of the underlying plate due to a decrease in flexural rigidity is suppressed by the development of a more narrowly tapered mountain belt the newly developed technique presented here is used to analyze the evolution of the tertiary taiwan orogeny and shows that the optimal flexural rigidity of the eurasian plate in taiwan is about 3 10 22 nm it also demonstrates that the collision between the eurasian plate and the luzon arc might have initiated 7 myr ago a finding that would account for the burial history of the foreland basin in south central taiwan

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

serpentinization of the fore arc mantle along the taiwan arc continent collision of the northern manila subduction zone inferred from gravity modeling abstract serpentinized peridotite in the fore arc has been observed in a number of subduction zones including the northern manila subduction zone which terminates northward in the taiwan arc continent collision how this zone of serpentinization changes northward from the subduction of thinned continental lithosphere to full arc continent collision in the taiwan orogeny is not well known in this paper we present 2 d gravity modeling along three p wave vp transects across the taiwan orogeny two of these transects were collected with ocean bottom seismometers these two transects provide good constraints on the velocity structure to the west of and on land southern taiwan conversion of vp to density in this area allows us to model the gravity anomaly with very little misfit along the subduction zone however the velocity models are poorly constrained in the upper mantle where an anomalous density unit has to be used in order to model the short wavelength gravity anomaly in this area a third transect across central taiwan that is derived from the taiger local tomography data provides good control on the crust and upper mantle vp structure that we use to place provide density constraints for modeling the gravity anomaly in this part of the collision zone in order to model the short wavelength gravity anomaly across the longitudinal valley and the southern longitudinal trough an anomalous density block is required beneath the fore arc region we interpret that the source of this anomalous density material could be serpentinized fore arc mantle similar to what is interpreted for the northern manila subduction zone farther south water released from the subduction of the extended crust of the continental margin results in the serpentinization of the fore arc area and may be driving the uplifting of the high pressure rocks

a viscoelastic earthquake cycle model for taiwan 1 while recent geophysical measurements including global positioning system gps surveys have clarified the mechanisms of active mountain building in the taiwan arc continent collision the geometry of faults beneath the island remains unclear in this paper we use gps data collected before the 1999 chi chi earthquake to image the geometry of active faults under taiwan and estimate slip rates on these faults we invert the data using kinematic and mechanical earthquake cycle models consisting of faulting in an elastic lithosphere overlying a viscoelastic asthenosphere in the kinematic model uniform slip is prescribed on the faults at a constant rate while in the mechanical model the faults slip in response to shear stresses in the lithosphere these models explain the essential features of both the horizontal and vertical gps velocity fields we find a 27 39 km thick elastic lithosphere overlying a viscoelastic asthenosphere with maxwell relaxation time of 15 80 years or viscosity of 0 5 4 1019 pa s the models produce a combined slip rate of 30 50 mm yr on the frontal thrusts in western taiwan and 60 80 mm yr of slip on the longitudinal valley fault inversion of the data using both models indicates a decollement dipping 10 under western taiwan and increasing in dip by no more than 10 under the central ranges and eastern taiwan

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

subsurface structure physical properties fault zone characteristics and stress state in scientific drill holes of taiwan chelungpu fault drilling project continuous cores and a suite of geophysical measurements were collected in two scientific drill holes to under stand physical mechanisms involved in large displacements during the 1999 chi chi earthquake physical properties obtained from wire line logs including p and s wave sonic velocity gamma ray electrical resistivity density and temperature are primarily dependent on parameters such as lithology depth and fault zones the average dip of bedding identified from cores and fmi or fms logs is about 30 degrees towards se nevertheless local azimuthal variations and increasing or decreasing bedding dips appear across fault zones a prominent increase of structural dip to 60 80 below 1856 m could be due to deformation associated with propagation of the sanyi fault a total of twelve fault zones identified in hole a are located in the plio pleistocene cholan formation pliocene chinshui shale and miocene kueichulin formation the shallowest fault zone occurs at 1111 m depth fza1111 it is a 1 m gouge zone including 12 cm of thick indurate black material we interpreted this zone as the slip zone during the chi chi earthquake fza1111 was characterized by 1 beddingparallel thrust fault with 30 dip 2 the lowest resistivity 3 low density vp and vs 4 high vp vs ratio and poisson s ratio 5 low energy and velocity anisotropy and low permeability within the homogeneous gouge zone 6 increasing gas co2 and ch4 emissions and 7 richness in smectite within the primary slip zone in situ stresses at the drill site were inferred from leak off tests borehole breakouts and drilling induced tensile fractures from borehole fms fmi logs and shear seismic wave anisotropy from dsi logs the dominant fast shear wave polarization direction is in good agreement with regional maximum horizontal stress axis particularly within the strongly anisotropic kueichulin formation a drastic change in orientation of fast shear polarization across the sanyi thrust fault at 1712 m depth reflects the change of stratigraphy physical properties and structural geometry

sedimentology of early pliocene sandstones in the south western taiwan foreland implications for basin physiography in the early stages of collision abstract this work presents sedimentological observations and interpretations on three detailed sections of the pliocene yutengping ailiaochiao formations deposited in the early stages of collision in taiwan seven facies associations record paleoenvironments of deposition ranging from nearshore to lower offshore with a strong influence of tidal reworking even in shelfal sub tidal environments and a pro delta setting characterized by mass flows the association of shallow facies of the upper offshore to lower shoreface with pro delta turbidite facies sourced in the orogen to the east suggests a peculiar setting in which turbidite deposition occurred below wave base but on the shelf in water depths of probably less than 100 m this adds to the examples of shallow turbidites increasingly commonly found in foreland basins and challenges the classical view of a deep early underfilled foreland basin time series analysis on tidal rhythmites allow us to identify a yearly signal in the form of periodic changes of sand supply energy and bioturbation that suggests a marked seasonality possibly affecting precipitation and sediment delivery as well as temperature the taiwan foreland basin may also present a potentially high resolution record in shallow sediments of the early installation of monsoonal circulation patterns in east asia we confirm partly the paleogeography during the early stages of collision in taiwan the chinese margin displayed a pronounced non cylindrical geometry with a large basement promontory to the west in place of the modern taiwan mountain range collision in taiwan may have happened at once along the whole length of the modern mountain range instead of progressively from north to south as classically considered

