Study on the site effects of trapezoidal sedimentary valleys under oblique incidence of SH waves based on the IBIEM

Abstract

Trapezoidal sedimentary valley sites exhibit significant differences in ground motion characteristics and a nonuniform distribution pattern under the oblique incidence of seismic waves. On the basis of the indirect boundary integral equation method (IBIEM), the scattered field of a sedimentary valley site is obtained by constructing and solving the Green's function of the scattering wave source from the valley sedimentary layer. This paper systematically and parametrically investigates the seismic ground motion characteristics and nonuniform distribution patterns of sedimentary valleys as they vary with changes in valley topographic parameters, oblique incidence angles of SH waves, and impedance ratios between bedrock and sedimentary layers. This study analyzed the differences between considering the scattering effects of sedimentary valleys and accounting only for the free-field seismic motion at the truncated boundary of the foundation. On the basis of this analysis, a more rational approach for ground motion input was proposed. The results indicate that as the valley height increases or the bottom width decreases, the sedimentary valley transitions from exhibiting edge effects to focusing effects. The oblique incidence of seismic waves significantly increases the amplification and nonuniformity of the peak displacements on the surface of the sedimentary valleys, with the maximum peak reaching more than five times the peak displacement of the incident wave. The change in slope has a significant effect on the seismic response at the inner and outer surfaces of the wedge. This phenomenon has been explained for the first time through ray theory. The amplification effect of sedimentary valley displacement becomes more pronounced with increasing impedance ratio. Compared with the free field on flat bedrock, the scattering effects generated by hollow and sedimentary valleys have a significant effect on the ground motion field at the truncated boundaries. The maximum errors at the bottom and right boundaries can reach 30.8 % and 54.8 %, respectively. Therefore, the total field that considers scattering effects should be used as the ground motion input.