P-wave velocity and gradient images beneath the Okinawa Trough

We determined the high-resolution 3-D P-wave velocity structure under the Okinawa Trough in the region between South Taiwan and North Kyushu (Japan) by inverting 102,511 absolute travel-time data from 5012 local earthquakes and 29,153 relative travel-time residuals calculated from 1873 teleseismic events. For the first time, a combined data set recorded simultaneously by three seismographic networks located in Kyushu, Okinawa Trough, Taiwan and Fujian province of China has been used, which enables us to determine the high-resolution P-wave velocity structure in the crust and upper mantle under the entire arc system. The resulting tomographic and gradient images provide a new insight into the structural heterogeneities related to the opening of the Okinawa Trough and dehydration of the Philippine Sea slab (PHS), and confirm some of the features proposed by previous studies. A high-velocity zone is imaged clearly at depths of 50–300 km along the Ryukyu Arc system from South Kyushu to North Taiwan, showing good agreement with the spatial distribution of seismicity under the subduction zone. The relative residuals from different quadrants also exhibit early relative arrivals at the stations corresponding to the high-Vp regions, and we therefore interpret the high-Vp zone as the subducting PHS slab. Along the volcanic front from South Kyushu to North Taiwan, low-Vp areas are visible, which are mainly caused by the fluids liberated by the extensive dehydration process of the subducting PHS slab. This fluid-related low-velocity zone is located at depths of 20–120 km along the arc with a total length of 1200 km from North Taiwan to South Kyushu. Using the inverted P-wave velocity model we also calculated the gradient images, which are independent of the selection of reference model or color scale, to provide further information and help us to understand and explain the tomoagraphic structures. Three sub-blocks of the Ryukyu Arc system are revealed from the inverted velocity images and the calculated gradient maps. These features of the velocity anomalies together with other parameters determined by the present study suggest that the Okinawa Trough extension, arc magmatism and seismicity are mainly attributed to the dehydration process associated with the PHS slab subduction and the oblique subduction direction of the plate.