how large are departures from lithostatic pressure constraints from host inclusion elasticity minerals trapped as inclusions within other host minerals will develop non lithostatic pressures during both prograde and retrograde metamorphism because of the differences between the thermoelastic properties of the host and inclusion phases there is only a single possible path in p t space the entrapment isomeke along which no residual pressure would be developed in a host inclusion system non lithostatic pressures are developed in inclusions as a result of the external pressure and temperature deviating from the isomeke that passes through the entrapment conditions with modern equation of state and elasticity data for minerals now available it is possible to perform precise calculations of the isomekes for mineral pairs these show that isomeke lines are not straight lines in p t space at metamorphic conditions we show that silicate inclusions in silicate hosts tend to have flat isomekes with small values of because of the small range of thermal expansion coefficients of silicate minerals as a consequence the general behaviour under decompression is for soft silicate inclusions in stiffer hosts to develop excess pressures whereas a stiff silicate inclusion in a softer matrix will experience lower pressures than lithostatic pressure the opposite effects occur for compression after entrapment on the prograde path the excess pressures in inclusions including allowance for mutual elastic relaxation of the host and inclusion are most easily calculated by using the isomeke as a basis analysis of the simplest possible model of a host inclusion system indicates that deviations from lithostatic pressure in excess of 1 gpa can be readily produced in quartz inclusions within garnet in metamorphic rocks for softer host minerals such as feldspar the pressure deviations are smaller because of greater elastic relaxation of the host the maximum pressure deviation from lithostatic pressure in the host phase around the inclusion is one third of the pressure deviation in the inclusion routines for performing these calculations have been added to the eosfit7c software package

prehistoric earthquakes along the shanchiao fault taipei basin northern taiwan abstract stratigraphic relations recorded in boreholes drilled along the shanchiao fault a normal fault that bounds the western margin of metropolitan taipei reveal evidence of early mid holocene subsidence and active extension of the taipei basin paleoseismic data suggest that the maximum displacement for each subsidence event ranges from 2 3 to 4 5 m despite large uncertainties the large offsets imply earthquake magnitudes that range in size from 6 9 to 7 1 our results suggest that large magnitude earthquakes along the shanchiao fault are one of the major seismic hazards in the metropolitan taipei area

structure of the 1999 chi chi earthquake rupture and interaction of thrust faults in the active fold belt of western taiwan abstract we summarize the structural characteristics of the surface ruptures of the 1999 mw 7 6 chi chi earthquake in western taiwan and discuss the geological interaction of the chelungpu fault with adjacent faults based on geological investigations seismological analyses and gps measurements of surface co seismic displacements we describe the regional 3 d fault plane and slip distribution of the chi chi earthquake and compare these to the geological features of the chelungpu fault we find that one key feature of the chelungpu fault is the stratigraphy controlled slip surface at the level of the uppermost few kilometers the chelungpu fault slip plane generally follows the bedding plane of the pliocene chinshui shale the second key feature of the chelungpu fault is the difference in structurally geometric configuration between its northern and southern segments the northern fengyuan segment shows a bedding parallel thrust fault within east dipping strata in both footwall and hanging wall in contrast the southern tsaotun segment exhibits east dipping strata are overthrust onto flat lying recent alluvial deposits these two features not only explain a hinterland imbricate thrusting on the hanging wall of the fengyuan segment but also explain the change in strike of the chi chi surface ruptures at the northern end the southern end of the 1999 chi chi rupture is interpreted to be linked to a series of nw trending strike slip faults in particular we propose that the luliao strike slip fault served as the lateral ramp of the chelungpu fault and the gukeng strike slip fault acted as a barrier to end the southern propagation of the 1999 rupture geomorphic features and paleoseismological data indicate that the range front chelungpu fault has generated large earthquakes during the last several thousand years alternatively in the miaoli area to the north and the chiayi area to south historical earthquakes as well as active geomorphic features are not restricted on the range front thrust faults instead more complicated structures including tightly spaced folds duplex structures and strike slip faults are involved in seismogenic processes a more detailed investigation of regional structural characteristics is needed for mitigation against the seismic hazards in the 300 km long active fold belt in western taiwan where several damaging large earthquakes have been documented during the last century

a study on the background and clustering seismicity in the taiwan region by using point process models 1 this paper investigates the shallow seismicity occurring in the taiwan region during the 20th century using a stochastic declustering method that has been developed on the basis of the theory of the epidemic type aftershock sequence model it provides a probability based tool to objectively separate the space time occurrences of earthquakes into a background and a clustering component on the basis of the background and clustering seismicity rates we discuss the correlation between the distribution of the cluster ratio and the regional seismotectonic structures specifically we find that the areas of the highest clustering ratio correspond to the major strike slip fault traces in and around taiwan additionally in the taiwan inland region during the period 1960 1990 the outputs for the stochastically declustered catalogue show a clear quiescence in background seismicity preceding the recovery of activation and the occurrences of the 1999 chi chi earthquake of ml7 3 while the other active regions show stationary background activity this could be interpreted as an effect of the aseismic slip in the chi chi rupture fault whereby the inland region around the chi chi source becomes a stress shadow

back arc rifting in the okinawa trough abstract geological and geophysical data reveal that the okinawa trough shows incipient continental rifting and crustal separation started from about 2 ma the early extensional movements in the trough are probably of miocene age in addition to the miocene phase two main periods of extension are recognized a pleistocene phase between 1 9 and 0 5 ma and the present day phase during the stage short central rifts central grabens were formed the opening however may have occurred only in the southern part of the trough basin having an average half spreading rate of 2 cm yr 1 since early pleistocene time producing its present width of several tens of kilometres these activities were well represented by igneous intrusions sedimentary facies and sedimentary structures in and around the okinawa trough the width of the zone affected by back arc extension defined as greater okinawa trough is larger than the present okinawa trough whose width is 200 250 km the present form of the greater okinawa trough started to form at the same time as that of the okinawa trough

frontal accretion an internal clock for bivergent wedge deformation and surface uplift 1 we propose a conceptual model of frontal accretion within bivergent wedges which is based on two dimensional sandbox simulations and the analysis of particle displacement fields each frontal accretion cycle consists of a thrust initiation an underthrusting phase and a reactivation phase the location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable therefore the frontal accretion cycle can be considered as an internal clock for wedge scaled deformation and surface uplift we further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle we estimate cycle duration to range between 104 and 105 years which is thus similar to the time frame of climatic cycles the proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic paleoseismologic and geologic estimates of fault slip

typhoon induced ground deformation geodetic instruments now offer compelling sensitivity allowing to investigate how solid earth and surface processes interact by combining surface air pressure data nontidal sea level variations model and rainfall data we systematically analyze the volumetric deformation of the shallow crust at seven borehole strainmeters in taiwan induced by 31 tropical cyclones typhoons that made landfall to the island from 2004 to 2013 the typhoon s signature consists in a ground dilatation due to air pressure drop generally followed by a larger ground compression we show that this compression phase can be mostly explained by the mass loading of rainwater that falls on the ground and concentrates in the valleys towards the strainmeter sensitivity zone further our analysis shows that borehole strainmeters can help quantifying the amount of rainwater accumulating and flowing over a watershed during heavy rainfalls which is a useful constraint for building hydrological models

