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Tectonic tremor in Taiwan is known for its short duration and weak amplitude. Recent observations from the mountain area (Central Range) allows us to document such type of tectonic tremor recorded at a close distance and to investigate a single station technique for a minute‐long tremor. To effectively distinguish tremor from earthquakes and noise, we applied the Fisher's class separability criterion to automatically select the optimal feature subset from a set of commonly adopted feature candidates. During the study period of 1 January to 16 September 2016 when a local seismic array was deployed, we successfully differentiated tremor from local earthquakes and noise with high accuracy of 86.6% to 98.9% from three stations using a k‐nearest neighbors classifier. Other than the maximum amplitude, number of peaks, and energy of the 2‐ to 8‐Hz passband, the spikiness of discrete Fourier transforms median in time is also found to be important to separate tremor from noise. Key Points: Recent observation from a mountain array allows us to document the tremor events recorded at the closest distanceWe evaluate the possibility of detecting minutes‐long tremor using a single stationWe obtain a collection of efficient seismic features for successful classification [ABSTRACT FROM AUTHOR]

In probabilistic seismic hazard analysis (PSHA), the standard practice is to select a set of appropriate ground motion prediction equations (GMPEs) and assign weights on the logic tree, especially for regions where strong motion data are sparse and where no indigenous GMPE exists. Subjectively assigning weights to a set of models usually has the disadvantage of not obtaining mutually exclusive and collectively exhaustive models because of sparse or unavailable data. Therefore, the development of logic tree weightings in PSHA remains a major challenge. In this study, a distance metric measure for GMPEs is first analyzed to show how a set of GMPE's prediction models can be partially reconciled by using high-dimensional information visualization techniques. Visualization of a large suite of GMPEs onto a 2-D graphical map provides a powerful theoretical framework that can guide the selection of a set of representative models. These models are considered mutually exclusive and collectively exhaustive, and have the ability to represent the center, body, and range of ground-motion distribution in a logic tree analysis. Second, determination a set of weights for PSHA are estimated based on the residuals, likelihood and EDR-index which are known as data - driven weight. The other weight type is non-data driven which is calculated based on the probability of a random model generated on a Sammon's map. The methods presented here, that improve consistency in the weight assignment, can help to reduce overall epistemic uncertainties and offer a way of assigning weight on the logic tree. Finally, an example for assigning the weights on the logic tree for PHSA are discussed. [ABSTRACT FROM AUTHOR]

Sediment flux signals from source to sink in foreland basins preserve a record of tectonics, sea level and climate through erosion and sedimentation. However, longitudinal sediment transport often occurs in foreland basins, thus removing part of the orogenic material flux from foreland basin records. Here we use mass balance calculation and stratigraphic simulations of sediment fluxes for the Taiwan orogen to provide an order of magnitude estimate of how much orogenic material may bypass a foreland basin. Our results indicate a significant, potentially more than 50%, mismatch between sediment volume currently preserved in the basin and the amount of material presumably eroded from the orogen since the onset of collision in Taiwan. This suggests either a significant overestimation of average erosion rates over the period concerned with orogenic development of Taiwan, or it supports previous paleogeographic work suggesting that longitudinal sediment transport in the paleo-Taiwan Strait served as a major bypass conduit of importance for the establishment of a steady state orogen. We identify candidate submarine topography in the South China Sea that may preserve Taiwan's missing erosional mass. [ABSTRACT FROM AUTHOR]

The Tiehchanshan structure is the largest gas-field in the outer foothills of northwestern Taiwan and has been regarded as the best site for CO2 sequestration. This study used a grid of seismic sections and wellbore data to establish a new 3-D geometry of subsurface structure, which was combined with lithofacies characters of the target reservoir rock, the Yutengping Sandstone, to build a geological model for CO2 sequestration. On the surface, the Tiehchanshan structure is characterized by two segmented anticlines offset by a tear fault. The subsurface geometry of the Tiehchanshan structure is, however, composed of two thrust-related anticlines with opposite vergence and laterally increasing fold symmetry toward each other. The folds are softly linked via the transfer zone in the subsurface, implying that the suspected tear fault in the surface transfer zone may not exist in the subsurface. The Yutengping Sandstone is composed of several sandstone units characterized by coarsening-upward cycles. The sandstone member can be further divided into four well-defined sandstone layers, separated by laterally continuous shale layers. In view of the structural and stratigraphic characteristics, the optimum area for CO2 injection and storage is in the structurally high in the northern part of the Tiehchanshan structure. The integrity of the closure and the overlying seal are not disrupted by the pre-orogenic high-angle faults. On the other hand, a thick continuous shale layer within the Yutengping Sandstone isolates the topmost sandy layer from the underlying ones and gives another important factor to the CO2 injection simulation. [ABSTRACT FROM AUTHOR]

The Meinong earthquake generated up to ~10 cm surface displacement located 10 - 35 km W of the epicenter and monitored by InSAR and GPS. In addition to coseismic deformation related to the deep earthquake source, InSAR revealed three sharp surface displacement gradients. One of them is extensional and is inconsistent with the westward interseismic shortening of ~45 mm yr-1 in this region. The gradient sharpness suggests slip triggering on shallow structures, some of which were not well documented before. To characterize these shallow structures, we investigated potential surface ruptures in the field. Sets of ~NS tension cracks distributed over 25 - 300 m width, with cumulative extension in the same order as InSAR observations, were found along 5.5 km distance along the extensional gradient and are interpreted as surface rupture. We build two EW regional balanced cross-sections, based on surface geology, subsurface data, and coseismic and interseismic geodetic data. From the Coastal Plain to the E edge of the coseismic deformation area, we propose a series of three active W-dipping back-thrusts: the Houchiali fault, the Napalin-Pitou back-thrust, and the Lungchuan back-thrust. They all root on the 3.5 - 4.0 km deep Tainan detachment located near the base of the 3-km-thick Gutingkeng mudstone. Further E, the detachment would ramp down to ~7 km depth. Coseismic surface deformation measurements suggest that, in addition to the deeper (15 - 20 km) main rupture plane, mostly the ramp, the Lungchuan back-thrust, and the Tainan detachment were activated during or right after the earthquake. Local extension is considered as transient deformation at the W edge of the shallow main slip zone. [ABSTRACT FROM AUTHOR]

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 θ 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 θ 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 θ 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 θ 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. [ABSTRACT FROM AUTHOR]

