seismotectonics of northeastern taiwan kinematics of the transition from waning collision to subduction and postcollisional extension 1 the northeastern region of taiwan is one of the few places on earth that experienced a transition from collision to subduction and back arc opening in a young orogenic belt this provides us a rare opportunity to study the structural characteristics in such a transition we attempt to analyze the seismicity and stress patterns in this transitional region using data obtained from a dense and high quality seismic network our results show that from south to north fault types change from thrust to strike slip then to normal faulting the transitional domain between ilan and hualien is dominated by left lateral strike slip faulting with a nnw sse minimum principal stress axis consistent with the opening of the okinawa trough to the north the ilan plain is formed by structural activities of a series of normal fault systems to the south the spatial distribution of an earthquake cluster north of taiwan s coastal range appears to parallel the subducting interface between the philippine sea and eurasian plates this implies the northern extension of the coastal range has subducted consequently the opening of the okinawa trough becomes an important tectonic driving force in northeastern taiwan the westernmost ryukyu arc is pushed by the opening okinawa trough and moves southward the northern part of taiwan s central range moves with the arc to bend southeastward and the ilan plain forms in the space north of the range the bending of the central range belt results in bedding plane slip with left lateral strike slip earthquakes in the transitional domain to accommodate the deformation

the two stage opening of the western central mediterranean basins a forward modeling test to a new evolutionary model abstract the algero provencal and the tyrrhenian extensional basins developed in two stages in a geodynamic setting characterized by the nearly n s convergence between africa and eurasia the spreading of the provencal basin occurred in the early miocene following a long period of rifting in the western mediterranean area a dramatic eastward shift of the active extensional deformation resulted in the tortonian to quaternary opening of the tyrrhenian basin in a companion paper carminati et al propose that a trench retreat process in a geodynamic setting locked by the continental collisions in the alps and in the betic chain is a viable mechanism for the late oligocene early langhian opening of the western mediterranean a langhian slab detachment episode along the north african margin is likely to have caused the end of the trench retreat along this part of the boundary determining the end of active expansion in the western mediterranean and the beginning of active extension in the tyrrhenian basin the objective of the present paper is to quantitatively test this proposed scenario we calculate by means of a thin shell model the effects of these plate boundary reorganizations on the european stress and strain field we show that the two stage opening of the western and central mediterranean can be explained by the evolution proposed by carminati et al and that in particular the eastward shift of the active extension which produced the termination of the first opening stage and the beginning of the second is likely to have been triggered by the slab detachment episode along the north african margin

recent advances in sar interferometry time series analysis for measuring crustal deformation abstract synthetic aperture radar sar interferometry is a technique that permits remote detection of deformation at the earth s surface and has been used extensively to measure displacements associated with earthquakes volcanic activity and many other crustal deformation phenomena analysis of a time series of sar images extends the area where interferometry can be successfully applied and also allows detection of smaller displacements through the reduction of error sources here we review recent advances in time series sar interferometry methods that further improve accuracy this is particularly important when constraining displacements due to processes with low strain rates such as interseismic deformation we include examples of improved algorithms applied to image deformation associated with the 2010 eruption of eyjafjallajokull volcano in iceland slow slip on the guerrero subduction zone in mexico and tectonic deformation in western anatolia turkey

origin and geodynamic implication of the dupal isotopic anomaly in volcanic rocks from the philippine island arcs newly acquired sr nd and pb isotope data combined with other recently available data in the literature clearly show that philippine arc volcanic rocks do not have a true dupal isotopic anomaly instead most of the volcanic rocks must have come from a mantle source with a dupal like indian ocean to enriched ocean island basalt composition this mantle source is present beneath the philippine sea plate and the south china sea plate on either side of the philippine arc systems the isotopic signature of philippine arc volcanics is most probably inherited from the previous southern locations of the different geologic terranes that compose the modern philippine island arcs the new data also show that the isotopic signature of philippine arc volcanics similar to volcanic rocks from other western pacific island arc back arc settings is mainly a function of the mantle source and was only recently modified by a component from subducted sediments and crust

