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A multicomponent ocean-bottom seismometer (OBS) data set collected by National Central University, Taiwan in the accreationary prism off southwestern Taiwan in 2013, 2014 and 2015, respectively. GI-gun shots located at 1 mile and 1.5 miles radius from the OBS, with spacing approximately 40 m along the sailing line that were analyzed as common receiver azimuthal gathers. The OBS recorded data at a sampling rate of 250 Hz and from a shot pattern that gave good azimuthal coverage around the OBS. Methods to obtain information about fractured sediments had developed from these data since anisotropy, an effect of parallel fracture trains, generates birefringence of P-S converted waves. The multicomponent seismic method allows recording the complete wave field, including P-S converted waves. Based on P and P-S converted waves recorded between the direct and multiple arrivals, this experiment targeted the top few hundred meters of sediment in the study area. After preliminary processing, including a static correction, the data were optimally rotated to radial (R) and transverse (T) components. The principal technique used to detect the anisotropy was azimuthal stacking of the radial and transverse horizontal geophone components. The R component shows azimuthal variation of travel-time indicating variation of velocity with azimuth; the corresponding T component shows azimuthal variation of amplitude and phase. From the radial component azimuthal gather and mode-converted wave amplitude variation for the first few layers and determined corresponding anisotropy parameter and Vp/Vs values. Significant result found that implies a presence of natural fracturing direction. We attribute the observed azimuthal anisotropy to the microcrack distribution and grain boundary orientation due to stress since fracture at this depth is not likely to occur.

After the occurrence of the 1999 magnitude 7.3 Chi-Chi earthquake, a cluster of NE-SW trending earthquakes,almost along the surface trace of the Lishan fault, has been detected in the northern portion of the Central Rangein northern Taiwan. From the spatiotemporal distribution of hypocenters based on cluster analysis, the Lishanfault cluster (LFC) can quantify the evolution of seismicity as aftershocks of the 1999 Chi-Chi earthquake. Theresults of seismic tomographic inversion indicate that the LFC extends down to about 10 km depth and seemsto be distributed in high Vp areas rather than in low Vp areas. This temporal expansion is attributed to fluiddiffusion. Seismic activity in the upper crust tends to be high above broad zone with low Vp in the lower crust. Ourtomographic images demonstrate a series of relatively high Vp/Vs anomalies dipping to the east which seems toform a fluid upwelling conduit beneath the Central Range. We thus suggest that the Lishan Fault might play a roleof an active fluid conduit, fluid or fluid fluxed a partial melt of the Philippines Sea plate would be released alongthe east-dipping conduit and rise gravitationally to the upper crust.

Crustal seismic velocity structure was determined for the northern Taiwan using seismic travel-time data to investigate the northeastern extension of the northern South China Sea’s high-magnetic belt. In order to increase the model resolution, a joint analysis of gravity anomaly and seismic travel-time data have been conducted. A total of 3385 events had been used in the inversion that was collected by the Central Weather Bureau Seismological Network from 1990 to 2015. The main features of the obtained three-dimensional velocity model are: (1) a relatively high Vp zone with velocity greater than 6.5 km/s is observed in the middle to lower crust, (2) the high Vp zone generally parallels to the north–south structural trending of the Chuchih fault and Hsuehshan Range, (3) at 25 km depth-slice, the high Vp zone shows structural trends change from northeastward to northward in central Taiwan, where the values of high-magnetic anomalies are rapidly decreasing to low values. A combination of seismic, GPS, and structural interpretations suggests that the entire crust has been deformed and demagnetized in consequence of the collision between the Philippine Sea plate and the Asian continental margin. We suggest that the feature of sharp bending of the high Vp zone would migrate southwestward and cause further crustal deformation of the Peikang High in the future.

Widely distributed Bottom-Simulating Reflectors (BSRs) have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In order to clarify the link between seismic site response and sedimentary properties of submarine slope, we evaluate the response of seafloor sediments in regard to passive dynamic loads. The local site effect produced by shallow marine sediments was characterized by estimating the horizontal-to-vertical (H/V) spectral ratios of data recorded by the short-period Ocean Bottom Seismometers (OBSs). The results show that the maximal H/V ratios appeared in the range of 3.66 - 9.28 Hz, suggesting that the fundamental frequency is dominated by the effect related to the very shallow sediments. For most stations, the H/V ratios estimated based on the earthquakes and noise records were characterized by different patterns. Relatively broad H/V pattern was obtained when the signals were extracted from earthquakes. This phenomenon may be related to soil nonlinearity when a stronger motion applies. In comparison with the available geological structures and bulk density distribution obtained from coring experiments, we found a relatively higher fundamental frequency of about 8 - 9 Hz for the more rigid material, such as mud diapir and folding axes. For most of the area along the slope, the fundamental frequency shows a relatively low value, about 6 - 8 Hz. Finally, when a site is characterized by thick or lowest bulk density sedimentary layer, we observed a fundamental frequency lower than 5 Hz, which is the lowest in our assessment.