Knowledge of the crustal structure is important for understanding the tectonic framework and geological evolution of southeastern China and adjacent areas. In this study, we integrated the datasets from the TAIGER (TAiwan Integrated GEodynamic Research) and ATSEE (Across Taiwan Strait Explosion Experiment) projects to resolve onshore-offshore deep crustal seismic profiles from the Wuyi-Yunkai orogen to the Taiwan orogen in southeastern China. Three seismic profiles were resolved, and the longest profile was 850 km. Unlike 2D and 3D first arrival travel-time tomography from previous studies, we used both refracted and reflected phases (Pg, Pn, PcP, and PmP) to model the crustal structures and the crustal reflectors. In total, data from 40 shots, 2 earthquakes, and approximately 1,950 stations were used; 15,612 arrivals were selected among three transects. Using these data, we determined the complex crustal evolution since the Paleozoic era , involving the closed Paleozoic rift basin in central Fujian, the Cenozoic extension due to the South China Sea opening beneath the coastline of southern Fujian, and the on-going collision of the Taiwan orogen. The shape of the Moho, which also reflects the crustal evolution, can be summarized as follows: ~ 30 km deep to the west of Fujian, deepening toward central Fujian (~ 35 km), becoming shallower toward the Taiwan Strait (~ 28 km), deepening again toward the mountain belt of Taiwan (~ 42 km), and becoming shallower toward the Pacific Ocean (~ 10 km). [ABSTRACT FROM AUTHOR]

The Longitudinal Valley in eastern Taiwan is generally considered as the suture of the collision between the Philippine Sea and the Eurasian plates, and has attracted numerous geologic and seismologic studies. In the northernmost part of the valley, however, constraints on how structures develop as the suture turns into the Ryukyu subduction system offshore are still very limited. Therefore, we analyzed relocated seismicity distributions and focal mechanisms of earthquake sequences, together with tectonic geomorphic investigations to further understand the seismotectonic characteristics of this area. In our seismologic observations, we found two previously unidentified reverse faults in the northernmost part of the Longitudinal Valley suture. One is an E–W striking, south-dipping reverse fault near the Liwu River fan delta, and the other is a N–S striking, east-dipping reverse fault near the eastern Central Range front. Both these structures connect with a detachment at ~ 10 km deep, and may connect with each other to form a curved structural system. The Meilun fault, a well-known active structure that ruptured in a M7.3 earthquake in October 1951, is not seismically active in the past two decades, and may just be part of a secondary branch of the major structural system. In the northernmost part of the Longitudinal Valley suture, we propose that as the Coastal Range bedrocks subduct northward beneath the Eurasian plate with the Philippine Sea plate, the shallow sediments of the Longitudinal Valley, being a buoyant block, do not subduct, but overthrust northward and westward instead. [ABSTRACT FROM AUTHOR]

In the foreland area of western Taiwan, some of the pre-orogenic basement-involved normal faults were reactivated during the subsequent compressional tectonics. The main purpose of this paper is to investigate the role played by the pre-existing normal faults in the recent tectonics of western Taiwan. In NW Taiwan, reactivated normal faults with a strike-slip component have developed by linkage of reactivated single pre-existing normal faults in the foreland basin and acted as transverse structures for low-angle thrusts in the outer fold-and-thrust belt. In the later stage of their development, the transverse structures were thrusted and appear underneath the low-angle thrusts or became tear faults in the inner fold-and-thrust belt. In SW Taiwan, where the foreland basin is lacking normal fault reactivation, the pre-existing normal faults passively acted as ramp for the low-angle thrusts in the inner fold-and-thrust belt. Some of the active faults in western Taiwan may also be related to reactivated normal faults with right-lateral slip component. Some main earthquake shocks related to either strike-slip or thrust fault plane solution occurred on reactivated normal faults, implying a relationship between the pre-existing normal fault and the triggering of the recent major earthquakes. Along-strike contrast in structural style of normal fault reactivation gives rise to different characteristics of the deformation front for different parts of the foreland area in western Taiwan. Variations in the degree of normal fault reactivation also provide some insights into the way the crust embedding the pre-existing normal faults deformed in response to orogenic contraction. [ABSTRACT FROM PUBLISHER]

We present a regional model of plate geometry and kinematics of Southeast Asia since the Late Cretaceous, embedded in a global plate model. The model involves subduction polarity reversals and sheds new light on the origin of the subduction polarity reversal currently observed in Taiwan. We show that this subduction zone reversal is inherited from subduction of the proto-South China Sea plate and owes its current location to triple junction migration and slab rollback. This analysis sheds new light on the plate tectonic context of the Taiwan orogeny and questions the hypothesis that northern Taiwan can be considered an older, more mature equivalent of southern Taiwan. [ABSTRACT FROM AUTHOR]

Ground motions from normal-faulting earthquakes are generally considered to be smaller than those of strike-slip and thrust events. On 11 April 2011 a crustal normal-faulting earthquake [the Fukushima earthquake (Mw 6.6)] occurred in Eastern Japan. The peak ground acceleration (PGA) observed was considerably higher than the predictions of several ground-motion prediction equations (GMPEs), which were derived mainly from thrust or strike-slip earthquakes. In northeast Taiwan, the tectonic structure of the Ryukyu Arc and the Okinawa Trough typically entail normal-faulting earthquakes. Because of the normal-faulting earthquakes relevance to ground motions and nuclear power plant sites in northeast Taiwan, we evaluated the impact of the ground motion of normal-faulting earthquakes in offshore northeast Taiwan using a newly constructed attenuation relationship for PGA and pseudo-spectral acceleration (Sa). We collected 832 records from 13 normal-faulting earthquakes with focal depths of less than 50 km. The moment magnitude (Mw) of the 13 events was between 4 - 6. The Sa and PGA of normal-faulting earthquakes offshore northeast Taiwan determined with the newly constructed attenuation relationship were higher and lower, respectively, than those obtained using attenuation equations commonly used in the Taiwan subduction zone. [ABSTRACT FROM AUTHOR]

Recent sedimentary facies and processes along the offshore slope of east Taiwan are investigated using a large set of geophysical and sedimentological data. The Taiwan orogen is often considered as one of the most tectonically active regions in the world and also suffers important climatic activity with an average of four typhoons per year. We have mapped in details the morphosedimentary features and characterized the sedimentary facies along offshore eastern Taiwan. There, we show that the slope is driven by a variety of erosional processes from mass wasting to turbidity current. Mass movements such as slides or mass transport deposits (MTDs) are ubiquitous and affect the whole east coast off Taiwan. Detailed core investigations, such as grain size analysis, chemical and mineralogical composition, revealed that turbidite facies range from thin fine-grained turbidites to thick massive turbidite facies. The detailed analysis of turbidite beds allow us to discuss the controlling factors of turbidity current generation. Turbidity currents represent an important sedimentary process that governs the slope morphology off east Taiwan. Triggering mechanisms likely range from slope instabilities related to earthquakes shaking to failure relative to climatic-controlled pulses of sediment supply. We propose that at least two end-members are characteristics in our turbidite records in term of controlling factor: 1) Turbidity currents likely generated by tectonic activity and earthquakes shaking in particular; and 2) Turbidity currents likely generated by climatic activity such as typhoon-induced floods. [ABSTRACT FROM AUTHOR]