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

crustal structure and deformation at the northern manila trench between taiwan and luzon islands abstract the philippine sea plate overrides the eurasian plate along the east dipping manila trench between taiwan and luzon islands from south to north the plate convergence gradually evolves from normal subduction of the south china sea lithosphere to initial collision of the taiwan orogen the subduction related earthquakes become diffusive close to taiwan the accretionary prism is dramatically wider toward taiwan to understand the plate convergent features of this subduction collision transition zone we have analyzed twelve seismic reflection profiles across the manila trench between luzon and taiwan the results show that the basement of northern south china sea basin generally dips toward east and south the northern manila trench accumulates more trench fill sediments in the south than in the north a sequence boundary t0 is suggested to distinguish the hemipelagic and trench fill sediments probably due to the collision in taiwan the sequence boundary t0 displays more gentle slope or has been uplifted in the north structural analysis shows that the subducting crust in the northern manila trench area can be characterized by three distinctive zones a normal fault zone nfz a proto thrust zone ptz and a thrust zone tz the nfz is defined by the distribution of numerous normal faults in the top or upper portion of the subducting crust the normal faults are gradually buried by trench fill sediments when they are closer to the deep trench it is suggested that normal fault may take place at the location where the crust starts to bend and induces gravity sliding of the upper sedimentary layers some buried normal faults could be reactivated to blind thrust faults because of stronger plate convergence near the accretionary prism the ptz is located between the nfz and the frontal thrust of the accretionary prism it contains blind thrust faults or folds instead of thrust faults the successive distribution of the crustal structures of normal faults blind thrust and thrust faults at the trench area suggests that the blind thrust faults develop along the location of pre existing normal faults the brittle deformation occurs at the lower part of the sedimentary layers probably because of stronger compaction and less water content eventually blind thrust faults may propagate upward and become thrust faults at the seabed

tectonic contraction across los angeles after removal of groundwater pumping effects after the 1987 whittier narrows and 1994 northridge earthquakes revealed that blind thrust faults represent a significant threat to metropolitan los angeles a network of 250 continuously recording global positioning system gps stations was deployed to monitor displacements associated with deep slip on both blind and surface faults here we augment this gps data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike slip faulting after removing these non tectonic signals we are left with 4 4 mm yr 1 of uniaxial contraction across the los angeles basin oriented n 36 degrees e perpendicular to the major strike slip faults in the area this indicates that the contraction is primarily accommodated on thrust faults rather than on the northeast trending strike slip faults we have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults in the 40 km long santa ana basin groundwater withdrawal and re injection produces 12 mm yr 1 of long term subsidence accompanied by an unprecedented seasonal oscillation of 55 mm in the vertical direction and 7 mm horizontally

the 2005 ilan earthquake doublet and seismic crisis in northeastern taiwan evidence for dyke intrusion associated with on land propagation of the okinawa trough summary northern taiwan underwent mountain building in the early stage of the taiwan orogeny but is currently subjected to post collisional crustal extension it may be related to gravitational collapse or to the rifting of the okinawa trough which lies offshore northeastern taiwan the ilan plain northeastern taiwan which is bounded by the normal fault systems and filled up with thick pliocene pleistocene sedimentary sequences formed under such an extension environment over there on 2005 march 5 two earthquakes with about the same magnitude m l 5 9 occurred within 68 s and produced intense aftershocks activity according to the records of central weather bureau seismic network of taiwan we relocated the earthquake sequence by the three dimension earthquake location algorithm with the newly published 3d vp and vp vs velocity model and determined the first polarity focal mechanisms of the earthquake doublet one major cluster of aftershocks which trends e w and dips steeply to the south can be identified and picked up as a potential fault plane the focal mechanisms of the two main shocks are both classified as normal type by first polarity but strike slip by centroid moment tensor inversion however two methods both yield consistent e w strike static coseismic deformation was additionally determined from global positioning system gps daily solutions at a set of continuous gps stations and from strong motion seismographs these data show nw se extension at high angle to the fault plane which cannot be explained from a simple strike slip double couple mechanism on the other hand the small vertical displacements and steep fault plane cannot be explained from a simple normal event as well we present from elastic dislocation modelling that the geodetic data are best explained by significant component of tensile source with centimetre scale of opening on a 15 km long fault extending from 1 to 13 km depth we therefore interpret the crisis as the result of dyke intrusion at the very tip of the okinawa trough which is reasonably driven by backarc spreading

