Three-dimensional P- and S-wave Velocity Structures of the Chingshui-tuchang Geothermal Area in Northeastern Taiwan

Crustal velocity structure of the Chingshui-Tuchang area is obtained from the analysis of Pand S-wave arrivals of 257 local earthquakes recorded by a temporary seismic network in Ilan, northeastern Taiwan. The results of one-dimensional structure show that a major discontinuity in the uppercrust is found at the depth of 3 km for both Pand S-wave velocities. In addition, a significantly low S-wave velocity layer is found at depth between 10-15 km. This feature is similar to that of a partially molten magma body found in other geothermal areas, such as Yellowstone. Threedimensional P-wave velocity structures show that lateral variations in P-wave velocity in the shallow depths are generally associated with the geological structures near the surface. The major geothermal areas such as Chingshui and Tuchang exhibit lower P-wave velocity. Relocated earthquakes using the threedimensional model show that many of the earthquakes are located in high velocity areas or areas with large gradient in the upper crust.