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

the determination of displacement fields from geodetic data along a strike slip fault observations of angles or distances between stations of a geodetic network are commonly used to infer information about the movement of the surface of the earth the absence of any observations external to the network leads to an ambiguous displacement field existing techniques for eliminating this ambiguity are all unsatisfactory in some respect the best technique an inner coordinate solution is not appropriate for networks located in a strike slip fault environment the inner coordinate solution zeros the rotation of all stations about their center of mass along a strike slip fault like the san andreas however motion normal to the fault is less likely than motion parallel to the fault the solution presented here an outer coordinate solution finds the rotation of the network that minimizes the components of displacement normal to the fault since motion along a strike slip fault is generally expected to be parallel to the fault the displacements obtained with the outer coordinate solution are more reasonable than those obtained with other techniques examination of a trilateration network near san francisco bay demonstrates the large effect that the choice of adjustment technique can have on the inferred relative motion of the two sides of the fault the inner coordinate solution gave a rate of about 1 mm yr whereas the preferred outer coordinate solution rate was 36 mm yr

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

marine geology in the arc continent collision zone off southeastern taiwan implications for late neogene evolution of the coastal range abstract marine geology in the region between taiwan and luzon is characterized by a convergent tectonics in the south and an oblique arc continent collision in the north in the collision zone six geologic provinces are recognized they are the manila trench the accretionary prism of the hengchun ridge the sutural basin of the southern longitudinal trough the subduction melange of the huatung ridge the forearc basins of the taitung and the north luzon troughs and the volcanic arc of the north luzon ridge the oceanic crust of the south china sea is subducting eastward beneath the philippine sea plate along the manila trench south of 20 n further to the north the trench has changed progressively its axis toward the west from the subduction melanges off southeastern taiwan at 3 ma to the hengchun peninsula of southern taiwan at 2 1 ma and then to the present day location off southwestern taiwan because of the continuing collision and the westward shifting of the manila trench sediments deposited in the region are passively incorporated into the accretionary prism and deformed to the hengchun ridge after the collision the southern longitudinal trough consisting of a 2 km thick of orogenic sediments represents a sutural basin superposed on the plate boundary between the accretionary prism of the hengchun ridge asian continent and the subduction melange of the huatung ridge philippine sea plate the huatung ridge is a bathymetric high connected to the southern coastal range and is characteristic of low negative free air gravity and magnetic anomalies the ridge contains deeply weathered sedimentary blocks with abraded deep water late pliocene foraminifera and identical clay mineral compositions as those in the onshore lichi melange this implies that at least a portion of the huatung ridge is composed of melange materials and represents a former subduction zone extending from the lichi melange of the southern coastal range the north luzon trough and the taitung trough are the constructed foreac basins with 2 km of orogenic sediments unconformably overlying the volcanic islands and the subduction melange the taitung trough is southward connected to the north luzon trough through a bathymetric passage which was formed by narrowing the north luzon trough as a consequence of clockwise rotation of the lutao volcanic island during the late pleistocene a detailed study of the four neogene tectonostratigraphic units in the coastal range shows that the overall history of the formation of the range can be related to the present day geologic provinces and the on going collision processes in the region off southeastern taiwan the range had evolved from a southward accretion of the shuilien loho remnant forearc basin and the chimei volcanic pair at 2 ma followed by the chengkung taiyuan remnant forearc basin and the chengkuangao volcanic island pair at 1 ma and the taitung trough and the lutao volcanic island pair presently in process by analogy the northern part of the north luzon trough and the lanhsu volcanic island will be accreted to the coastal range in the next million years the paired forearc basin volcanic island accretion model proposed in this study may thus have implications for mountain ranges that evolved from similar tectonics of oblique arc continent collision

dislocation model for crustal deformation in the longitudinal valley area eastern taiwan abstract three trilateration and leveling networks traversing the longitudinal valley eastern taiwan have been surveyed frequently during 1983 1988 average rates of length change on each line and height difference on each leveling section are used to study the crustal deformation of this area we model the aseismic deformation near the fault as the sum of a steady rigid block motion and the effect of an upper locked crust due to frictional resistance the effect of the locked part is equivalent to that of a negative dislocation on the fault for the northernmost hualien and central yuli networks we simply assume two blocks and one fault patch while the southernmost taitung network is modeled by three blocks and three fault patches the fault aforementioned is presumed to be the main trace of the longitudinal valley fault lvf we invert the geodetic data to determine the block motion and dislocation model parameters by utilizing a bayesian nonlinear inversion method parameters to be estimated include the three components of velocity for each block and the vertical and horizontal dislocation rates dip angle and width of each rectangular fault patch the estimated block motion parameters and fault widths are mostly well resolved by the observed data the fault widths are all very shallow tectonophysics vol 42 is concentrated on a quite narrow zone several kilometers wide centered at the lvf however due to the limited aperture of our geodetic networks the total width of the deformation zone and the velocity of relative plate motion at this active collision boundary are not resolved the new gps satellite survey technique can be used to solve this ambiguity

