Lateral variations of Pn velocity and anisotropy in Taiwan from travel-time tomography

In this study, the P arrivals of 1726 ray paths with the epicentral distances longer than 200 km from 539 earthquakes that occurred in Taiwan are used to investigate the variation of P-n velocity. The results show that the lateral variation of P-n velocity in Taiwan is quite similar to the pattern of gravity anomaly and strongly related to several features of Taiwan's geological structure. The crust is a little thicker (about 39 km) in the Central Range and somewhat thinner toward the east and west. A relatively thin crust (about 35 km) is obtained in the area of Peikang High; however, an unexpected thick crust of greater than 40 km is also found at the corner of southwestern Taiwan. The degree of anisotropy of P-n velocity is found less than 10% and the fast direction is generally in the E-W direction. This direction is parallel to the axis of compressional stress or the direction of plate motion, indicating that the anisotropy results from the deformation of the upper mantle.