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

Abstract

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 Pn velocity. The results show that the lateral variation of Pn 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 Pn velocity is found less than 10% and the fast direction is generally in the EW 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.