USING OBS TELESEISMIC RECEIVER FUNCTIONS TO INVERT THE LITHOSPHERIC STRUCTURE—A CASE STUDY OF THE SOUTHWESTERN SUBBASIN IN THE SOUTH CHINA SEA

Abstract

Due to the particular seafloor environment and the special outer mechanical structure of the ocean bottom seismometer (OBS), there are some problems that have not been solved in the lithospheric structure inversion using the OBS teleseismic receiver functions. Based on the analysis of these problems, we combined the Fourier transform and wavelet analysis to suppress the non-stationary noise to obtain a better signal-to-noise ratio and clearer seismic phases recorded by the OBSs at the southwestern subbasin in the South China Sea (SCS). The equipment is a broadband OBS of I-4C type produced by the Institute of Geology and Geophysics, Chinese Academy of Sciences. We then inverted the lithospheric structure by successfully applying the receiver functions to the teleseismic data recorded at the southwestern subbasin in the SCS. The results show that it is feasible to use the receiver functions for the inversion of the lithosphere structure based on the passive observation data collected via the OBS, in which the key is to suppress the non-stationary noise. The Moho depth at the southwestern subbasin in the SCS is 10∼12 km, with a crust thickness of 6∼8 km. The shallow crust is a low velocity layer with sediment thickness 1∼2 km, which consists of sediment and volcanic clastic breccia produced by the magma eruption after the spreading cessation. In the spreading axis center, a low S-wave velocity zone exists at the depths of 6 km to 12 km above the Moho. We interpreted this zone as the result of partial melting of the lower crust or the presence of a magma chamber. In the same area at the depths of 17 km to 30 km, the vertical S-wave velocity gradient is negative. We interpreted it as the result of the hot magma supply moving upward in the mantle.