3-DIMENSIONAL SEISMIC VELOCITY STRUCTURE OF THE CRUST AND UPPERMOST MANTLE BENEATH TAIWAN

Taiwan is located in a complex, tectonically active region where the P hilippine Sea plate is subducting northwestward along the Ryukyu Trenc h while the Eurasian plate is subducting eastward along the Manila Tre nch. The Central Weather Bureau Seismographic Network (CWBSN), which i s composed of 75 stations with three-component instruments, has been o perated over the island of Taiwan since March of 1991. This dense seis mic network provides high-quality P- and S-wave arrival times of local earthquakes, and, thus, gives us a good opportunity to determine the velocity structure of the crust and upper mantle structures beneath Ta iwan. We have applied a new tomographic method to infer the fine 3-D P - and S-wave velocity structures beneath Taiwan. Results show the exis tence of a low velocity zone in the uppercrust of west Taiwan. This zo ne is consistent with the thick sediments in the area. A low-velocity zone has been inferred underneath the extinct volcano regions, and it is due to the remnant geothermal effect after past volcanic activities . A low-velocity zone with a relatively homogeneous distribution of P- wave velocities, extending to a depth of about 30-35 km, is found bene ath the Central Range. We infer that this feature might have resulted from the heat intrusion from the oceanic upper mantle of the Philippin e Sea plate, which has been colliding with the Eurasian plate, and the effect of partial melting. The thickness of the crust of Taiwan is es timated to be 30-35 km and that for the Philippine Sea plate is 15-20 km.