In order to improve our understanding of the marine slope instability of hydrate-bearing sediments in the offshore southwestern Taiwan, P- and S-waves seismic data generated by P-S conversion on reflection from airgun shots recorded from a multi-component Ocean Bottom Seismometer (OBS) survey were used to construct two-dimensional velocity model. The investigated profile lies above a structural high of the Yuan-An ridge, proposed as a high priority drilling site for gas hydrate investigations off southwest Taiwan. The locations of the OBSs were determined with high accuracy by an inversion based on the shot traveltimes. Traveltime inversion and forward modeling of seismic data result in general trends P- and S-wave velocities of sediments. Generally, P- and S-wave velocities are high beneath topographic ridges which might represent a series of thrust-cored anticlines develop in the accretionary wedge. P-wave velocities of the sea floor are about ~1.58 km s-1, increasing to the bottom simulating reflectors (BSR), reaching values of about ~2 km s-1. Below it, a low velocity layer (1.62 - 1.74 km s-1) is observed, which indicates the presence free gas in the sedimentary layer. S-wave velocities of the sediments over the entire section range from 0.3 to ~0.6 km s-1. Significant lateral velocity variations were found beneath the eastern flank of the Yuan-An ridge, probably represents thrust faults that extend from seafloor to hydrate-bearing layer. We suggest that the BSR has been disturbed by the thrust faults and further rupture of the fault could potentially trigger failures in the study area.

After the occurrence of the 1999 magnitude 7.3 Chi–Chi earthquake, a cluster of NE-SW trending earthquakes, almost distributed along the surface trace of the Lishan fault, has been detected in northern Taiwan. From the spatiotemporal distribution of hypocenters based on cluster analysis, the Lishan Fault cluster (LFC) can quantify the influence of crustal fluids on the occurrence of seismicity after the 1999 Chi–Chi earthquake. In order to investigate the possible causes of the seismicity clusters and their relationship to the movement of fluids through part of the collision/subduction system, high-resolution 3-D tomographic images of the crust are determined under the entire region of northern Taiwan by inverting a large number of arrival time data of P-waves and S-waves. The results of seismic tomographic inversion indicate that the LFC extends down to about 10 km depth and seems to be distributed in or around the low Vp, high Vp/Vs zones. This temporal expansion of the focal area during the first week may be attributed to fluid diffusion. The b-value of the LFC is about 0.98 close to the average value, 1.0 of the entire Taiwan region. Our tomographic images demonstrate a series of relatively high Vp/Vs anomalies dipping to the east which seems to form a fluid upwelling conduit beneath the Central Range. We thus suggest that the Lishan Fault might play a role of an active fluid conduit, transferring additional fluids of the Philippines Sea plate along the east-dipping fault zone into the upper crust.

Data from P-waves and from S-waves generated by P–S conversion on reflection from airgun shots recorded along four lines of ocean bottom seismometers were used to construct 2-D velocity sections near the Yuan-An Ridge, off southwest Taiwan. The locations of the ocean bottom seismometers were determined to high accuracy by an inversion based on the shot traveltimes. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P-wave (Vp) and S-wave (Vs) velocities of hydrate-bearing sediment layers. The inversion indicates a relatively high P-wave velocity beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. S-wave velocities of the sediments over the entire section, down to 400 m below seafloor, range from 320 to 570 ms−1. We suggested the lateral variation in Vp/Vs profiles in the hydrate-affected zones may be related to the migration conduit of gas-rich fluid and a characteristic of hydrate content. We model Vp using equations based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ∼200 m below the seafloor to 10–15% of pore space close to the bottom simulating reflector (BSR) in the survey area.

