[1] In this study, a large collection of 41,141 S-P times from the untapped records of the Taiwan Strong Motion Instrumentation Program (TSMIP) network is combined with the P and S wave arrival times from the Taiwan Central Weather Bureau Seismic Network (CWBSN) to image the V p and V p /V s structures beneath Taiwan. The records from the 680 TSMIP stations throughout Taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously. Our result for the V p structure largely confirms previous studies but brings better constraint on the V p /V s structure. The colliding Luzon volcanic arc is characterized by a belt of high V p and high V p /V s with high seismicity that includes the offshore islands of Lutao and Lanyu and the Coastal Ranges in eastern Taiwan, at the depth between about 13 and 25 km. This high V p /V s belt can be traced to the subduction zone in the region between Hualian and Ilan in the deeper portion. The shallow portions of the southwestern coastal plain and the Pingtung region are also characterized by a belt of high V p /V s with lower seismicity. Most of the events occurred at the base of the high V p /V s zones. We suggest that material strength in those regions may be too low to accumulate stress, which may indicate water-saturated young sediments. Finally, the Central Range region is characterized by a low V p /V s belt.

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[1] The 2003 Chengkung earthquake (Mw 6.8) provided diagnostic evidence for a source model showing the deformation process of the seismogenic Chihshang fault in eastern Taiwan. The aftershocks show a fault-bend at a depth of 18 km. Coseismic ground displacements recorded by strong-motion records allow us to deduce instant rupturing of this event. Our resulting model shows a fault length of $33 km and dip-slip dominant rupture on fault-plane deeper than 18 km. Estimated coseismic displacements constrain two fault planes: one at 5 -18 km depth dipping 60°E and 18-36 km depth dipping 45°E. The uppermost fault-plane of the Chihshang Fault (0 -5 km) did not break immediately after the main shock; however, it may have a major role in after-slip and even interseismic ground deformation. The Taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements.

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S U M M A R YTo improve the resolution in imaging the 3-D V P and V P /V S structures in the Taiwan region, especially offshore eastern Taiwan, we combine the arrival times from eleven ocean bottom seismometers (OBSs) and from seismic stations of the Japan Meteorological Agency (JMA) with those from the permanent stations of Taiwan Central Weather Bureau Seismic Network (CWBSN) and Taiwan Strong Motion Instrumentation Program (TSMIP). By doing so, we have obtained a new 3-D tomographic model for Taiwan and its surrounding regions with a better resolution, especially in the area offshore northeastern Taiwan. We also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern Taiwan and determined the focal mechanisms of relatively large events. Our tomography results indicate that in the region northeast of Taiwan, the subducting oceanic Philippine Sea Plate beneath the Eurasian continental lithosphere is characterized by a high V P layer, surrounded by lower V P areas. This P-wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface, especially in its shallower portion. In the hangingwall (Eurasian) block above the subduction interface, a vertically elongated high V P /V S body appears to originate from the interface at depths between 100 and 140 km. We suggest that it represents the partially melted materials that are related to the magmatic activity of the Ryukyu volcanic arc.A good understanding of the 3-D seismic structure is a very important foundation for a variety of earthquake research topics, such as earthquake relocation, earthquake source study and seismotectonic investigations. In the area of Taiwan, an active collision zone between the Eurasian and the Philippine Sea plates, a detailed 3-D velocity model is crucial to the understanding of the active structural characteristics and the tectonic evolution of the collision zone. To this end, there has been a long history of attempts in imaging the seismic velocity structure in the Taiwan region. One of the first tomographic studies for Taiwan was carried out more than two decades ago by Roecker et al. (1987) using the P-wave arrival times observed by the Taiwan Telemetered Seismographic Network (TTSN), which was operated by the Institute of Earth Sciences, Academia Sinica.Initially, the TTSN consisted of only 25 stations, equipped with vertical-component, short-period seismometers. Since the incorporation of TTSN into the Central Weather Bureau Seismic Network (CWBSN) in 1991, many more stations have been installed, and now the CWBSN involves 71 telemetered stations, equipped with three-component S13 seismometers. Including the retired stations, the CWBSN has a total of 91 different sites. Fig. 1 shows the distribution of the CWBSN stations. The CWBSN offers a better station coverage for the Taiwan region, leading to a series of P-and S-wave velocity models (e.g. Shin & Chen 1998;Rau & Wu 1995;Ma et al. 1996;Kim et al. 2005).Recently, we obtained regiona...

ABSTRACT1 Institute of Geophysics, National Central University, Chung-Li, Taiwan, ROC The Tatun volcano group is located adjacent to the Taipei metropolitan area in northern Taiwan and was a result of episodic volcanisms between 2.8 and 0.2 Ma. Earthquake data collected over the last 30 years are analyzed to explore seismicity patterns and their associated mechanisms of faulting in the area. Using a Joint Hypocenter Determination (JHD) method, a few sequences of relocated earthquake hypocenters are tightly clustered; these seemed to be blurry in the original catalog locations. Numerous earthquakes, previously unnoticed and not reported in the CWB catalog, have been identified from careful examination of the continuous recordings of a nearby broadband seismic station. These newly identified earthquakes show similarities in waveforms and arrival time differences between direct P-and S-waves indicating that their hypocenter locations are very close to each other and their source mechanisms are similar. A relatively high b-value of 1.22 is obtained from the analysis of crustal earthquakes (depth < 30 km) in the region, which may suggest that clustered local seismicity in the Tatun volcanic region probably resulted from subsurface hydrothermal or volcano-related activities. Focal mechanism solutions determined in this study are dominated by normal faulting. Thus, these earthquake clusters are most probably associated with hydrothermal/magmatic activities in a back-arc extensional environment. TAO, Vol. 16, No. 3, August 2005 580

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