Numerical Modeling and Monitoring of the Seismic Response of Coral Reef Island in Xisha Islands, South China Sea

The seismic response analysis of the coral islands in South China Sea has practical significance for the seismic safety of the islands. This study establishes a two-dimensional FEM model of a typical coral island in Xisha Islands considering the unsaturated zone caused by the fluctuation of daily tidal. The accuracy of the model is then verified based on the seismic event recording captured by the seismometers installed on the coral island. Then we explore the nonlinear seismic response of the coral island with different earthquake signal inputs. Major conclusions are summarized as follows: (1) the PGA amplification is more significant at the edge of the island because of the bowl-shape interface between the coral sands and reef limestone, decrease from the surface of the island (loose coral sand layer) with depth and bend at the lithological and the unsaturated-saturated interfaces; (2) the resonance periods of the acceleration response spectra coincide with those of the input ground motion. However, the surface ground motion durations are significantly enlarged as compared to those of bedrock; (3) the saturation degree impact the PGA amplification factor and Arias intensity, but has less effect on PSA. The findings provide significant guide for the seismic stability evaluation and seismic design on the coral sand sites in Xisha Islands.