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 74 km. 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.

article i nfo This paper investigates velocity structure of the active plate boundary in southern Taiwan by joint analysis of gravity anomaly and seismic arrival time data. P and S-P arrival time data from 3238 earthquakes were used. In addition to CWBSN permanent networks, seismic data include the Central Weather Bureau permanent net- works and a temporary network consisting of 11 ocean bottom seismometers (OBSs) that was deployed to detect the aftershocks of the 2006 (ML7.1) Henchun earthquake occurred beneath southern Taiwan. The total available OBS data set consists of ~700 detected earthquakes, from which around 500 could be well lo- cated where about 450 events have been used in simultaneous inversion for hypocenters, three-dimensional Vp and Vp/Vs models for the study area. The main objective of incorporating gravity analysis is used to im- prove the velocity model for the offshore area, where it is poorly sampled by local earthquakes. This study found a low-velocity zone existing above the subducting South China Sea slab in the mantle wedge. Based on gravity modeling and our resulting velocity and Poisson's ratio models suggest that the subduction com- plex, which is characterized with a low P-wave velocity and low Poisson's ratios beneath the southern Tai- wan. This duplex structure is characterized by a zone of low P-wave velocities in the range of 6.2-6.8 km/s between 25 and 40 km depth. It also shows that earthquake hypocenters do not fall within this low velocity zone. We have also used seismic tomography velocities to estimate the volume percentage of serpentinite and silica concentrations in southern Taiwan. The calculated serpentinite is about 30% and the volume per- centage of quartz estimated is about 20% at the base of the forearc lower crust.

This paper presents results of a seismic tomography experiment carried out on the accretionary margin off southwest Taiwan. In the experiment, a seismic air gun survey was recorded on an array of 30 ocean bottom seismometers (OBS) deployed in the study area. The locations of the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocity structure for the survey area. The inversion indicates a relatively high P wave velocity (Vp) beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. The bottom-simulating reflectors (BSR) closely follow the seafloor and lies at 325 ± 25 m within the well-constrained region. Mean velocities range from ~1.55 km/s at the seabed to ~1.95 km/s at the BSR. We model Vp using an equation based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200 m below the seafloor to 25% of pore space close to the BSR in the survey area.

This paper investigates crustal features of the plate boundary in eastern Taiwan by joint analysis of gravity anomaly and seismic arrival time data. We found two crustal anomalies: (1) two prominent high-velocity/high Poisson's ratio anomalies in the mid- to lower crust beneath the offshore area; (2) several volumes of relatively high-velocity/high Poisson's ratio rocks in the upper- to mid-crust beneath the Central Range. The former is interpreted as the Luzon arc and forearc blocks from east to west. The latter is interpreted as uplift material from the oceanic crust scraped from the Luzon forearc, which forms the core of the Central Range. The features of northward-narrowing in dimension and deepening in depth of the forearc block are revealed in the velocity solution, implying a subducting Luzon forearc fragment in eastern Taiwan. The values of Poisson's ratios and earthquake activity in the northern domain are generally larger than that in the southern domain, suggesting that the crustal accretion process has not been completed in the northern domain of the study area. Our resulting velocity and Poisson's ratio models provide some insight into the tectonic processes presently operating in eastern Taiwan. It's also suggested that the subducting Luzon forarc acts as a backstop against which accreted rocks are thrust, and this contact may have seismogenic potential.

The genetic algorithm method is combined with the finite‐element method for the first time as an alternative method to invert gravity anomaly data for reconstructing the 3D density structure in the subsurface. The method provides a global search in the model space for all acceptable models. The computational efficiency is significantly improved by storing the coefficient matrix and using it in all forward calculations, then by dividing the region of interest into many subregions and applying parallel processing to the subregions. Central Taiwan, a geologically complex region, is used as an example to demonstrate the utility of the method. A crustal block 120 × 150 km2 in area and 34 km in thickness is represented by a finite‐element model of 76 500 cubic elements, each 2 × 2 × 2 km3 in size. An initial density model is reconstructed from the regional 3D tomographic seismic velocity using an empirical relation between velocity and density. The difference between the calculated and the observed gravity anomaly (i.e., the residual anomaly) shows an elongated minimum of large magnitude that extends along the axis of the Taiwan mountain belt. Among the interpretive models tested, the best model shows a crustal root extending to depths of 50 to 60 km beneath the axis of the Western Central and Eastern Central Ranges with a density contrast of 400 or 500 kg/m3 across the Moho. Both predictions appear to be supported by independent seismological and laboratory evidence.

