基于三重震相拟合的华南地区上地幔P波与S波速度结构

利用中国数字测震台网（CDSN）记录到的台湾地区两个地震事件6°~30°震中距范围的三重震相波形资料，基于观测与理论波形拟合法，获到华南地区上地幔P波和S波的最佳波形拟合速度模型及其V_P/V_S比值.与AK135模型相比，华南地区410 km深度上方存在明显低速层：S波低速区厚度约为70 km，速度降为2%~5%；而P波低速区厚度为70~230 km，速度降为5%~6%.另外，410 km间断面整体表现为一个梯度层，厚度约为10~40 km，V_P跃增量为4.0%~5.4%，而V_S跃增量为2.6%~11.7%.研究区内，低速层的V_P和V_S异常值大小和410 km间断面速度跃变量由北向南逐步减小.结合以往的接收函数和地震层析成像结果，华南地区410 km间断面上方的低速区可能与太平洋俯冲板块脱水有关.

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° and 30° 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%~5% based on the AK135, while for P-wave velocity structure, the LVL thickness is 70~230 km with a P-wave velocity drop of 5%~6%. The 410 km discontinuity is a gradient layer with the thickness of 10~40 km, in which V_P and V_S drops are 4.0%~5.4% and 2.6%~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.