High seismicity with spatial heterogeneity in Taiwan makes this region one of the best natural laboratories for seismological researches. Numerous seismicity studies, including the b-value, have been performed for more than one century. One of the possible ways to mitigate seismic risk is predicting an impending earthquake through various kinds of seismic precursors observations. The first seismic precursors project in Taiwan started in 1978. The temporal variation in b-value prior to a forthcoming earthquake has been considered a significant seismic precursor for earthquake prediction. In this review study, we focus on the studies of b-values of earthquakes in Taiwan, including the spatial distribution of b-values, temporal variation in b-values, correlation between the b-value and fractal dimension, and correlation between the b- and p-value of Omori law for aftershocks. Also included is the relation between the spatial distribution of b-values and regional geotectonics. The properties and controlling factors of b-value and earthquake monitoring in Taiwan will also be briefly described. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ambient noise analysis in Northern Taiwan revealed obvious lateral variations related to major geological units. The empirical Green’s functions extracted from interstation ambient noise were regarded as Rayleigh waves, from which we analyzed the group velocities for period from 3 to 6 s. According to geological features, we divided Northern Taiwan into seven subregions, for which regionalized group velocities were derived by using the pure-path method. On average, the group velocities in mountain areas were higher than those in the plain areas. We subsequently inverted the S -wave velocity structure for each subregion down to 6 km in depth. Following the analysis, we proposed the first models of geology-dependent shallow S- wave structures in Northern Taiwan. Overall, the velocity increased substantially from west to east; specifically, the mountain areas, composed of metamorphic rocks, exhibited higher velocities than did the coastal plain and basin, which consist of soft sediment. At a shallow depth, the Western Coastal Plain, Taipei Basin, and Ilan Plain displayed a larger velocity gradient than did other regions. At the top 3 km of the model, the average velocity gradient was 0.39 km/s per km for the Western Coastal Plain and 0.15 km/s per km for the Central Range. These S -wave velocity models with large velocity gradients caused the seismic waves to become trapped easily in strata and, thus, the ground motion was amplified. The regionalized S -wave velocity models derived from ambient noises can provide useful information regarding seismic wave propagation and for assessing seismic hazards in Northern Taiwan. [ABSTRACT FROM AUTHOR]

The Tulungwan-Chaochou Fault system in southern Taiwan represents the boundary between a slate belt of moderate metamorphic grade and a relatively unmetamorphosed fold-and-thrust belt. The offset between hanging wall and footwall of this fault ranges from 7 to 11 km and is considered one of the major tectonostratigraphic faults in Taiwan. This 75-km-long fault system is also one of the most conspicuous topographic features in Taiwan. The geometry, kinematic history and associated subsidiary structures have not been resolved. Field mapping of fabrics and brittle faults show that a 45-km-long west-northwest-vergent antiform defined by folded slaty cleavage exists in the hanging wall of the fault. This antiform has not been previously described and apparently formed in a brittle environment. The flat crest and tight forelimb of the antiform suggests a two-stage deformation model composed of a fault-bend fold followed by a trishear fold. We infer that regional scale fold is associated with a thrust that splays upward from the main detachment. [ABSTRACT FROM AUTHOR]

Highlights: [•] Onset of Taiwan foreland basin development at 4.4Ma, younger than that proposed in some previous studies. [•] Three rapid subsidence events at 4.4–4, 2–1.8 and 0.45Ma in Taiwan foreland basin. [•] Tectonostratigraphic model for back-and-forth migration of the basin margin in the distal part of Taiwan foreland basin. [•] Slower migration rate of the forebulge in the early stage than that of the steady migration of the orogenic belt. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

River networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels. We demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides. Assessing this metric throughout a landscape maps the dynamic states of entire river networks, revealing diverse conditions: Drainage divides in the Loess Plateau of China appear stationary; the young topography of Taiwan has migrating divides driving adjustment of major basins; and rivers draining the ancient landscape of the southeastern United States are reorganizing in response to escarpment retreat and coastal advance. The ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape-scale interactions among tectonics, erosion, and ecology. [ABSTRACT FROM AUTHOR]

Highlights: [•] Geological structure of the site was revealed by seismic reflection surveys. [•] CO2 evaluation at the CCIP was simulated with locally refined grids. [•] Migration of CO2 plume did not penetrate low permeability formation in 500 years. [•] Solubility and residual trapping account for 46% of injected CO2 after 500 years. [ABSTRACT FROM AUTHOR]

The offshore area of northern Taiwan is currently in an extensional regime, known for a post-collision area of the Taiwan mountain belt that has formed due to the oblique collision of the Philippine Sea Plate against the Eurasian continental margin since the late Miocene. The active collision has undergone a progressive migration of the Taiwan mountain belt from the northeast to the southwest direction. After the relaxation of the compressional stress due to the westward subduction of the Philippine Sea Plate, the northern Taiwan orogen has collapsed together with the opening of the southern Okinawa Trough. Here, we show the transitional process from compressional to extensional regime in the area off northern Taiwan and reconstruct the tectono-sedimentary evolution by using multi-channel reflection seismic data. We process and interpret multi-channel seismic profiles and use borehole data to determine the ages of sedimentary units and stratal surfaces/unconformities. Based on the seismic facies, we have recognized two distinctive domains separated by the Offshore Shanjiao Fault (OSF): a shelf basin to the northwest and a collapsed zone to the southeast. The tectono-sedimentary evolution off northern Taiwan since the late Miocene can be described in 4 stages. (1) The compressional structures off northern Taiwan were formed during the collision period starting around 6 Ma, a series of folds-and-thrusts aligned in the NE-SW direction. (2) Due to the oblique collision, some NW-SE-trending strike-slip faults were developed across the collision belt to accommodate the differential displacements between two adjacent segments of the folded belt. (3) During the transition from compressional to extensional regime at about 2.7 Ma, a post-collisional erosion and subsidence occurred in the collapsed zone to form the basal unconformity. This post-collisional collapse is associated with the opening of the southern Okinawa Trough, a back-arc basin, due to the westward migration of the Philippine Sea Plate subduction beneath the southern Ryukyu arc. (4) The offshore area of northern Taiwan has subsided continuously with sediment accumulation since the early Pleistocene (~2.0 Ma), accompanying the rifting of the southern Okinawa Trough. [ABSTRACT FROM AUTHOR]

