Lithospheric Velocity Structure Beneath the Northern Margin of the South China Sea

Imaging the lithospheric structure beneath the South China Sea (SCS) not only provides crucial constraints on the tectonic history of southeastern Asia, but also provides an important sample for understanding the evolution of the oceanic lithosphere. However, due to the lack of seismic sampling, the lithospheric structures beneath the SCS are not well resolved. Here, with China National Seismic Network, we observe the "Lid signal" generated by earthquakes below the lithosphere, which propagates through the fast mantle Lid and acts as the first arrival. The Lid signal provides robust constraints on the structure of the lithosphere. Through modeling the Lid signals of events occurred at different depths, we find that the thickness of the lithosphere beneath the northern margin of the SCS is similar to 65 km. Across the lithosphere-asthenosphere boundary, the SH velocity decreases by 6% similar to 8% within 60 km. Such a large velocity contrast may be due to the presence of melt in the asthenosphere. We also detect another velocity jump at a depth of similar to 250 km in both SH and P wave data, which correlates with the depth of the X-discontinuity. A comparison between the lithospheric structure beneath the SCS basin and other oceanic plates further suggests a hot asthenosphere beneath the SCS, which could be related to the existence of the Hainan Plume nearby.