P- and S-wave velocity structures of upper mantle beneath South China derived from seismic triplications

The P- and S-wave triplications from two Taiwan earthquakes recorded by the Chinese Digital Seismograph Network at epicentral distances between 6 degrees and 30 degrees are used to image P-wave (V-p) and S-wave (V-s) velocity structures of the upper mantle and V-p/V-s ratios beneath South China by matching the observed and synthetic seismograms with possible models. Comparing with the Earth model AK135, there is an obvious low-velocity layer (LVL) atop the 410 km discontinuity beneath South China. Matching the observed and synthetic S-wave triplications suggests that the LVL thickness is 70 km with a drop of S-wave velocity 2%similar to 5% based on the AK135, while for P-wave velocity structure, the LVL thickness is 70 similar to 230 km with a P-wave velocity drop of 5%similar to 6%. The 410 km discontinuity is a gradient layer with the thickness of 10 similar to 40 km, in which V-p and V-s drops are 4.0%similar to 5.4% and 2.6%similar to 11.7%, respectively. The P-wave and S-wave velocity structures and the thickness of the gradient 410 km discontinuity decline gradually from the north to the south in South China. Combined with previous results of seismic tomography and receiver functions, lateral variations of the LVL and the 410 km discontinuity might be related to the dehydration of the subducting Pacific Ocean slab.