On 26 February 2012, an inland ML 6.4 earthquake occurred near Wutai Township in Pingtung County, Taiwan and caused massive shaking in southern Taiwan. After the 2010 ML 6.4 Jiasian earthquake caused extensive damage in southern Taiwan, deep, complicated structures were revealed. This earthquake provided us an opportunity to further investigate related tectonic issues and hazard mitigation. Although the focal mechanisms of the Wutai earthquake from various institutes's readings were quite consistent, a large uncertainty was present with regard to focal depth. The relocated source parameters suggest a deeper depth of 32.3 km and a focal mechanism of a 326° strike, a 35° dip, and a 57° rake. Based on the tectonic seismic profile in southern Taiwan, the hypocenter of the Wutai earthquake is therefore likely located in the lower crust and implied that the deep blind fault system under the southwestern Central Range could even go deeper southward, revealing complicated thick-skinned tectonics. In addition, due to the abundance records of the P-wave first-motion polarities collected, we also tested the feasibility of searching the source depth by a polarity fitting for which we demonstrated several synthetic tests and showed its strength. In applying the polarity fitting method to the real case of Wutai earthquake, the results indicated that this approach may be affected by the uncertainty of the epicenter sensitively. However, the application showed the potential that a large number of polarity data existing in earthquake catalog does not only dedicate to determine the focal mechanisms conventionally but may also contribute to the source location in the future. [ABSTRACT FROM AUTHOR]

Late Quaternary shortening along the length of the Western Foothills of Taiwan highlights the tectonic segmentation of the foreland and raises questions about the relationship between erosion and the thickness of synorogenic foreland basin fill, and their influence on thrust kinematics. We compare measurements of shortening with geodetic observations and numerical model results, and relate these to regional topography. Predictions of shortening from numerical modeling and observed strain gradients within central Taiwan are generally similar in terms of their scaling and kinematic behavior. Within this framework, the current deformation field is likely related to the inheritance of older passive-margin structures in the foreland, as well as to the easily erodible nature of the 5-km-thick sequence of synorogenic sediment accreted at the leading edge of the orogen in the last 1.1 m.y. Additionally, available constraints on the timing of recent activity of faults suggest that infilling of piggy-back or wedge-top basins there occurred rapidly, contemporaneously with the activation of the frontal thrust and a possible increase in the rate of shortening across the adjacent thrust sheet toward the center of the orogen. [ABSTRACT FROM AUTHOR]

Abstract: Distribution and architecture of slope basins across a continental slope vary as a consequence of accommodation forming, sediment dispersal rates, canyon cutting, sediment filling and different sediment transporting mechanisms. The area offshore Southwestern Taiwan is generally recognized as having active tectonics and high sediment deposition rates. In the Gaoping Slope, slope basins are formed by the developments of folds, faults and diapiric intrusions. Portions of the sediments discharged from the Taiwan mountain belt have been trapped in these basins in the Gaoping Shelf and Gaoping Slope. The rest of the sediments were transported to deep sea areas through submarine canyons. This complex system of folds, faults, diapirs, slope basins, submarine canyons, and sediment deposits has also readjusted the morphology of the Gaoping Slope. This study examines the linkage between accommodation spaces of tectonic and sedimentary processes in the Gaoping Slope through seismic facies analysis. Four seismic facies which include convergent-symmetrical facies, convergent-baselapping facies, chaotic facies, and parallel and drape facies, and different deposition patterns have been recognized in the Gaoping Slope basins. The thick mud layers which are regarded as the source of diapiric intrusions are first observed beneath the basin. Strata records show that the accommodation spaces in various slope basins have increased or decreased during different stages of basin evolution. Because of the competition between regional tectonism (accommodation space variations) and sediment routing distance from provenance to depository (sediment input variations), most under-filled basins lie in the lower slope domain in the Gaoping Slope, but also in the upper slope domain east of the Gaoping Submarine Canyon. This observation suggests that in the inner Gaoping Slope west of the Gaoping Submarine Canyon, sediment deposition rate is higher than the basin subsidence rate, the topography of the upper slope domain there is “healed”, and most sediments are overfilled in the slope basins now. Besides the sequential steps of sedimentary disposal in the filling-and-spilling model, we have also observed evidences which indicate that mass movements and submarine canyons in the area have significantly changed the sediment dispersal patterns in the slope basins of the Gaoping Slope. We suggest that although filling-and-spilling is a key sedimentary process in the Gaoping Slope, tectonic activities, mass wasting events and canyon feeding processes have diversified sediment transporting mechanisms from the inner to outer slopes in the area offshore Southwest Taiwan. [Copyright &y& Elsevier]

Abstract: Three-dimensional (3-D) attenuation structures were determined for Taiwan region by inversion of earthquake intensity data set. The seismic intensity which is assumed to be a measure of the maximum acceleration of the S-wave at the seismic station is used to estimate the attenuation structure. The intensity data set consists of about 5500 intensity readings for 1410 earthquakes reported by the Central Weather Bureau of Taiwan from 1987 to 2010. The 3-D attenuation maps consisting of five layers were constructed to a depth of 74km. The obtained model reveals the following features: first, a high absorption zone exists in the westernmost Okinawa trough. These high-attenuation anomalies seem to extend from upper crust to mantle beneath the Yilan Plain and Kueishantao Island. We interpret the high absorption zone as the presence of the relatively hot lower crust and uppermost mantle in response to the local opening of the Okinawa trough. Second, strong lateral variation of seismic attenuation is observed from the source region of the Chi–Chi earthquake in the central Taiwan. Most aftershocks were occurred beneath the Western Foothills which is characterized by a transition from high attenuation to low attenuation between the Chelungpu fault and Chuchih fault. However, the region east to the Chuchih fault is relatively quiescent. This study suggests that the high Vp/Vs and lateral variation of attenuation features existing beneath the Western Foothills could be associated with porous materials containing fluid which might affect the generation of the Chi–Chi earthquake. [Copyright &y& Elsevier]

Abstract: The present study is proposed to investigate temporal variations of soil–gas composition in the vicinity of different fault zones in Taiwan. To carry out the investigations, variations of soil–gases compositions were measured at continuous earthquake monitoring stations along Hsincheng and Hsinhua faults in Hsinchu and Tainan areas, respectively. Before selecting a monitoring site, the occurrence of deeper gas emanation was investigated by the soil–gas surveys and followed by continuous monitoring of some selected sites with respect to tectonic activity to check the sensitivity of the sites. Based on the results of long term geochemical monitoring at the established monitoring stations we can divide the studied area in two different tectonic zones. We proposed tectonic based model for earthquake forecasting in Taiwan and tested it for some big earthquakes occurred during observation period i.e. 2009–2010. Based on the anomalous signatures from particular monitoring stations we are in a state to identify the area for impending earthquakes of magnitude ≥5 and we have tested it for some earthquakes which rocked the country during that period. It can be concluded from above results that the stress/strain transmission for a particular earthquake is hindered by different tectonic settings of the region under study. [Copyright &y& Elsevier]

