Crustal Structure of Hainan Island and Surrounding Seabed Based on High-Resolution Airborne Gravity

Hainan Island and its surrounding seabed are located at the intersection of the Eurasian, Indochina, and South China Sea tectonic plates with active Quaternary volcanism and intensive seismicity, such as the 7.6-magnitude earthquake that occurred in northern Hainan in 1605. Based on the newest airborne gravity data of Hainan Island and its adjacent areas, this paper uses wavelet multiscale decomposition followed by power spectral analysis to estimate the average depth of each layer of the source field. We use the Parker–Oldenburg method to invert the Moho structure, incorporating constraints from seismic data to investigate the fine crustal structure and deformation characteristics to elucidate the deep seismogenic mechanism. The regional Moho depth decreases from 30 km in the northwest to 16 km in the southeast. The map of the Moho surface shows three Moho uplift zones, located in the northern Hainan Island, the southern Qiongdongnan Basin, and the southwestern tip of Hainan Island. The following findings are revealed: Firstly, a series of northeastward high-gravity anomaly strips are discovered for the first time in the middle and lower crust of Hainan Island, which may be the remnants within the continental crust of the ancient Pacific northwestward subduction during the Mesozoic era. Secondly, under the Leiqiong volcanic rocks, there is a pronounced northeastward high-value anomaly and shallower Moho depth, which may indicate the deep-seated mantle material that rose and intruded during the activity of the Hainan mantle plume. Thirdly, the seismogenic structure is discussed by combining the wavelet multiscale decomposition results with natural seismic data. The results show that earthquakes occur in the place where the NE-trending gravity anomaly is cut by the NW-trending fault in the upper crust. That place also lies in the gravity anomaly gradient or high-value anomaly in the middle and lower crust. These features reveal that the earthquakes on Hainan Island are controlled by the left strike-slip activity of the Red River Fault and deep mantle upwelling caused by Hainan Plume.