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 is of practical significance for the seismic safety of the islands. A two-dimensional FEM model is established for a typical coral island in Xisha Islands considering the unsaturated zone caused by the fluctuation of daily tides. The accuracy of the model is then verified based on the seismic event records captured by the seismometers installed on the coral island. Then the nonlinear seismic response of the coral island with different earthquake signal inputs is explored. The major conclusions are drawn as follows: (1) The PGA amplification is more significant at the edge of the island because of the bowl-shaped interface between the coral sands and reef limestone, decreases from the surface of the island (loose coral sand layer) with depth and bends 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 impacts the PGA amplification factor and the Arias intensity, but has less effects on the PSA. The findings provide significant guide to the seismic stability evaluation and seismic design of the coral sand sites in Xisha Islands.