Abstract: A blind thrust fault with a unique strike, which is orthogonal to the strike of most tectonic structures in Taiwan, triggered the Jiashian earthquake on March 4, 2010 (M =6.4; 22.96°N, 120.70°E). This study utilizes 100 global positioning system stations to examine changes of surface displacements during the Jiashian earthquake. We mitigate effects of short-term noise and long-term plate movements from surface displacement data using a frequency dependent filter via the Hilbert–Huang transform and compute the horizontal azimuth (i.e. GPS-azimuth) using residual data at the NS component relative to residual data at the EW component. Analytical results show that orientations of horizontal azimuths were aligned and orthogonal to the strike of the blind thrust fault. Meanwhile, inverse orientations are observed before and after the earthquake that agrees well with the seismic rebound theory. As stress disturbed on strata a few days before the earthquake, an impeded region can be clearly identified by disordered orientations of horizontal azimuths for anticipating the mainshock. These results provide an additional view to explore stress disturbance associated with earthquakes and offer more information to examine diverse models of tectonic evolution in this region. [Copyright &y& Elsevier]

The Tseng-Wen Reservoir Transbasin water tunnel has broken the historical record of the maximum overburden of a tunnel within Taiwan. The three-dimensional in situ stresses at the tunnel were evaluated using acoustic emission (AE) and deformation rate analysis (DRA) with an under-coring technique. This is the first time that the AE and DRA have been used in evaluating an in situ stress in Taiwan. As cores drilled in different directions are required for the three-dimensional stress evaluation, but the in situ borehole drilling is usually limited to the vertical direction, small samples 2.28 cm in diameter and 4.56 cm in length were under-cored from the borehole cores in six directions to evaluate the in situ stresses. Cyclic uniaxial compressive loadings were artificially applied to the Changchikeng sandstones under-cored from the outcrops. The experimental results validate the definitiveness of using under-cored samples at different pre-stresses with AE and DRA. Then, cores taken from the Tseng-Wen Reservoir Transbasin water tunnel site at a great depth of 416-739 m underground were under-cored. The experimental results indicate that the major principle in situ stress is from 30.8 to 39 MPa and is in the directions of SE-NW and SW-NE at different depths. [ABSTRACT FROM AUTHOR]

Abstract: This paper discusses submarine morphology as a means of understanding the avulsion of the Fangliao canyon. Multichannel seismic profiles and bathymetric data were used to perform a morphological analysis. In response to tectonic uplifting, the Fangliao canyon was divided into two separate segments and two distinct trends. Initially, in the shelf edge–upper slope region the canyon extends downslope in a N–S direction. Then the canyon course bends to the SW at a depth of about 630m because of a mud diapir uplift and then returns to and maintains a N–S course with the canyon mouth at a depth of 900m, which is its present-day course. The Fangliao ridge, a mud diapiric ridge, lies east of the lower Fangliao canyon and extends upslope into the upper Fangliao canyon with a generally straight course and a nearly N–S trend. Because of the uplift of the Fangliao ridge a major step of the convex-upward profile, ~150m above the canyon floor, resulted in the east Fangliao ridge canyon. The gravity flows are unable to rise up against the high gradient and, thus, have had to abandon the segment of paleocanyon once flowing along the trend of the Fangliao ridge. The internal seismic characteristics of the terraces located along the eastern wall of the Fangliao canyon suggest deposition of terrace mainly occurs in the inner bend of the Fangliao canyon. The sedimentary sequences of the terrace are divided into two units: (1) the lower sedimentary unit is characterized by oblique, high-amplitude reflections and is considered as coarse-grained canyon fills resulting from aggradation related to the canyon avulsion events; and (2) the upper sedimentary unit consists mainly of low-amplitude reflections that are interpreted as inner levee resulting from spillover deposition. Numerical simulations were performed to discuss avulsion of the Fangliao canyon. Initially, the course of the Fangliao canyon had a N–S trend and followed the regional slope (i.e., the direction gravity currents would flow). Subsequently, the mud diapir uplifted the segment of the paleocanyon of the Fangliao canyon once flowing along the present-day Fangliao ridge and diverted its course to the present-day Fangliao canyon. The model suggests that the uplift rate exceeds the erosion rate in the east Fangliao ridge canyon, and hence the sediment flows in the canyon may be unable to pass through the 150-m major step. This new model suggests that the avulsion of the Fangliao canyon is marked by a significant bend at the boundary between the upper and lower canyons. [Copyright &y& Elsevier]

Abstract: Large gneiss bodies have been reported in the metamorphic complex in northern and eastern Taiwan for decades. Some of them are cut or intruded by granitic pegmatite dikes. However, increasing evidence suggests that the gneiss bodies are more likely to be granites or meta-granites. To validate the existence of the granites/meta-granites and propose their potential distribution in the metamorphic complex of northeastern Taiwan, a geological reconnaissance along with a crooked long-distance VLF-EM survey line of 19km and a 4.4km controlled experimental line were conducted in the Hoping geological area of the northeastern Taiwan mountain belt. The VLF-EM data were initially processed by using the Fraser linear filter and a nonlinear filtering method based on the ensemble empirical mode decomposition (EEMD) technique to enhance the signal and to evaluate the data quality. A skin-depth added Karous–Hjelt filter was performed to generate the equivalent current density model. With the aid of the 3-D topographic representation, the equivalent current density model clearly indicates that a vast area of granites/meta-granites in the survey area is highly possible. In spite of a large uncertainty of the pseudo-quantitative model, the geological implication of our finding agrees with the tectonic framework that Taiwan and the adjacent southern Ryukyu arc system could be part of the rifted China continental margin before the collision of the Luzon and Ryukyu arcs started in late Cenozoic. [Copyright &y& Elsevier]

Abstract: We have investigated the structure beneath the Tatun volcanic zone north of Taipei metropolitan area of Taiwan, area of five million people. We used the data collected from a seismic network deployed for 5years over the volcanic zone. This plus another data from Taiwan regional networks allow us to carry out tomographic inversions for V p, V p/V s and Q p structures beneath the Tatun volcanic zone. Based on our results and other geological, tectonic, and seismic findings, we reconstruct the structural evolution of the crust in the Tatun volcanic zone, and discuss the implication to the surrounding faults, fractured zones, and discuss potential future volcanic activities. From the tomographic results, there appear to exist a tube-shaped, highly fractured ancient magma passage with high seismic velocities that parallel to the Chinshan fault, and magma passage extends to the southeast at the depth about 20km. This structure suggests plutonic intrusion passage beneath the Tatun volcano group that may have been associated with the earlier subduction of the Philippine Sea plate, melting of the subducted plate at depth has generated the magma intrusion that has brought about the Tatun volcanic activities. The high seismicity today also implies a highly fractured crust due to the hydrothermal activities and induced crustal stress. The hydrothermal fluid-rich upper crust as indicated by the low V p/V s ratio may have important bearing on the potential hazards associated with the two active faults cutting through both the Taipei Basin as well as the Tatun volcanic groups. [Copyright &y& Elsevier]

