石岛地震台远震记录反演研究

Inversion of far-field seismic waves at Shidao Seismographic Station

利用石岛地震台的远震体波记录，采用旋转相关函数法和接收函数法分别反演了台站下方介质的各向异性特征和速度结构．（1）对震中距25°～35°且记录良好的5次地震的ScS震相，采用旋转相关函数法反演了岩石圈的剪切波分裂参数．对深源地震的反演结果表明，石岛地震台快波偏振方向为N94°E，这意味着西沙附近处于近东西向微偏南的拉张或地壳下方的地幔流方向为近东西微偏南，西沙地区地壳是过渡性的，其底部的驱动力主要来自与欧亚板块运动一致的物质流．快慢波时间延迟为1．3s，估算各向异性层厚度为100km左右．（2）对震中距20°～60°的9次远震P波波形三分向记录，采用接收函数法反演了地壳和上地慢的S波速度结构．反演结果表明，石岛地震台下方地壳分为3层：约5km以上有一速度梯度带，S波速度从1．5km／s逐渐增加到3．5km／s，其间有若干小的分层；在5～16km的平均速度为3．8km／s左右，其间有若干小的分层；在16．0～26．5km的速度为3．6km／s左右，这是一个明显的低速层；莫霍面埋深为26．5km，莫霍面以下平均速度为4．7km／s，也有若干小的分层，尤其是在莫霍面之下有一个明显的低速层．根据转换波到时分析和速度剖面左右摆动现象，认为反演结果中的小分层可能是不真实的，但在16．0～26．5km的低速层的真实程度还是较高的，表明下地壳具有一定的塑性．

Far-field seismic waves recorded at the Shidao Seismographic Station, located in the South China Sea, are used to inverse earth anisotropy and shear wave velocity vertical distribution under the station respectively. （1）The approach of rotated correction function is applied to five satisfactory ScS wave recordings at distance of 25°-35° to inverse shear wave splitting parameters of lithosphere. The inversion result of the deepest earthquake shows that the polarization of fast shear wave is N94°E and the extensional stress or the moving direction of the mantle mass in the Xisha region is nearly EW. It suggests that the crust in this region is a transitional one and driving force beneath comes from the mass flow whose flowing direction is consistent with the moving of Eurasian plate in the South China Sea. The time delay between fast and slow shear waves is 1.3 s, from which the anisotropy effective thickness is estimated about 100 km. （2）The method of receiver function is used for nine high quality P wave recordings with three components at distance of 20°-60° to inverse vertical distribution of shear wave velocity beneath the station. The result indicates that the crust can be divided into three layers： above 5 km where there is a gradient zone including several small layers, in which shear wave velocity increases from 1.5 to 3.5 km/s gradually; between 5 and 16 km the velocity is about 3.8 km/s in average including several small layers; between 16.0 and 26.5 km the velocity is about 3.6 km/s which is an obvious low velocity layer. The buried depth of Moho discontinuity is 26.5 km and beneath the Moho the velocity is about 4. 7 km/s in average, including several small layers, especially an obvious low velocity layer just beneath the Moho. The analysis of the arrival time of transformed waves and the velocity swinging variation around initial model suggest the smaller layers maybe are not reliable, but the velocity layer between 16.0 and 26.5 km maybe is real one implying the plasticity of the lower crust.