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

coseismic deformation revealed by inversion of strong motion and gps data the 2003 chengkung earthquake in eastern taiwan summary a moderate earthquake of mw 6 8 occurred on 2003 december 10 it ruptured the chihshang fault in eastern taiwan which is the most active segment of the longitudinal fault as a plate suture fault between the luzon arc of the philippine sea plate and the eurasian plate the largest coseismic displacements were 13 cm horizontal and 26 cm vertical we analyse 40 strong motion and 91 gps data to model the fault geometry and coseismic dislocations the most realistic shape of the chihshang fault surface is listric in type the dipping angle of the seismic zone is steep about 60 70 at depths shallower than 10 km and then gradually decreases to 40 50 at depths of 20 30 km thus the polygonal elements in poly3d are well suited for modelling complex surfaces with curving boundaries using the strong motion data the displacement reaches 1 2 m dip slip on the chihshang fault and decreases to 0 1 m near surface the slip averages 0 34 m releasing a scalar moment of 1 6e26 dyne cm for gps data our model reveals that the maximal dislocation is 1 8 m dip slip the dislocations decrease to 0 1 m near the surface the average slip is 0 48 m giving a scalar moment of 2 2e26 dyne cm regarding post seismic deformation a displacements of 0 5 m were observed near the chihshang fault indicating the strain had not been totally released as a probable result of near surface locking of the fault zone

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

tectonic wedging along the rear of the offshore taiwan accretionary prism abstract the structural geometry kinematics and density structure along the rear of the offshore taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling deformation between the offshore prism and forearc basin at the point of incipient collision and southward into the region of subduction has been interpreted as a tectonic wedge similar to those observed along the front of mountain ranges this tectonic wedge is bounded by an east dipping roof thrust and a blind west dipping floor thrust an east dipping sequence of forearc basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge section restoration of the roof sequence yields an estimate of 4 km of shortening which is small compared with that inferred in the collision area to the north based on the variation in distance between the apex of the prism and the island arc previous studies propose that either high angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the central range in the collision zone on land to better constrain the initial crustal configuration we tested 350 crustal models to fit the free air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land the gravity anomaly observed in the region of subduction 20 2 n can be modeled with the arc basement forming a trenchward dipping backstop that is overlain by materials with densities in the range of sedimentary rocks near the point of incipient collision 20 9 n however the free air gravity anomaly over the rear of the prism is approximately 40 mgal higher compared with the region of subduction and requires a significant component of high density crustal rocks within the tectonic wedge these results suggest that the forearc basement may be deformed along the rear of the prism associated with the onset of collision but not in the subduction region further to the south

1999 chi chi earthquake a case study on the role of thrust ramp structures for generating earthquakes the 21 september 1999 chi chi earthquake m w 7 6 occurred on east dipping shallow thrust faults that produced a high relief surface rupture extraordinary surface breaks appeared that could be clearly traced for about 100 km across many counties these thrust faults the chelungpu and shihkang are part of an active fold and thrust belt related to ongoing recent arc continent collision measurement of slip vectors along the earthquake rupture indicates that the orientation of the maximum shear stress changed from a westward direction n70 90 w on the chelungpu fault to a northwestward direction n30 40 w on the shihkang fault the stress field underwent a clockwise rotation of about 40 during the chi chi earthquake near rupture vertical displacements in the hanging wall of the shihkang fault have more cumulative displacement than on the chelungpu fault which is consistent with global positioning system gps measurements maximum vertical offset on the rupture was found to be about 10 m by the surficial rupture and gps measurements in addition analysis of crustal deformation by gps measurements on the hanging wall defines a coseismic uplift related to a fault ramp structure our synthesis of geological and geodetic analyses shows the importance of ramp structures associated with thrust faults for generating large earthquakes and provides a general framework for understanding earthquake in fold and thrust belts large surficial coseismic uplift and strong asperities appear to be a function of fault ramp geometry our analysis also indicates that in general ramp structures in fold and thrust belts may have a high potential in generating large earthquakes manuscript received 14 november 2000

upstream facing convex surfaces bedrock bedforms produced by fluvial bedload abrasion bedrock bedforms are a common sight in many bedrock river channels but few process form linkages have been established for the myriad of forms recognised in the literature direct observations from bedrock channels on rates and distributions of erosion involved in bedrock bedform development are rare because of difficulties in making observations in these high energy environments upstream facing convex surfaces ufcs are common in bedrock rivers and are partnered with downstream facing surfaces we have made observations of bedrock morphology aspect and surface texture of ufcss in bedrock river channels exposed at low river stage in taiwan a distinct contrast in morphology and surface texture exists between upstream and downstream facing surfaces on a millimetre scale of observation ufcss have a three dimensional brittle fracture dominated granular surface texture but on a decimetre to metre scale have an overall convex form downstream facing surfaces in contrast have millimetre to micrometre scale rugosity characterised by smooth polished surfaces that on a centimetre to metre scale are concave or undulating with intricately sculpted features a curvilinear topographic feature or crest line marks the change in slope aspect and surface textures between upstream and downstream facing surfaces and is orientated approximately perpendicular to the reach azimuth and therefore mean flow direction we used a system of removable datums and terrestrial laser scanning to monitor the three dimensional erosion of nine bedforms at sites in the liwu river taiwan over a period of one year 2007 2008 all sites had an asymmetric distribution of erosion with higher rates of erosion on upstream facing surfaces and minor to undetectable erosion on downstream facing surfaces with local complexity the crest line mapped the boundary between relatively high and low erosion rates between the upstream facing and downstream facing surfaces respectively observations from unscaled flume experiments show that bedload particles transit a flow obstacle by travelling up the upstream facing surface where erosion is focused and hop over the top of the obstacle our field and flume observations indicate that ufcss observed in natural rivers are produced by bedload abrasion which by being decoupled from the flow can only impact the stoss surface to any significant extent micropitting found on polished downstream facing surfaces indicate that they are dominantly formed by suspended load abrasion the aspect of a surface its position in a channel cross section the topography of the channel upstream and the local topography of the bedrock bedform are significant controls on the location and magnitude of abrasion by bedload and resultant surface textures because of their effect on steering a bedload particle stream the high erosion rates observed indicate that abrasion by bedload is the controlling process in the formation of this variety of bedrock bedform and possibly the channel at large in this setting

