Comparison and analysis of the influence of different shapes of 3D concave topographies on site ground motion

The seismic responses of three-dimensional (3D) concave topography sites were calculated by the combination method of finite element discrete model, the viscoelastic artificial boundary and the central difference integral formula, and the characteristics and differences of the influence of pyramid-shaped, hemisphere-shaped and prism-shaped concave topographies in homogeneous elastic half space on site ground motion were analyzed under the vertical incidence of P wave and SV waves. (1) The influence of different shapes of concave topographies on site ground motion is significantly different, namely, the prism-shaped concave topography>the hemisphere-shaped concave topography>the pyramid-shaped concave topography, but the differences between the different concave topographies are more manifested in the medium frequency range. (2) Concave topographies have a complex scattering effects on seismic waves. Whether it is vertically incident P wave or vertically incident SV wave (only vertical or horizontal seismic motion input), it can produce significant ground motions in both directions in and near the sag area, and while SV wave is vertically incident, the wave scattering of concave topography is more intense, and furthermore the amplitude of ground motion in vertical direction generated by SV wave is even greater than that in horizontal direction. (3) The concave topography may cause the topographical edge effect of ground motion, and the ground motion at the edge of prism-shaped concave is significantly amplified, while this effect is relatively weak in the pyramid-shaped and hemisphere-shaped concave topographies. The study reveals that the steep degree of the concave edge has a significant impact on site ground motion, and that the edge effect of concave topography may exist and is closely related to the steep degree of the concave edge. This research result gives an important hint for the analysis of practical engineering problems, that is, the rational treatment of concave edge parts should be paid special attention to when establishing a simplified analysis model of concave topography. The research results can be applied to the engineering construction in mountainous areas, especially bridge and dam construction, and provide a reference for its seismic fortification to consider the characteristics and differences of different shapes of concaves on site ground motion, especially the possible topography edge effect.