measurement of interseismic strain across the haiyuan fault gansu china by insar the haiyuan fault is part of a major left lateral fault system at the northeastern edge of the tibet qinghai plateau two m8 earthquakes 1920 and 1927 occurred along the fault bracketing an unbroken section of the fault identified as the tianzhu seismic gap we use interferometric synthetic aperture radar data from descending orbits of the ers satellites acquired between 1993 and 1998 along two adjacent tracks covering the gap to measure the current surface movements and better understand the present day mechanical behavior of this fault section the analysis of the radar data involves first the combined correction of orbital errors and errors associated with the phase delay through the troposphere a subset of the data is then selected based on the analysis of the residual noise spectra for each pair of data the selected interferograms are stacked and the average phase change rate is converted in fault parallel velocity assuming that the ground movement is horizontal and parallel to the fault velocity maps from both tracks show a zone of high velocity gradient across the fault a few kilometers wide consistent with left lateral slip on the haiyuan fault the average velocity field from the two tracks in their overlapping area is well fit with a single screw dislocation model in an elastic half space the derived fault slip rate at depth 4 2 8 mm yr is consistent with recent gps results the corresponding shallow apparent locking depth 0 4 2 km can be explained by a current low stress accumulation on the fault due to creep almost on the entire fault plane however unless it is transient this creep would be paradoxical with the occurrence of past large earthquakes along this fault section as revealed by paleoseismology an alternative model implying both shallow creep in the brittle upper crust and deep aseismic slip beneath the seismogenic layer separated by a locked section would be consistent with insar observations and with the potential for large earthquakes on the fault as well a two dislocation model with slip at 5 mm yr beneath 15 km and a transient creep rate of 11 mm yr between 2 and 7 km fits the insar data however the width and creep rate of the shallow creeping zone and their possible along strike variations are still poorly resolved with the present data set

late holocene paleoearthquake activity in the middle part of the longitudinal valley fault eastern taiwan abstract the longitudinal valley fault in eastern taiwan is the most important arc continent collisional boundary fault between the philippine sea plate and the eurasian continent the 50 amount of collisional deformation partitioned on the fault has considered implications for seismic hazard our paleoseismological work on the middle segment the juisui fault of the longitudinal valley fault has shown that the fault occurred intensive seismicity of four paleoearthquakes of 1951 ad p2 event

relocating the historical 1951 hualien earthquake in eastern taiwan based on tide gauge record summary a sequence of large earthquakes happened along the longitudinal valley in eastern taiwan in 1951 thefirstthree andlargest earthquakestookplaceneartheharbourcityofhualienatthe northerntipofthevalleyduringthefirstday thesecondofwhich eventii severelydamaged the city the epicentre of event ii had previously been allocated at some 15 km offshore which would result in a vertical subsidence of the hualien city according to model calculations for the supposed seismic source mechanism however the hualien tide gauge record speaks the opposite the site experienced a co seismic uplift by 255 mm due to event ii we conduct a series of numerical grid search simulations and determine a most likely scenario for the earthquake sequence we assert that event ii s hypocentre was actually on land rather than offshore right along the recognized active milun fault below the city of hualien at a very shallow depth this result advances our understanding of the spatio temporal seismotectonic behaviour associated with these historical events and is important towards the assessment of the potential seismic and tsunami hazards of the region

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

kinematic analyses of the hsuehshan range taiwan a large scale pop up structure the hsuehshan range exposed in the northern and central taiwan slate belt is a fault bounded structural high cored by biotite grade slates and metasandstones syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern hsuehshan range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland near the eastern boundary of the hsuehshan range however pressure shadows record noncoaxial strain histories consistent with a top to the east sense of shear along a steep nw dipping shear zone this noncoaxiality is attributed to se directed backthrusting on the lishan fault which separates the higher grade eo oligocene rocks of the hsuehshan range from the lower grade miocene rocks of the backbone range because it is bounded to the east by the se vergent lishan fault and to the west by a series of nw vergent thrusts e g the chuchih fault the hsuehshan range is envisaged as a pop up structure strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces e g the lishan fault the hsuehshan range tapers in width to the south thus the pop up may be buried or die out to the south where the collision is younger

systematic insar tropospheric phase delay corrections from global meteorological reanalysis data 1 despite remarkable successes achieved by differential insar estimations of low tectonic strain rates remain challenging in areas where deformation and topography are correlated mainly because of the topography related atmospheric phase screen aps in areas of high relief empirical removal of the stratified component of the aps may lead to biased estimations of tectonic deformation rates here we describe a method to correct interferograms from the effects of the spatial and temporal variations in tropospheric stratification by computing tropospheric delay maps coincident with sar acquisitions using the era interim global meteorological model the modeled phase delay is integrated along vertical profiles at the era i grid nodes and interpolated at the spatial sampling of the interferograms above the elevation of each image pixel this approach is validated on unwrapped interferograms we show that the removal of the atmospheric signal before phase unwrapping reduces the risk of unwrapping errors in areas of rough topography