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

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

an introduction to the global positioning system and some geological applications receivers equipped to measure dual frequency carrier phase signals from satellites of the global positioning system gps have been capable under special conditions of determining relative horizontal positions among stations separated by one to a few hundred kilometers with a precision of one to several millimeters since the early 1980s the major obstacles to making this capability routine extending it to all parts of the globe and extending it to longer station separations have been equipment cost limitations in the gps satellite constellation arduous data analysis uncertainties in satellite orbits uncertainties in propagation delays associated with variable tropospheric water vapor and difficulties in resolving carrier phase cycle ambiguities recent improvements have occurred in all these areas the increasing ease and reduced cost of gps data acquisition and analysis are having a significant impact on studies of near fault crustal deformation and earthquake processes until recently the province of conventional terrestrial geodetic techniques the enhanced satellite constellation improved models and establishment of global tracking networks have extended several millimeters horizontal positioning capability to station separations of 1000 km or more in virtually all parts of the world this enables study of new classes of tectonic problems that previously were difficult to attack with any geodetic technique examples include a complete kinematic description of ongoing crustal deformation in broad complex continental plate boundary zones and measurement of relative plate motion at convergent boundaries where global models may be poorly constrained

coseismic slip resolution and post seismic relaxation time of the 1999 chi chi taiwan earthquake as constrained by geological observations geodetic measurements and seismicity summary we analysed the surface displacements estimated by global positioning system gps mea surements and systeme probatoire d observation de la terre spot image correlation and the seismicity change induced by the mw 7 6 1999 september 20 chi chi earthquake we first modelled the seismogenic zone for which we introduced a realistic geologically constrained ramp and flat geometry and estimated by joint inversion of the gps and spot data the co seismic slip distribution stressing the resolution of the model parameters the fair adjustment indicates that the ramp and flat geometry is consistent with the geodetic measurement the cal culated coseismic slip increases both towards the surface and northwards with maximum slip of approximately 15 m in good agreement with the observed surface rupture and previously ob tained slip models based on seismic waveforms this distribution may be represented to a good approximation by a linear increase of slip with a constant azimuth in the plate convergence direction special emphasis is given to the analysis of the model resolution which sharply decreases with depth in spite of the exceptionally well distributed and high quality inverted data this resolution decrease expected for any slip distribution based on surface observations only led us then to examine the seismicity as independent data to test our model following the static stress triggering hypothesis the model is accurately defined in terms of coulomb stress change in the central part of the rupture where the fault geometry is well constrained by seismic profile in contrast at the southern and northern ends it is less appropriate however the static stress triggering approach is not precise enough to determine whether fault geometry or slip distribution is less suitable we thus conclude that the analysis of seismicity in terms of coulomb stress change might not allow us to examine small scale features of seismogenic zones finally we discuss post seismic relaxation from the temporal variation of seismicity rate in contrast to the relaxation time of 0 23 0 90 yr inferred from post seismic permanent gps measurements we found that the decay of aftershock sequence follows omori s law with an 8 14 yr relaxation time we suggest that this discrepancy results from the coexistence of several different processes not all associated with the production of aftershocks

uplift shortening and steady state topography in active mountain belts we present a tectonic surface process model used to investigate the role of horizontal shortening in convergent orogens and the effects on steady state topography the tectonic model consists of a specified velocity field for the earth9s surface and includes a constant uplift rate and a constant horizontal strain rate which varies to reflect the relative importance of frontal accretion and underplating in an orogenic wedge the surface process model includes incision of a network of rivers formed by collection of applied precipitation and diffusive hillslope mass transfer three non dimensional parameters describe this model a ratio of the maximum horizontal velocity to the vertical velocity a peclet number expressing the efficiency of the hillslope diffusion relative to the uplift rate and a fluvial erosion number reflecting the fluvial incision efficiency relative to the uplift rate a series of models are presented demonstrating the resultant steady state landforms parameterized by these three numbers a finite velocity ratio results in an asymmetric form to the model mountain range although the magnitude of the asymmetry also depends on the peclet number topographic steady state is achieved faster for models with no horizontal component to the velocity field with finite horizontal velocity topographic steady state is achieved only at the scale of the entire mountain range even the first order drainage basins are unstable with time in the presence of horizontal shortening we compare our model results to topographic profiles from active mountain ranges in taiwan new zealand and the olympic mountains of washington state all these examples exhibit asymmetric topographic form with the asymmetry consistent with the polarity of subduction suggesting that horizontal tectonic motion is affecting the macro geomorphic form of these ranges

mountain building in taiwan insights from 3 d geodynamic models taiwan is widely considered to be a typical example of an arc continent collision surrounded by two opposite dipping subduction zones the manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood various hypotheses have been proposed but the geodynamic feasibility of those remains to be tested here we present 3 d thermomechanical models to study the geodynamic evolution process of a taiwan like setting after an initial transform fault was consumed in our model setup the boundary between the eurasian plate and the south china sea is northeast trending the results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small scale mountain belt with high topography and crustal exhumation occurs in most cases the eurasian continental crust is exhumed in a dome like manner exposing higher grade metamorphic rocks facilitated by high erosion rates and a weak continental lower crust rheology but inhibited by the presence of a weak arc a high topography within the orogen as well as continental slab detachment can develop for the convergence direction of n307 degrees and large convergence rates our modeling results are thus generally consistent with the eurasian slab tearing model proposed for taiwan based on seismic tomographic studies and we suggest that the main characteristic features in taiwan can be explained by the combined effects of fast erosion a weak lower crust fast convergence and a small convergence azimuth

tectonic analysis and the evolution of a curved collision belt the hsuehshan range northern taiwan abstract detailed studies of fracture patterns and paleostress reconstructions based on analyses of fault slip data sets allow identification of four main patterns of compressional paleostress orientations in the cenozoic rocks of northern taiwan these patterns correspond to five successive tectonic events dominated by 1 n s 2 wsw ene 3 wnw ese 4 nw se and 5 wnw ese directions of compression the first two events are characterized by relatively minor structures in the foreland fold and thrust belt the first event probably reflects major left lateral shear along nne ssw trends which occurred in the adjacent eastern metamorphic belt during the middle late miocene these first two events do not affect the plio pleistocene in the localities studied in contrast the last three events created the major compressional structures of the foreland belt during the plio pleistocene collision in taiwan preliminary studies suggest that the magnetic fabric of the rocks has been significantly influenced by compressional deformation along wnw ese and nw se trends especially in the slate belt thus confirming the importance of these last three events on the basis of paleomagnetic work in progress the northernmost segment of the taiwan belt has been affected by a clockwise rotation of about 20 taking into account the distribution of successive compressional paleostress patterns we conclude that of the 55 angle of the sharply curved belt of northern taiwan about 20 probably correspond to actual clockwise rotation during the plio pleistocene whereas the remaining 35 reflect the initial shape of the curved plio quaternary belt the paleostress history of northern taiwan is examined in terms of geodynamic evolution and kinematic reconstructions the plio quaternary development of the collision between the luzon arc and eurasia followed a late miocene predominantly transform motion from south to north of the philippine sea plate relative to eurasia the succession of plio pleistocene paleostress patterns in northern taiwan is found to be consistent with the longitudinal shift from northeast to southwest and the transverse migration from the inner to the outer zones of compressional phenomena related to the evolution of oblique collision some major changes in compressional trends are also accounted for by variations in the kinematics of plate convergence the most recent charge comprises a counterclockwise change in the direction of philippine sea plate eurasia convergence