A joint analysis of gravity anomaly and seismic travel-time data has been used to construct a three-dimensional velocity structure for the northeastern extension of the northern South China Sea’s high-magnetic belt in the Taiwan region. The earthquake data used in this study was collected by the Central Weather Bureau Seismological Network from 1991 to 2002, while the gravity data around Taiwan was compiled by Hsu et al. (1998), Terr. Atmos. Oceanic. Sci., 9, 509–532, and Wang et al. (2002), >Terr. Atmos. Oceanic. Sc., 13, 339–354. A modified velocity model obtained by local earthquake tomography was used to construct an initial three-dimensional gravity model, using a linear velocity–density relationship. To derive a crustal velocity–density model that accounts for both types of observations, this study performed a sequential inversion of travel-time and gravity data. The main features of our three-dimensional velocity model are:(1) an uplifted zone with velocity greater than 6.5 km/s being observed in the lower crust, (2) the width and the shape of the uplifted zone being found to be strongly correlated with the high-magnetic belt, (3) a trend by which the lower crustal high-velocity zone turns from northeast to north in central Taiwan, where the high-magnetic zone was truncated. A combination of seismic, gravity, and structural interpretations suggests that the crustal deformation relating to the magnetic truncation observed in northwestern Taiwan could be correlated closely with the collision between the Philippine Sea plate and the Asian continental margin.

The excessive travel distances have been observed for the landslides with abnormal large volume. The dominating mechanisms are still controversial. It is evidence that the volume of landslide is highly related to the thickness of the sliding mass. Accordingly, it is reasonable to speculate that the normal stress on the sliding surface could dominate the kinematics of landslides. This study tries to testify this conjecture experimentally. Series of rotary shear tests of dry sandstones and wet gouges under different normal stress are performed with an equivalent slip rate of 1.3 m/s. Interestingly, the steady-state friction coefficients of the tested samples, which is highly related to the kinematics of landslides, are decreased with increasing normal stress. The steady-state friction coefficients of the tested sandstones will decrease from 0.69 to 0.35 when the applied normal stresses were increased from 0.5 to 3.0 MPa. Moreover, the steady-state friction coefficients of the tested wet gouges will decrease from 0.26 to an extremely low value of 0.04. Simplified rigid block model predicted the travel distance increased with increasing normal stress. A normal stress threshold of 1.5 MPa on the sliding surface of the dry sandstone was observed to induce excessive travel distance. On the other hand, even with a low normal stress of 0.5 MPa, the sliding surface composed of wet gouges experienced large excessive travel distance. The results revealed that the normal stress on the sliding surface, as well as the sheared materials and their water content, of the large landslides dominating travel distance.

The Philippine Sea plate moves northwestward, with respect to the Eurasian plate, and produces the Ryukyu subduction system (including the Ryukyu trench, the Ryukyu arc and the Okinawa trough) and the Manila subduction system (including the Manila trench and the Luzon arc) with opposite polarity of subduction on the plate boundary along the Ryukyu-Taiwan-Luzon region. The transition zone between these two subduction systems is the Taiwan island, with vigorous mountain building in the south and gravitational collapsing in the north. Thus, the Taiwan island propagates southwestward as a solitary wave along the Eurasian continental plate margin, with a speed of about 7 cm/y. In this paper, we describe the occurrence environment for this phenomenon, with morphological, magnetic and seismological evidence. The shifting oblique lithospheric convergence of the Philippine Sea plate with the Eurasian continental plate margin causes the transient solitary wave of the Taiwan island. The convergence has squeezed the tectonic zones near the plate boundary tightly and uplifted the transition zone that is the Taiwan island. When the convergence site migrates southwestward along the continental plate margin, the squeezed parts of these tectonic zones are released again and spread, by the following tensional stress due to the northward subduction of the Philippine Sea plate. Therefore, the area of the Taiwan island does not necessarily increase substantially with geological time. In order to better understand the lithospheric interaction in the Taiwan area, we also proposed a lithospheric convergence model in this paper.

The Chi-Chi earthquake in Taiwan ( ML =7 .3) occurred on 21 September, 1999 and caused widespread damage over a large area in central Taiwan. A distinct rupture appeared along the Chelungpu fault, which marks the transition from a sedimentary basin to the thrust terrain of the Western foothills. In order to investigate the subsurface geometry of the Chelungpu fault and the structural features that may have influenced the sistribution of Chi-Chi aftershocks, a three-dimensional P-wave velocity model has been developed. The velocity model was determined by simultaneously inverting 41,347 P-wave arrival times from 2,582 pre-mainshock “background” earthquakes. The most striking feature imaged by the velocity model is the high velocity wedge extending from surface to about a 15 km depth beneath the Western foothills. The Chi-Chi mainshock just occurred in the transition area from the high velocity wedge to a basin-type low velocity material where the sub-surface geometry of the Chelungpu fault is proposed. The model also images an elongated high velocity volume at midcrustal depths beneath the eastern flank of the Central Range. West of the Chelungpu fault, another high velocity volume is found at the similar depths which could be associated with the Peikang basement high. Within these two high velocity volumes, the Western foothills and the Central Range exhibit relatively low velocity in the middle-crust. A possible explanation for the nucleation of the Chi-Chi earthquake, the thrust faulting and deformation in central Taiwan is attributed to the interactions between the three high-low-high velocity volumes in response to regional compression.

