Nonlinear seismic response characteristics of a coral island site

The seismic safety evaluation of the coral islands in the South China Sea is an urgent and fundamental scientific issue. For a coral island in the South China Sea, the cyclic triaxial tests on 16 sets of saturated coral sand samples are performed. The test results show that the modified Matasovic constitutive model is suitable to characterize the shear modulus ratio reduction and damping ratio increase of coral sand, and the parameters of the Matasovic model are also proposed. Considering the engineering geology characteristics of the coral island, nonlinear dynamic behaviors of the coral sand, layout of nonuniform mesh of the free-field site and artificial boundary conditions as well as the special emphasis given to the irregular unloading-reloading rules for the stress-strain hysteresis loop, a two-dimensional nonlinear seismic response analysis model for the coral island is established. The spatial variation characteristics of the peak acceleration amplification factors, shapes of ground surface acceleration response spectra and durations of the coral island are analyzed. The results show that: (1) The peak accelerations of the coral sand site tend to increase as the elevation increases, the peak acceleration amplifications are obvious for the shallow depth less than 10 m, and the amplification effects are especially strong in lime-sand island and harbor basin regions. (2) The shapes of ground surface acceleration response spectra are closely related to the characteristics of the input bedrock motions. The ground surface spectral acceleration is very significant in the period less than 0.7 s. (3) The ground motion durations are shortened to different degrees, which are closely related to the characteristics of seismic bedrock motions, and to some extent related to the topographical and geomorphic characteristics of the coral island. The results will be of use in the construction of future coral islands in the South China Sea.