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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.

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.