Improved seismic tomography offshore northeastern Taiwan: Implications for subduction and collision processes between Taiwan and the southernmost Ryukyu

To 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.