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

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

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

identification and tectonic implications of nano particle quartz 50 nm by synchrotron x ray diffraction in the chelungpu fault gouge taiwan abstract to determine the ultrafine nano scale grains in a fault gouge the chelungpu fault gouge was sampled and analyzed it was separated into different particle size ranges and was analyzed by synchrotron x ray diffraction and transmission electron microscopy the minerals of gouge are predominantly composed of quartz plagioclase smectite illite chlorite and kaolinite the mineral association of

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

plate tearing in the northwestern corner of the subducting philippine sea plate the philippine sea plate phs simultaneously subducts northwestward and collides eastward with the eurasian plate eu in northeast taiwan these two tectonic events induce high seismic activity which makes northeastern taiwan one of the most seismically active zones in the world to understand the mechanical processes at work we used existing geophysical data and the aftershocks recorded following a recent large strike slip event occurring within the phs oceanic crust during this event a nw se trending left lateral sub parallel to the phs eu convergence vector was active as a consequence of the collision subduction plate geometry we show that the lithosphere of the northwestern corner of the phs has been torn in a nw se orientation this tectonic feature is associated with an abrupt tectonic stress boundary and could generate large intra plate earthquakes

assessment of global positioning system measurements for studies of crustal deformation the u s geological survey has been using the global positioning system gps to perform repeated surveys of networks of different sizes since january 1986 the intersite distances from these various networks are 223 km for a single line connecting the palos verdes and vandenberg sites in southern california 30 50 km for a network of three sites south of the san francisco bay near loma prieta 9 30 km for a network located near hebgen lake montana 7 12 km for a network of sites in the vicinity of parkfield california and 240 m for a line located at the goldstone antenna complex in the mojave desert the lengths quoted do not include the intersite distances for the fiducial sites included in the solutions for orbit determination which can be up to 8000 km we have attempted to assess the long term precision and accuracy of determinations of relative site position for a range of line lengths this assessment has consisted of several different studies in assessing precision we have generally investigated repeatability usually about a best fit linear temporal trend the smallest fractional repeatability was yielded by the measurements on the palos verdes to vandenberg line the root mean square rms residual about the best fit line for these determinations was 6 mm 0 03 parts per million or ppm for the north component 11 mm 0 05 ppm for the east component and 40 mm 0 18 ppm for the vertical component for these determinations gps data from 3 4 consecutive days were combined to produce a single estimate of the vector separation we also used the day to day scatter from these measurements to perform an analysis of variance and found that these multiday determinations can be significantly more precise than estimates determined from a single observing session the worst results were obtained for the loma prieta network where we obtained rms residuals of 11 12 mm 0 3 0 4 ppm for north 14 18 mm 0 3 0 6 ppm for east and 18 21 mm 0 4 0 7 ppm for vertical we cannot definitely explain these poor results but we have evidence to indicate that this poor scatter is due to the short observing session 4 5 hours compared to 6 5 hours for the palos verdes to vandenberg results in assessing accuracy we compared the gps results to those from independent techniques at parkfield we compared the gps estimates of fault parallel motion to those from creep meters and alignment arrays and found excellent agreement within 1 2 mm yr 1 not only for the slip but also for the variation of slip along the san andreas fault we compared the loma prieta and hebgen lake line length estimates to those obtained from a geodolite a high precision laser distance measurement device that the u s geological survey has used for 20 years for studies of crustal deformation if we model the geodolite minus gps line length differences l as l l where l is the line length then we find 0 6 0 5 mm and 0 1 0 1 mm km 1 the uncertainties quoted are one standard deviation from this comparison we conclude that at this level of significance there is no detectable difference between lengths determined from geodolite and gps data finally we compared the gps estimates of the palos verdes to vandenberg line length to those obtained from very long baseline interferometry vlbi the mean difference between the six gps estimates of line length and a best fit line through the vlbi estimates was 1 mm and the rms difference was 12 mm 0 05 ppm close to the standard deviation of the gps measurements for this predominantly east west line

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

paleoseismology of the rueisuei segment of the longitudinal valley fault eastern taiwan paleoseismic trench excavations in the dafu area of eastern taiwan have provided data on the rupture history of the rueisuei segment of the longitudinal valley fault during the late holocene the 1951 taitung earthquake ruptured the dafu site which is characterized by several terrace raises with late holocene sediments uplifted by the westward thrust fault trenches across the northwest facing fault scarp exposed fluvial and alluvial deposit sediments although nearly all of the 23 radiocarbon ages vary somewhat within each layer the overall age determined for each layer is in good accord with the stratigraphic ordering of these layers based on stratigraphic ordering and a statistical comparison of radiocarbon dates using the oxcal program we estimate that two pre 1951 earthquake surface ruptures at the dafu site occurred in the periods a d 1736 1898 and a d 1564 1680 the same oxcal model constrains the past two recurrence intervals to about 165 and 140 yr although with sizable uncertainties 55 215 and 90 260 yr respectively which are 95 ranges through the correlation of three trenches across the longitudinal valley fault we are able to identify evidence for at least three earthquakes with moment magnitudes of about 7 0 7 2 that occurred up to 390 yr prior to and during 1951 furthermore based on the radiocarbon dates the mean recurrence interval is roughly 150 yr uncertainty is indeterminate with a minimum vertical uplift rate of 8 5 12 2 mm yr

motion of the philippine sea plate consistent with the nuvel 1a model summary we determine euler vectors for 12 plates including the philippine sea plate ph relative to the fixed pacific plate pa by inverting the earthquake slip vectors along the boundaries of the philippine sea plate gps observed velocities and 1122 data from the nuvel 1 and the nuvel 1a global plate motion model respectively this analysis thus also yields euler vectors for the philippine sea plate relative to adjacent plates our results are consistent with observed data and can satisfy the geological and geophysical constraints along the caroline cr ph and pa cr boundaries the results also give insight into internal deformation of the philippine sea plate the area enclosed by the ryukyu trench nankai trough izu bonin trench and gps stations s102 s063 and okino torishima moves uniformly as a rigid plate but the areas near the philippine trench mariana trough and yap palau trench have obvious deformation