SUMMARY Microearthquakes with magnitude down to 0.3 were detected by the Taiwan Chelungpu-fault Drilling Project Borehole Seismometers (TCDPBHS). Despite the large coseismic slip of 12 m at the drill site during the 1999 Chi-Chi earthquake, our studies show very little seismicity near the TCDPBHS drill site 6 yr after the Chi-Chi main shock. The microearthquakes clustered at a depth of 9-12 km, where the Chelungpu thrust fault turns from a 30° dipping into the horizontal decollement of the Taiwan fold-and-thrust tectonic structure. Continuous GPS surveys did not observe post-slip deformation at the larger slip region and no seismicity was observed near the drill site. Therefore we suggest that the thrust belt above the decollement is locked during this interseismic period. We further investigated source parameters of 242 microearthquakes by fitting ω−2-shaped Brune source spectra to our observation data using a frequency-independent Q model. We find that the static stress drop increases significantly with increasing seismic moment. However, due to the intense debate on this topic of scaling-relations and the related self-similarity of earthquakes, we further improve the data analysis and correct for path and site effects using the Projected Landweber Deconvolution (PLD) method for events within some clusters. The PLD method analyses the source time functions of the larger and the smaller event by an iterative technique. As a result we received source dimensions and stress drops of larger events including path and site effect corrections. The results from the PLD method are less scattered and also show a positive relation between static stress drop and seismic moment. We find a similar positive trend for the apparent stress scaling with seismic moment. [ABSTRACT FROM AUTHOR]

Abstract: We use GPS data, trench parallel gravity anomaly (TPGA), and bathymetry to infer plate coupling patterns along the Manila subduction zone. Using a block model and a fault geometry constrained by seismicity, we simultaneously solve for the location of Euler pole and angular velocity between the Sunda and Luzon blocks as well as the slip-deficit rate on plate interface. Our estimates show that the Euler pole between the Sunda and Luzon blocks is situated at southern Palawan near 8.3°N and 119.4°E with the angular velocity of 4.6Myr−1. The estimated convergence rate along the Manila Trench continuously decreases southward from 91mm/yr at the northern tip of Luzon to 55mm/yr north of Mindoro. The inversion of GPS data reveals partially locked fault patches extending from the West Luzon Trough to the east of Scarborough Seamount chain. The slip-deficit rate in this region is in the range of 20–30mm/yr corresponding to a coupling ratio of 0.4. However, the fault slip behavior is not well resolved near the North Luzon Trough. Based on a good correlation between locations of large subduction zone earthquakes and areas possessing gravity low, we investigate a variety of TPGA-based plate coupling models assuming different scaling between TPGA values and plate coupling ratios. The TPGA-based plate coupling models offer plausible rupture scenarios which are not constrained by current GPS data. The partially locked fault zone near 15–16.5°N may be associated with the subducted Scarborough Seamount wherein oceanic floor is highly fractured. The great subduction zone earthquake propagates beneath the Scarborough Seamount seems to be unlikely. The densification of GPS network in central Luzon and seafloor geodetic observations close to trench axis are crucial to distinguish the detailed fault coupling patterns and the role of subducted seamounts. [Copyright &y& Elsevier]

Typhoon Morakot caused serious damage to southern Taiwan. Almost all major mountain highways located in the affected region suffered severe damage from landslides, rock falls, and debris flows. Among these highways, Ali Mountain Highway is one of the most severely affected. Ali Mountain Highway, i.e., T18 route, was constructed in the early 1980s. For the past 30 years of operation, it has suffered substantial damage from different natural hazards. The continued damage-rehabilitation cycle is implemented to maintain the serviceability of this highway. Ali Mountain Highway was seriously damaged by the strike of typhoon Morakot on August 8, 2009. Traffic through this highway was interrupted for over 3 months until temporary rehabilitation was completed. This paper provides a summary of the damage suffered along the Ali Mountain Highway and presents suggestions for mitigation strategies to minimize the likelihood of similar levels of damage during other extreme typhoons such as the typhoon Morakot. The manuscript summarizes extensive field surveys and aerial photo analysis carried out by the authors to investigate the mechanisms of typical slope failures along the Ali Mountain Highway. The engineering geology perspectives are emphasized in the analyses. Geological and hydrogeological features for the sites in this study were found to be the most influential factors dominating the failures. The fault-related weak zones and impermeable layers dominated the global stability of the slopes. In summary, geotechnical engineers should consider carefully the characteristics of geological structures for mitigation and rehabilitation of landslides. Extreme scenarios, such as typhoon Morakot, should also be considered. [ABSTRACT FROM AUTHOR]

Abstract: Taiwan formed during the Plio-Pleistocene collision of Eurasia with the outboard Luzon arc. Its pre-Tertiary basement, the Tananao metamorphic complex, consists of the western Tailuko belt and the eastern Yuli belt. These circum-Pacific belts have been correlated with the high-temperature/low-pressure (HT/LP) Ryoke belt and the high-pressure/low-temperature (HP/LT) Sanbagawa belt of Japan, respectively. To test this correlation and to reveal the architecture and plate-tectonic history of the Tananao metamorphic basement, detrital zircons were separated from 7 metasedimentary rock samples for U–Pb dating by LA-ICPMS techniques. Results of the present study, coupled with previous data, show that (1) the Tailuko belt consists of a Late Jurassic to earliest Cretaceous accretionary complex sutured against a Permian–Early Jurassic marble±metabasaltic terrane, invaded in the north by scattered Late Cretaceous granitic plutons; the latter as well as minor Upper Cretaceous cover strata probably formed in a circum-Pacific forearc; (2) the Yuli belt is a mid- to Late Cretaceous accretionary complex containing HP thrust sheets that were emplaced attending the Late Cenozoic Eurasian plate–Luzon arc collision; (3) these two Late Mesozoic belts are not coeval, and in part were overprinted by low-grade metamorphism during the Plio-Pleistocene collision; (4) accreted clastic sediments of the Tailuko belt contain mainly Phanerozoic detrital zircons, indicating that terrigenous sediments were mainly sourced from western Cathaysia, whereas in contrast, clastic rocks of the Yuli accretionary complex contain a significant amount of Paleoproterozoic and distinctive Neoproterozoic zircons, probably derived from the North China craton and the Yangtze block±eastern Cathaysia, as a result of continent uplift/exhumation after the Permo-Triassic South China–North China collision; and (5) the Late Jurassic–Late Cretaceous formation of the Tananao basement complex precludes the possibility that the early Yanshanian (Early Jurassic) granitoids in southern China represent a landward arc contemporaneous with the later, outboard Tananao accretionary event. [Copyright &y& Elsevier]

