The seismic response of sediment-filled valleys. Part 1. The case of incident SH waves

Abstract

abstract In this study is presented the extension to time domain calculations of the Aki-Larner method (Aki and Larner, 1970), developed to investigate the scattering of plane waves at irregular interface. Seismograms computed at the surface of a soft basin for SH waves vertically incident are compared with results obtained by finite difference, finite element, and asymptotic ray theory methods. The method is then applied to a study of the seismic response of sediment-filled valleys to incident SH waves. Various geometries and rheological parameters are considered. The study shows the important role played by the nonplanar interface, which, when the incident wavelengths are comparable to the depth of the valley, results in the generation of Love waves which may have much larger amplitude than the disturbance associated with the direct incident signal. In the presence of a high-velocity contrast between the sediments and the underlying bedrock, these local surface waves can be reflected several times at the edges of the valley, resulting in a long duration of the ground shaking in the basin. In the case of a lower impedance contrast, these waves may produce disturbances on the outer sides of the valley.