measurement of crustal deformation using the global positioning system during the past several years there has been a tremendous surge of interest in geodesy among geophysicists interested in the kinematics and dynamics of crustal deformation i n 1 980 there were 30 talks concerning geodesy applied to tectonics given at the spring and fall meetings of the american geophysical union agu of these over half were authored by scientists at government agencies charged with geodetic missions fewer than five were by academic scientists who had collected the data that they discussed in contrast in 1990 almost three times as many papers were given at agu almost halfby academic scientists addressing data that they themselves had participated in collecting the total number of authors increased even more dramatically from 75 in 1980 to 325 in 1 990 this remarkable increase in geodetic activity in the past decade par ticularly by academics has largely resulted from the opportunities pro vided by the development of a new geodetic tool high accuracy relative vector positioning made possible by analyzing radio signals broadcast by the global positioning system gps satellites operated by the united states department of defense the development of gps geodesy was greatly facilitated by advances in the space geodetic techniques of very long baseline interferometry vlbi and satellite laser ranging slr

revealing coseismic displacements and the deformation zones of the 1999 chi chi earthquake in the tsaotung area central taiwan using digital cadastral data 1 the 1999 chi chi taiwan earthquake mw 7 6 was the largest earthquake to strike taiwan in the twentieth century this earthquake is associated with a 100 km long surface rupture in order to reveal the details of displacement near the surface rupture we use a digital cadastral system to calculate coseismic displacement around the tsaotung area central taiwan the digital cadastral system was originally conceived to survey land and building boundaries in the tsaotung area taiwan authorities have taken digital cadastral measurements before and after the chi chi earthquake the cadastral system affords high density control points that reach 1421 points km2 a system denser than that of the gps accuracy is to within 11 cm a level that is higher than spot imaging and one that allows us to study surface deformation in detail coseismic displacement is 4 3 4 6 m at distance from the surface rupture and decreases to 3 4 m near the surface rupture the azimuth of horizontal displacements is 310 315 and rotates to 280 305 near the surface rupture this produced a compression left lateral deformation zone with 10 3 compression strain near the surface rupture coseismic displacement of the footwall is 1 1 2 m in 110 which is similar to that from using gps data in the tsaotung thrust slice we observed that the azimuth of horizontal displacement rotates from a nw trend to a south trend as a result of slip partitioning and gravity slide effect

detecting rock uplift across southern taiwan mountain belt by integrated gps and leveling data abstract rock uplift on the earth surface is a key observation for studies of tectonics and geodynamic processes geodetic measurements from global positioning system gps and leveling are able to track vertical deformation over a wide range of spatial and temporal scale taiwan mountain belt is subject to a rapid uplift rate of 20 mm yr as revealed by gps and leveling measurements in previous studies the extremely high rates motivate us to analyze a set of newly processed gps and leveling data along a nw se transect across southern taiwan by considering the effects of the earthquake related deformation as well as hydrological and surface processes our estimates of rock uplift rates from gps and leveling are from 12 to 14 mm yr across the southern central range of taiwan vertical velocity changes on the small spatial scale are primary related to fault locking in the seismic cycle the large spatial feature showing the mean uplift rate of 6 mm yr between the eastern central range and the frontal thrust faults can be explained by a large scale pop up structure assuming the depth of 15 20 km in the orogenic wedge and the horizontal shortening of 30 mm yr our results draw attention to carefully study geodetic vertical deformation in the mountain belts and seek relevant causes

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

geodetic measurement of horizontal crustal deformation in eastern taiwan abstract four medium aperture trilateration networks in eastern taiwan have been surveyed three to four times since 1981 one is the ilan network crossing an elongated active seismic zone the others are the hualien yuli and taitung networks located at the northernmost middle and southernmost portions of the longitudinal valley respectively based on changes of the observed line length the three components of the surface strain rate tensor for each of the networks are obtained by a least squares adjustment technique then the principal strain rates are calculated the ilan network gives a principal strain rate of uniaxial extension at 2 3 strain yr in the direction of n45 w the hualien network has a principal strain rate of 1 2 strain yr extension in n62 e and 1 2 strain yr contraction in n152 e the yuli network yields essentially a principal strain rate of uniaxial contraction at 8 4 strain yr in n117 e whereas the taitung network has a principal strain rate of 1 5 strain yr extension in n24 e and 3 9 strain yr contraction in n114 e the directions of contractionof both the yuli and taitung networks are consistent with the direction of the maximum compressive stress of this area furthermore average velocities of the relative motion between geodetic stations in the central range and the coastal range are estimated from the average rates of changes in line length stations in the hualien network show a left lateral relative motion in a direction more or less parallel to the strike of the longitudinal valley while stations in the yuli and taitung networks move toward each other in the direction approximately perpendicular to the trend of the longitudinal valley

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

application of sar interferometry to a large thrust deformation the 1999 mw 7 6 chichi earthquake in central taiwan summary application of the interferometric method to four ers2 sar images acquired before and after the 1999 chichi earthquake has allowed determination of the coseismic surface displacement in the footwall area of the chelungpu fault the interferometric results revealed a relative shortening in the round trip distance between the radar antenna and the ground of the footwall side of chelungpu fault during the earthquake this shortening progressively increased from the west to the east and reaches the maximum amount of approximately 26 cm near the central segment of the chelungpu fault our interferometric results have been precisely examined using a dense gps network in the investigated area we mapped the gps coseismic measurements into the radar line of sight and implemented a forward simulation of sar interferogram from this synthesized result to control our unwrapping performance in this study these two observations are compared with a 3 d dislocation model of the fault finally a deformation analysis based on our interferometric result has indicated that a segment with irregular deformation behaviour can be distinguished in the footwall area of the chelungpu fault this segment may result from either the influence of inherited basement faults or the presence of a structural terrace that provide local opportunities for superficial deformation