Abstract: Here we tackle a tectonically important question – the upper mantle velocity structure beneath central Taiwan – with seismically interesting observations – receiver-side slab waveform effects. We use teleseismic P waveforms of the NS broadband array deployed by the TAIGER project to examine patterns of variation in arrival time, pulse width, and amplitude – measuring the first two by Gaussian fitting – and contrast measurements of earthquakes to the southeast (SE earthquakes) with those of one Sumatra earthquake in order to focus on upper mantle heterogeneities. Overall variation patterns as a function of earthquake are compatible with ray-tracing predictions. Relative reduced arrival times and amplitudes at central Taiwan stations suggest the existence of a deep aseismic slab below. From simulations of 2-D wave propagation, we conclude that lateral heterogeneity of crust and uppermost mantle primarily contributes to variations in arrival time and only secondarily to variations in amplitude and pulse width. Furthermore, discrepancies between source-side and receiver-side waveform effects, where the latter are not always amplitude-reduced, are explained by constructive interference between the fast and slow phase. Thus, the use of full waveform information can provide independent constraints to complement results of previous studies. A future extension will be to incorporate S waves and apply waveform inversion to yield quantitative constraints. [Copyright &y& Elsevier]

Abstract: The southern Ryukyus represents an area where different tectonic stress regimes result in high seismicity and increased seismic hazard for nearby areas such as Taiwan. On 18 December 2001 at 04:03 (GMT) a strong earthquake (Mw 6.8) occurred in the forearc area of the southern Ryukyu subduction zone. Revised moment tensor solutions published by GCMT and BATS groups show a normal faulting mechanism with some strike-slip component and also point to a shallow focal depth (∼12km). We use arrival times picked at both Taiwanese and Japanese stations along with a 3D geo-realistic a priori velocity model in order to obtain accurate absolute locations for the mainshock and 153 of its aftershocks. Locations are derived by using the Maximum intersection (MAXI) algorithm which has been used in many previous seismicity studies in the southern Ryukyus. These improved locations indicate that the mainshock was caused by the failure of a NE–SW oriented fault that extends from the edge of the Nanao forearc sedimentary basin to the Ryukyu arc basement. Far-field P and SH waveforms of the mainshock recorded at stations surrounding the source and at distances 30–100°, were inverted for the purpose of investigating its rupture process. A non-negative least-squares inversion technique utilizing multiple time windows was used to derive the spatio-temporal slip distribution. The preferred slip distribution model shows that there is one large area of high slip (∼0.9m) at 5–15km depth that essentially represents the crystalline rocks of the Ryukyu arc basement. Another smaller area with lower slip (∼0.4m) extends at 10–15km depth beneath the Nanao basin. Most aftershocks are located in areas of low slip (<0.4m) filling the regions of slip deficit. It is likely that the 18 December 2001 earthquake was caused by a stress field interaction generated by the oblique subduction of the Gagua ridge and the gravitational forces acting at its landward flank. [Copyright &y& Elsevier]

GPS observations have revealed important information for studying active tectonics and plate motion and are a useful tool for monitoring crustal deformation. The CWB continuous GPS array consists of approximately 150 stations with dense spatial coverage throughout Taiwan and can be used not only to monitor crustal deformation and seismic activity, but also to analyze the earthquake precursors in Taiwan. [ABSTRACT FROM AUTHOR]

Field investigations following the 2003 Mw = 6.8 Chengkung earthquake in eastern Taiwan revealed some interesting observations of surface geological processes closely related to the co-seismic deformation. We discovered that the Tama Fault, which is about 15 km east of the causative Chihshang Fault, underwent shortening of about 15.5 mm locally in 2001 - 2006, particularly during the 2003 earthquake. This shows that ESE-WNW compression affects the upper crust of the Coastal Range and produces significant shortening in addition to that of the major Chihshang Fault. On the hanging wall of the Chihshang Fault, we also found vigorous activities of the two major mud volcanoes during the main shock, lasting several days. To the north, the Luoshan Mud Volcano, a large mud basin, erupted noisily with water and gases during the earthquake. To the south, in the Leikunghuo Mud Volcano, two sets of fractures, one aligned with the N16°E right-lateral fault and the other with the N80°E left-lateral fault, occurred during the earthquake. This conjugate system revealed a strike-slip stress regime with NE-SW compression and NW-SE extension. We interpret it to be the result of local stress permutation rather than regional tectonic stress. We conclude that deformation did occur inside of the Coastal Range, especially during the co-seismic event. Therefore, a better understanding of the internal deformation of the Coastal Range is an important target for future studies, particularly across three mapped faults: the Yungfeng, Tuluanshan and Tama faults. We also want to draw attention to the stress analysis in the mud volcanoes area, where the local stress perturbation plays an important role. [ABSTRACT FROM AUTHOR]

Abstract: A very common but important observation of collided and accreted volcanic arcs is that the forearc regions of these arcs are largely or even entirely missing. The processes and mechanisms responsible for the removal and transport of the forearc materials from the collisional belts are thus important issues in understanding tectonics and crustal growth. The young and ongoing collision between the Luzon volcanic arc and the Eurasian continental margin that forms the island of Taiwan provides a rare opportunity to examine these processes and mechanisms as they occur. From observations of a new detailed 3-D tomography combined with relocated hypocenters of two earthquake sequences occurred in 2003 and 2006, we found that the Luzon forearc lithosphere initially underthrusts westward after the collision began. As the collision proceeds, the forearc basement then subducts eastward beneath the colliding and accreting Luzon arc along a major fault system in eastern Taiwan. Thus the Luzon forearc lithosphere appears to be removed by tectonic erosion and is being transported eastward into the mantle. Our results from the active Taiwan orogen will provide important insights for interpreting rock records from many old arc-continent collisional belts in the world. [Copyright &y& Elsevier]

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 37km 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 17km. 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 30–35km 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. [Copyright &y& Elsevier]

Abstract: Mud volcanoes are important conduits for deep fluids to migrate upward, providing an important window to study fluid/sediment interactions at depth in accretionary prisms. Fluids emitting from mud volcanoes in southern and eastern Taiwan were analyzed for dissolved chemical composition and B isotopes to delineate the sources and chemical alteration occurring during fluid migration. Concentrations of dissolved major elements (Cl, Na, K, Ca, Mg, and SO4) in most of the mud volcano fluids were lower than seawater concentrations, except for the high Ca samples in eastern Taiwan, which were probably affected by fluids/igneous rock interactions. The high concentrations of B, Ba, and Li in the mud volcano fluids are consistent with a scenario that involves intense sediment alteration and clay dehydration at depth. The reaction temperatures estimated using chemical thermometers fall within the range of 70–170°C, with some variation by location. In nearby mud volcano sites, the concentration of dissolved major elements varied little, but the B stable isotope compositions varied by as much as 20‰, supporting the argument that dissolved B concentrations were over-printed with effects of sediment adsorption. This explains the occurrence of B enriched and heavy δ11B fluids at moderate depths in subduction zones. It is probable that the fluids in mud volcanoes in Taiwan originated from 2 to 5km depth and are influenced by processes involving tectonic shortening, sediment alteration, and clay dehydration, and subsequent retrograde adsorption during fluid migration. [Copyright &y& Elsevier]