In the early morning of 21 September, 1999, Taiwan experienced its most powerful inland earthquake (with Richter magnitude 7.3) of the century. The earthquake occurred near the small town of Chi-Chi in central Taiwan, in an area with a low recent background of seismicity although it is surrounded by belts of high seismicity. It caused an extensive surface ruptures totaling about 100 km long and a tremendous number of aftershocks. Using seismological and morphological data, we investigate tectonic environment in central Taiwan and the associated plate structure in the Taiwan region, with emphasis on the cause of the earthquake and its aftershock sequence. Taiwan island is situated on the northwestern corner of the Philippine Sea plate, where its interaction with the Eurasian plate is complicated. The Philippine Sea plate moves northwestward with respect to the Eurasian plate. The former subducts northwestward, in general, along the Ryukyu ward and underthrusts westward in the Taiwan region, with the meridianline of l2l.5°E being the present western boundary of the active subductlng lithosphere. The western boundary is migrating westward, at a rate of about 6 cm/y, along with the northwestward movement of the Philippine Sea Plate. The Philippine Sea plate, like a plow, pushes and uplifts Taiwan island situated on the Asian continental margin, as demonstrated by a seismic wedge to the east of 121°E. In central Taiwan, there exist two obvious linear belts of frequent earthquakes, almost perpendicular and individually trending NW and NE. These linear belts form the boundaries of a triangular area with low background seismicity in front of the seismic wedge, including the Peikang basement high, an uplifted pre-Miocene basement beneath the Western Coastal Plain and its offshore zone. The Chi-Chi earthquake occurred in a subarea of the triangular area near the vertex, con. Fined by the seismic belts and the Peikang basement high. ‘The northwestward movement of the subarea has been impeded by the Peikang basement high, and the associated crustal strain energy may have thus been stored, and then released to trigger the Chi-Chi earthquake, the ruptures and the aftershocks.

The Taitung Canyon originates at the southern end of the Longitudinal Valley.It runs southward along the axis of the Taitung Trough.Thecanyon turns eastward into the Huatung Basin,after crossing the Luzonarc between the volcanic islands of Lanyu and Lutao.It then flows northeastward for about 170km and merges with the Hualien Canyon near theRyukyu Trench,thus forming the largest canyon in the Huatung Basin.High resolution bathymetric data,and several multi-channel seismic profiles are used to describe the structural fabric in the vicinity of the TaitungCanyon and the factors controlling its path within the Huatung Basin.Crossing the eastern slope of the Lanyu-Lutao volcanic ridge,the head of theTaitung Canyon is fan shaped and its entrenchment rapidly increasesdownslope.Throughout the canyon's upper and central portions,the channel depth ranges from 300 to 500m with respect to the surrounding seafloor. The canyon's levees are generally asymmetric and turbidite overbankdeposits are observed.Then,a basement high,trending parallel to the GaguaRidge,forces the path of the Taitung Canyon to turn 90°toward the northwest. As the canyon reaches the Ryukyu Trench,the width of the mainchannel has decreased from 14 km near the Luzon arc,to less than 200m atthe outer-slope of the Ryukyu Trench.The importance of structural controls, such as basement highs and faulting,on the canyon's developmentare examined.Thus,the existence of a strike-slip fault system,affecting theoceanic basement in the Huatung Basin,and its role in controlling the pathof the canyon are discussed.

The southwestern part of the Ryukyu arc-trench system to the west of126°E is obviously bent northwards horizontally.The mechanism of thishorizontal bending has been discussed in the literature,but it is still ambiguous.The West Philippine Basin(WPB)is generally believed to be produced by spreading along the Central Basin Ridge(CBR).However,theCBR does not extend to the northwestern corner of the WPB.Previousgeomagnetic and geomorphological studies suggested that the CBR terminates or is displaced at its northwestern end.However,the detailed pictureis still unclear.In this paper,we have examined bathymetry,gravity andseismicity in and around the northwestern corner of the WPB,and discovered a NE-trending right-handed strike-slip fault system which can be usedto interpret the horizontal bending of the southwestern part of the Ryukyuarc-trench system.A plausible model showing how the right-handed faultsystem results in the bending of the Ryukyu arc-trench system is demonstrated.As the southeastern part of the right-handed strike-slip fault system,the Luzon-Okinawa Fault Zone,which is an obvious fault zone between the Luzon and Okinawa islands,is described in terms of bathymetriccontours and profiles along some cruise lines.The Luzon-Okinawa FaultZone is the main cause of the termination of the CBR.