Deep structure beneath the west margin of Philippine Sea Plate and South China Sea from P and S wave travel time tomography

The velocity model V_pf from a data set of P waves, and 3-D models of V_p and V_s estimated from P and S wave data with the same source-receiver pairs, were respectively determined for the area of Philippine Sea Plate (PHP) and South China Sea (SCS). The results obtained revealed the following features: (1) The interactions between the PHP and Eurasia plate (EUAP) are different in different parts of the west boundary of PHP. Along the Ryukyu Trench, the PHP subducts beneath EUAP down to a depth of 400 km. Under Taiwan island, the EUAP steeply subducts down to about 400 km depth, and then toward PHP until it reaches a depth of about 660 km. At the north segment of Manila Trench, a nearly vertical slab inserts down to about 660 km depth. Underneath Philippine Trench, the subducting PHP slab inserts into EUAP, and penetrates below the 660'-discontinuity. (2) Under the SCS, there is a low-velocity area (LVA) of large width which is visible at depths of 300 - 400 km. It wriggles and reaches to another LVA at depth about 700 km under Yingqiong Basin. Within the LVA under SCS, there are two small but prominent low velocity anomalies: one beneath Haikou Volcano, and another beneath Zhujiangkou Basin. (3) The statistical results of the perturbations of V_s and V_b (the bulk-sound velocity calculated from V_p and V_s) show that: Firstly, the change rates of average perturbations of V_s and V_b with depth are reverse within some depth ranges. Secondly, the averages of relative perturbations of V_s are bigger than that of V_b in the old subducting Pacific slab. In the young subducting PHP slab, however, this contrast relationship is opposite.