temporal and spatial variation of stress field in taiwan from 1991 to 2007 insights from comprehensive first motion focal mechanism catalog article i nfo the total amount of 4 761 focal mechanisms was determined based on p wave first motion polarities from 1991 to 2007 in taiwan region this dataset offers us a good opportunity to examine temporal and spatial variability of the stress field we find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the chi chi earthquake the orientation of the maximum horizontal compressive stress axes sh shows a general agreement with the direction of plate motion between a depth range of 0 30 km the 20 anticlockwise rotation of sh from the longitudinal valley lv to western taiwan is probably caused by the left lateral motion on the longitudinal valley fault lvf that has consumed part of the oblique motion of plate convergence on the other hand part of the oblique convergence is transferred into the central range and the hsuehshan range judging from counterclockwise rotation of sh from east to west and strike slip faulting in the hsuehshan range most events with a depth greater than 30 km occur offshore eastern taiwan and the azimuth of sh is close to e w directed different from nw se directed at shallow depths this may infer the existence of the transition of lithosphere rheology in offshore eastern taiwan the trends of sh in the depth of 0 10 km are strongly affected by the coseismic stress change of the chi chi earthquake in the northern half of the chi chi rupture area the trends of sh rotate 30 clockwise and the stress ratio increased by a factor of six after the mainshock the orientations of sh still differ by 30 in 2007 comparing to that in the period before the chi chi earthquake the variation of sh trend is more diverse in the southern half of the rupture area showing 20 counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation the trend of sh returns to the pre seismic direction of 110 in 2001 these notable changes of sh before and after the chi chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop we also recognize a significant 10 counterclockwise rotation of sh in the entire chi chi rupture area between 1991 and 1999 before the earthquake took place however to the south of the chi chi rupture the trends of sh remain little changed before and after the chi chi earthquake

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

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

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

is the lishan fault of taiwan active abstract the lishan fault has been characterized alternately as a major discontinuity in stratigraphy structures and metamorphism a ductile shear zone a tectonic suture or non existent in addition to being a geological boundary it also marks transitions in subsurface structures thus the seismicity to the west of the fault permeates through the upper and mid crust while beneath the central range it is noticeably less and largely concentrated in the upper 12 km a prominent west dipping conductive zone extends upward to meet the lishan fault also the eastward increase of crust thickness from 30 km in the taiwan strait quickens under the lishan fault to form a root of over 50 km under the central range in the past the small magnitude seismicity along the lishan fault has been noticed but is too diffuse for definitive association with the fault recent processing of aftershock records of the 1999 mw 7 6 chi chi earthquake using central weather bureau data and especially data from three post chi chi deployments of seismic stations across central taiwan yielded hypocenters that appear to link directly to the lishan structure the presence of a near 4 km long vertical seismic zone directly under the surface trace of the lishan fault indicates that it is an active structure from the surface down to about 35 km and the variety of focal mechanisms indicates that the fault motion can be complex and depth dependent

geodetic evidence of back arc spreading in the mariana trough 1 repeated gps surveys in the mariana islands show that the mariana block is moving apart from the philippine sea plate the velocities of the islands relative to the philippine sea plate range from about 15 mm yr in the northern islands to about 45 mm yr near guam the data also suggest convergence rates for the mariana forearc with respect to the pacific plate of 35 45 mm yr at 19 n increasing to 55 70 mm yr at 13 5 n in addition the velocity vectors show a slight north south expansion of the arc the estimated location of the euler pole of the mariana forearc with respect to the philippine sea plate is well south of the geographical point where the back arc basin narrows to zero width

satellite geodetic imaging reveals internal deformation of western tibet 1 it is often assumed that the majority of the interseismic strain accumulates around the major mapped geological block boundaries however numerous recent earthquakes in the continents have occurred on faults that were previously unidentified existing deformation data from tibet are insufficiently dense to map the distribution of interseismic strain here we combine 265 interferograms formed from 166 radar images with gps data to constrain a high resolution velocity field covering 200 000 km2 of western tibet we confirm that the slip rate of the karakoram fault is low 6 mm yr but show that areas of focused strain do not coincide with the major fault structures some of this strain is due to postseismic relaxation after a mw 6 8 earthquake that occurred in 1996 on a structure that is difficult to identify in satellite imagery models of seismic hazard that rely on imperfect knowledge of the boundaries of crustal blocks can therefore underestimate hazard from unknown faults