Abstract:This study aims to address the issue of the three-dimensional seismic motion simulation for the mountain-sedimentary river valley in Chengguan Town, Wen County, Gansu Province, at the town scale. By leveraging the advantages of boundary element method in reducing dimensionality and automatically satisfying far-field radiation condition for elastic waves when solving elastic wave propagation problems in an infinite domain, the indirect boundary element method (IBEM) was extended to simulate the three-dimensional seismic ground motion in the mountainous and river valley sites of Chengguan Town. Based on the drilling data and high-precision terrain data, a three-dimensional refined model was established. Plane P waves and SV waves were used as inputs to analyze the amplification effect of the mountain river valley terrain in Chengguan Town on the seismic motion in the frequency domain. In order to improve calculation efficiency, OpenMP (Open Multi-Processing) parallel programming was used in the time-consuming processes of constructing the scattered wavefield and solving virtual loads. The accuracy of the method was verified by comparison with the semi-analytical solutions. The simulation results of Chengguan Town showed that: (1) both the sedimentary river valleys and the mountains amplified seismic motion, but the amplification effect in the sedimentary river valleys was more pronounced. Under P wave incidence, the maximum displacement amplification coefficients (the ratio of seismic motion displacement to incident wave displacement amplitude) at observation points in the sedimentary river valleys and the mountains were 19.64 and 3.66, respectively, while under SV wave incidence, they were 14.13 and 3.48, respectively. (2) The reflection of the seismic waves by the mountains on both sides would aggravate the edge effect of the mountain river valley, with the maximum seismic motion amplification coefficient at the edge of the river valley reaching 19.64. (3) The spatial distribution of seismic motion in the sedimentary river valleys varied significantly with frequency. As the frequency increased, the strong seismic zone gradually shifted from the river valley center to the edge. The findings can provide certain reference for seismic zoning and earthquake resistant design for such sites.