present day surface deformation and tectonic insights of the extensional ilan plain ne taiwan abstract taiwan s mountain belt is an ideal location to address major questions regarding mechanisms of lithospheric deformation in convergent settings mountain building processes from oceanic subduction to continental subduction and post orogenic extension in the northeast of this belt the ilan plain is a triangular deltaic plain characterized by a flat topography close to the sea level and surrounded by the high mountains of the hsuehshan range to the northwest and the central range to the southeast its eastern coast faces the western tip of the okinawa trough the back arc basin of the ryukyu subduction zone in this study we analyzed the present day surface deformation of the ilan plain aiming at deciphering its relationships with basement faults and the regional geodynamic setting our approach is mainly based on surface vertical displacements revealed by persistent scatterer interferometry synthetic aperture radar psi which indicate that there is an area of active subsidence 18 mm yr located in the southern part of the plain in probable connection with active basement faults and in agreement with previous geodetic measurements and existing geophysical data our psi results also suggest that the subsidence occurring in the ilan plain has moved from north to south during quaternary in relation with extrusion of the belt due to the westward propagation of the okinawa trough through the taiwan mountains

experimental insights into the partitioning of motion within zones of oblique subduction abstract in zones of oblique convergence the motion is often partitioned into subduction and an associated strike slip component fitch 1972 in this paper we first describe the geodynamic setting of the philippine region as a case study of deformation by strike slip faulting behind parallel subduction zones then we present six small scale laboratory experiments with analogue materials to investigate how partitioning may be controlled by internal boundaries between oceanic lithosphere and continental or island arc lithosphere during the early stages of a typical experiment deformation is taken up on oblique thrusts subduction zones which root into internal boundaries paired strike slip faults develop at later stages thereafter subduction zones and paired strike slip faults operate simultaneously the partitioning occurs without any need for complex constraints on external boundaries

geological and morphological study of the jiufengershan landslide triggered by the chi chi taiwan earthquake the jiufengershan rock and soil avalanche is one of the largest landslides triggered by the chi chi earthquake taiwan 1999 the landslide destabilized the western limb of the taanshan syncline along a weak stratigraphic layer it involved a flatiron remnant which was almost entirely mobilized during the earthquake the avalanche was slowed down by ns trending ridges located downstream along the jiutsaihu creek the landslide affected a 60 m thick and 1 5 km long sedimentary pile composed of shales and sandstones which dip 228se toward a transverse valley the triggering mechanism and the sliding process were analyzed by means of geological and morphological data from aerial photographs and observed in the field a high resolution airborne light detection and ranging lidar image taken 2 5 years after the landslide allows the identification of morphological structures along the sliding surface and the landslide accumulation the sliding surface shows several deformation structures such as fault scarps and folds these structures are interpreted in terms of basal shear stresses created during the avalanche three major joint sets were identified at the sliding surface the isopach map of the landslide was calculated from the comparison between elevation models before and after the earthquake the coseismic volume of mobilized material and landslide deposit data are 4210 6 m 3 and 5010 6 m 3 respectively the geometry of the landslide accumulation in the field has an irregular star shape the morphology of the deposit area shows a sequence of smooth reliefs and depressions that contrast with the neighboring ridges d 2005 elsevier b v all rights reserved

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

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

seismicity controlled by a frictional afterslip during a small magnitude seismic sequence m l 5 on the chihshang fault taiwan we report evidence for frictional afterslip at shallow depths about 5 to 7 km during a small magnitude seismic sequence with ml 5 along the chihshang fault a main active structure of the longitudinal valley in southeast taiwan the afterslip which was recorded by a nearby borehole dilatometer lasted about a month with a cumulative geodetic moment magnitude of 4 8 0 2 the afterslip comprised two stages and controlled the aftershock sequence the first postseismic stage which followed a ml 4 6 earthquake lasted about 6 h and mostly controlled the ruptures of neighboring asperities e g multiplets near the hypocenter then a 4 week duration large afterslip event following a ml 4 9 earthquake controlled the rate of aftershocks during its first 2 days through brittle creep the study presents a rare case of simultaneous seismological and geodetic observations for afterslip following earthquakes with magnitude lower than 5 furthermore the geodetic moment of the postseismic phase is at least equivalent to the coseismic moment of the sequence

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

an introduction to the tectonophysics special issue geodynamics and environment in east asia tribute to jacques angelier 1947 2010

tectonic foliations and the distribution of landslides in the southern central range taiwan abstract geometry of planner geological structures can interact with topographic slope and cause different types of slope failure e g wedge failure dip slope failure and toppling etc this study focus on how the tectonic foliation influences the landslide pattern in the southern central range of taiwan the tectonic foliations in this area range from pencil cleavage to schistosity depending on the increasing of metamorphic grade and different foliations have different influences on the landslide behaviors to describe the relation between foliation and topographic slope we create the d value as the angle between the foliation dip direction azimuth and the topographic downslope direction azimuth at a given location the location of landslides in the area with pencil cleavage has no preferential d value and it suggests the landslide distribution has no correlation with foliation the location of landslides in the area with well developed slaty cleavage is mostly in the lower d value which suggests the landslides are more likely located in the area where the foliation dip direction is parallel to the topographic downslope direction the location of landslide in the area with schistosity is mostly in the high d value area which suggests the landslides are more likely located in the area where the foliation dip direction is opposite to the topographic downslope direction

discrete element simulation of the jiufengershan rock and soil avalanche triggered by the 1999 chi chi earthquake taiwan we present contact dynamics discrete element simulations of the earthquake triggered jiufengershan avalanche which mobilized a 60 m thick 1 5 km long sedimentary layer dipping similar to 22 degrees se toward a valley the dynamic behavior of the avalanche is simulated under different assumptions about rock behavior water table height and boundary shear strength additionally seismic shaking is introduced using strong motion records from nearby stations we assume that seismic shaking generates shearing and frictional heating along the surface of rupture which in turn may induce dynamic weakening and avalanche triggering a simple slip weakening criterion was adopted to simulate shear strength drop along the rupture surface we investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers incipient deformation forms a pop up structure at the toe of the dip slope as the avalanche propagates the pop up deforms into an overturned fold which overrides the surface of separation along a decollement simultaneously uphill layers slide at high velocity 125 km h and are folded and disrupted as they reach the toe of the dip slope the avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear we simulated five cross sections across the jiufengershan avalanche which differ in the geometry of the surface of separation topographic and simulated surface profiles are similar the friction coefficient at the surface of separation determined from back analysis is abnormally low mu ss 0 2 possibly due to lubrication by liquefied soils the granular deposits of simulated earthquake and rain triggered avalanches are similar

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