Ambient noise tomography and deep structure in the crust and mantle of the South China Sea

Using the continuous seismic ambient noise data recorded by 28 land seismic stations in the area surrounding the South China Sea and 2 island stations on Dongsha Island and Taiping Island during January 2011 and December 2016, we calculated the cross-correlation functions between station-pairs utilizing the cross-correlation method, and extracted the Rayleigh surface wave group velocity and phase velocity dispersion curves based on it. The group velocity and phase velocity images of South China Sea over period range of 12 similar to 40 s were inverted by using Fast Marching and Subspace method. Then, the 3D S-wave velocity structure up to 60 km in depth was inverted by joint inversion. During the inversion process, we added a water layer into the inversion model, considering that several kilometers seawater layer could strongly change the surface wave dispersion behavior. In fact, it is proved that an additional water layer can significantly improve the reliability of the inversion results. From the inverted result, there exists strong lateral variations in the crust and upper mantle structures within the study areas. These variations spatially correlate with the main structural units in this area. At depth of 5 similar to 10 km, the Yinggehai-Song Hong Basin is of low velocity, which may be related with the thick sediments layer under the sea. At depth of 5 similar to 15 km, the high velocity anomaly in our model correlates well with location of the South China Sea Basin, which may infer that the crust thickness of the sea basin area is much thinner than that of the continental margin area. At depth of 20 similar to 30 km, the high-velocity feature of the sea basin extended further to the continental margin area. This reflects the gradual thickening of the crust thickness from the sea basin to the continental margin, which is consistent with the deep seismic profile result obtained using OBS data. In the depth range over 35 similar to 60 km, the high-velocity feature of the South China Sea Basin is still obvious and the velocity increases with the depth in general, by which we surmise that the lithosphere thickness of the sea basin should be greater than 60 km.