Inversion of teleseismic waves at Shidao Seismographic Station

Teleseismic datasets at the Shidao Seismographic Station, located in the northwestern South China Sea, are used to determine the earth anisotropy and the vertical distribution pattern of the shear wave velocity by inversion approaches.The rotated correction function is applied to analyzing high quality ScS records from five earthquakes at distance of 25°~35° to obtain shear wave splitting parameters of the lithosphere.The result from the deepest earthquake among the five events indicates that the polarization of the fast shear wave is N94°E, which means the direction of extensional stress or the moving of the upper mantle mass in Xisha Islands is nearly west to east and confirms that the crust in this region is a transitional one and the driving force beneath the crust is from the moving mass consistent with the Eurasian plate.The anisotropy effective thickness is estimated about 100 km based on the time delay of 1.3 s between the fast and slow shear waves.The receiver function is applied to analyzing high quality P wave records from nine earthquakes at distance of 20°~60° to obtain the vertical distribution pattern of shear wave velocity beneath the station.The result indicates that the crust could be divided into three layers:the uppermost crust (5 km above) is a velocity gradient zone consisting of several small layers, where the shear wave velocity increases from 1.5 to 3.5 km/s gradually; the 5~16 km depth interval also consiss of several small layers of which the mean velocity is about 3.8 km/s; and the lower crust (16.0~26.5 km) is an obvious low velocity layer with a velocity of about 3.6 km/s.The buried depth of the Moho discontinuity is 26.5 km, the mean velocity of the layers beneath the Moho is about 4.7 km/s and there is an obvious low velocity layer just beneath the Moho.Moreover, analysis of the arrival time of converted waves and the swinging variation of velocity around the initial model suggests that smaller layers in the model maybe are not reliable but the low velocity layer between 16 and 26.5 km maybe is the real one that implies the plasticity of the lower crust.