Abstract: The direct and feedback relationships between tectonics, climate and denudation are a matter of debate. A better understanding of these relationships requires quantifying rates of denudation in a wide range of climate and tectonic settings, as well as at various time and space scales. Because of an ongoing active collision implying high uplift rates and a climate prone to extreme rainfall events and frequent tropical typhoons facilitating strong erosion dominated by mass movements and high degree of fluvial transport, the Taiwan environment is highly dynamic. In this work, spatially-averaged denudation rates determined along the network of one of the major rivers in Taiwan (Lanyang River) from in situ-produced cosmogenic nuclides (10Be) measured in river-borne quartz minerals are compared to the erosion rates determined from the statistical analysis of modern sediment load data. Integrated over the last several hundreds of years, the denudation rate derived from in situ-produced 10Be concentrations averages 2 ± 1 mm/yr within the Lanyang watershed. Integrated over the last 50 years, the erosion rate given by modern sediment load data is 5–7 mm/yr within the same catchment area. The studied catchment being characterized by a relatively low-level of human activity, the discrepancy between the two rates is most probably due different sensibilities to high-frequency, stochastic erosional events (typhoons and earthquakes). The cosmogenic-derived denudation rates can thus be regarded as more representatives for quantifying erosion processes on the short-time scale, and be strictly compared to the long-term exhumation rates derived from low-temperature chronological data. [Copyright &y& Elsevier]

In 2008, the TAiwan Integrated GEodynamics Research (TAIGER) project conducted an active-source experiment in which seismic records from 10 explosions were collected at nearly 1400 locations on the island of Taiwan. We use the first-arrival times manually picked from the explosion records to examine the P-wave travel-time variations resulting from the complex crust and uppermost mantle structures beneath Taiwan. Distributions of the observed first-arrival times exhibit characteristic patterns associated with vertical and lateral velocity variations present in different tectonic domains in the region. Aided by numerical simulations, we compare the observed first-arrival times with those predicted from a one-dimensional (ID) model and three three-dimensional (3D) tomography models for Taiwan that portray the underlying variations in seismic wave speeds frequently utilized to decipher the regional tectonic framework in Taiwan. We evaluate the quality of the four models by the goodness of fit between the observed and model-predicted travel times that demonstrates significant improvement of the 3D models over the ID model. Statistical distributions of the differential travel-time residuals allow us to inspect quantitatively the overall effectiveness of these models in replicating the observed ground-truth first-arrival times so as to make an assessment of the strength and deficiency of the resolved velocity structures that agree with or contradict the travel-time observations from the explosion experiment. [ABSTRACT FROM AUTHOR]

Abstract: Probing the lateral heterogeneity of the upper mantle seismic velocity structure beneath southern and central Taiwan is critical to understanding the local tectonics and orogeny. A linear broadband array that transects southern Taiwan, together with carefully selected teleseismic sources with the right azimuth provides useful constraints. They are capable of differentiating the lateral heterogeneity along the profile with systematic coverage of ray paths. We implement a scheme based on the genetic algorithm to simultaneously determine the relative delayed times of the teleseismic first arrivals of array data. The resulting patterns of the delayed times systematically vary as a function of the incident angle. Ray tracing attributes the observed variations to a high velocity anomaly dipping east in the mantle beneath the southeast of Taiwan. Combining the ray tracing analysis and a pseudo-spectral method to solve the 2-D wave propagations, we determine the extent of the anomaly that best fits the observations via the forward grid search. The east-dipping fast anomaly in the upper mantle beneath the southeast of Taiwan agrees with the results from several previous studies and indicates that the nature of the local ongoing arc–continent collision is likely characterized by the thin-skinned style. [ABSTRACT FROM AUTHOR]

Abstract: Seismic profiles provide evidence that there has been strong transport by deep-water bottom currents and drift deposition on the northern slope of the South China Sea. Earlier geochemical studies suggest that the drift sediments originated primarily from Taiwan. However, the transport process, history and origin of the deep-water bottom deposition in the northern South China Sea, on both glacial–interglacial and tectonic time scales, remain unclear. Here, we show new high-resolution records of clay minerals, grain size and mass accumulation rate (MAR) of terrigenous materials from Ocean Drilling Program (ODP) Site 1144, together with trace element concentrations in siliciclastic sediments from ODP Site 1146. Combined with other published data, we find that the primary source for sediments at ODP Sites 1144–1148 since 3Ma is from Taiwan, and not from Pearl River as previously thought. Before 3Ma, however, sediment source to ODP Sites 1146 and 1148 was mainly from the Pearl River. Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since ~3Ma may be related to the formation of the Taiwan orogen and strengthening of deep-water bottom current transport in the northern South China Sea. Variations in clay mineralogy and sedimentology at ODP Site 1144, located on a sediment drift, shows strong glacial–interglacial cyclicity. This suggests that bottom current deposition is highly dependent on sea-level fluctuations, which control the terrigenous supply to the deep sea. [ABSTRACT FROM AUTHOR]

The Chi-Chi earthquake ( M = 7.6) took place in central western Taiwan in 1999. The earthquake caused reactivation of the Chelungpu Fault and resulted in 100-km-long surface ruptures. The fault strikes mostly north-south to NNE-SSW; however, the northern tip of the southern segment of the surface ruptures rotates clockwise to define an east-west trend, then jumps to a shorter NNW-trending rupture. The largest vertical displacement is recorded in the Shihkang area of the Shihkang-Shangchi Fault Zone, where vertical slips are up to 8-10 m. The Shihkang-Shangchi Fault Zone displays a complex fault pattern as a linkage damage zone between two fault segments with the greatest concentration of faults and fractures. Our new interpretation, based on recent published geometric, kinematic, and geophysical studies on the Chi-Chi earthquake fault, suggests that the Shihkang-Shangchi Fault Zone is not a simple termination zone, but may be an 'overstep zone' or a 'transfer zone'. Slip analysis along the surface ruptures indicates that they are composed of three fault segments and the amount of slip partly depends on the intersection angle between slip direction and fault strike. Our numerical modeling for the area indicates that Coulomb stress changes are mainly concentrated on tips and bends of the surface ruptures. Slip patterns indicate that the fault propagates toward the northeast. Therefore, this study suggests high potential for future earthquake activity along the unruptured Shangchi segment. Hence, future geohazard studies should focus on the Shangchi segment to evaluate potential earthquakes, determine recurrence intervals, and reduce future earthquake hazards. [ABSTRACT FROM AUTHOR]