Comparison of Inversion Method of Wide Angle Ocean Bottom Seismometer Profile: A Case Study of Profile OBS973‐2 Across Liyue Bank in the South China Sea

Abstract

AbstractDeep crust structure of the wide angle seismic profile OBS973‐2, which crosses the central sea basin and Liyue bank in the southern of South China Sea (SCS), represents one of the direct evidences for conjugate spreading and evolution of SCS. For this reason, we remodeled the data by using both forward and inversion methods for a detailed crustal velocity structure, and also compared results with depth profile of multi‐channel seismic (MCS) close to OBS973‐2 for more reliable crust thickness. Comparing to former works, and according to the new velocity models, we divided the profile to three parts of continental crust (0∼200 km in the profile), continent and ocean transition (COT, 200∼280 km in the profile) and ocean crust (280∼370 km in the profile). And their crust structures are obviously different. Sediment layer in the continental crust shows a feature of horizontal variations in both thickness and velocity, and total crust thickness in continental part is thick (about 20 km). In the COT, both thickness and velocity are horizontally smooth, and the total thickness of crust is thinner (about 8 km). And in the ocean crust, the crust thickness thins to 6 km. There are two main conclusions, (1) the method of using data collected by wide angle seismic and multi‐channel seismic and using both forward and inversion software in modeling will obtain more reliable information of crust structure, (2) the new insights on the crust structure are as follows. (a) We get the seaward thinning of Moho and its depth of about 12∼18 km below the sea level in COT from both wide angle and multi‐channel seismic profiles. (b) No high velocity layer was found beneath COT, which represents non volcanic passive continental margin and its conjugate spreading point is Zhongsha massif. (c) Heavy extension occurred in upper crust of continental part, which shows horizontal velocity anomalies and low velocity zone in the velocity model and a large number of normal faults in the multi‐channel seismic profile.