Group velocity tomography of Rayleigh waves in South China Sea and its geodynamic implications

The South China Sea is one of the marginal seas of West Pacific where the Eurasian plate, Philippine Sea plate, Pacific plate and Indo-Australian plate interact, and therefore has complex geological structures. In this study, we give a 3D shear wave velocity structure of South China Sea deduced from surface wave tomography and analyze its geodynamic implications. Due to the newly deployed seismic stations in western and southern South China Sea, we have a better ray path coverage when using the single station method. This is especially true for the coastal region of southern China, where earthquakes occur less frequently and the newly added stations can increase the ray density in this region. We used earthquakes distributed on the periphery of South China Sea and collectedearthquake data from 48 stations. We first calculated the group velocity dispersion curves of fundamental mode for Rayleigh waves with periods from 14 s to 130 s using multiple filter technique. Then we conducted subspace inversion to get group velocity distributions for different periods in the region. Finally, on the basis of the relationship between shear wave velocity and group velocity under certain layer structure of the Earth, we obtained the 3D shear wave structures in the form of depth slices and vertical profiles by using a damped least square algorithm. The results show: ① High velocities exist in sea basins where velocity image delineates the shape of sea basins: the high velocities in shallow parts may indicate oceanic characteristics of the sea basin crust, while high velocities in deeper parts may come from high velocity materials which remained after the formation of oceanic crust at expanding ocean ridge. The velocity differences among sea basins are consistent with their heat flow values as well as their ages. The high velocities disappear at depths greater than 60 km, and are replaced by a low-velocity zone in a certain depth range. Beneath the low-velocity zone, a NE-SW high-velocity belt is observed at a depth of 200 km, and may be related to the ancient subduction in this region. ② Surrounding the South China Sea, there are obvious high velocities representing peripheral subductions. These high-velocity features are segmented and show differences in velocity values,implying nonuniformness of these subducting plates as well as their different subducting angles. ③ Above these high-velocities, we see discontinuous low-velocity zones. At shallow depths, these low velocities reflect the thickness of sedimentary layers and the crust, and in mantle depths, the low velocities may correspond to either mantle wedges of the ancient Pacific subduction zones or residual mantle melting anomalies after the cease of South China Sea opening. ④ The lithosphere thickness of the South China Sea basin tends to be 60mdash;85 km.