Seismic Soil Effect in an Embanked Deep Alpine Valley: A Numerical Investigation of Two-Dimensional Resonance

This article presents a 2D numerical study of the seismic response of a deep embanked sediment-filled valley (Rhône valley, Switzerland) to incident SH and SV waves. The 2D modeling is performed with a program developed by Pedersen et al. (1995), based on the indirect boundary element method technique. This valley exhibits variation in subsurface geometry. Therefore, three representative cross sections are considered in order to investigate the influence of symmetric and asymmetric morphology on the seismic response. Comparison between 1D and 2D modeling shows that both resonant frequencies and amplification values differ to a large extent. Looking in the central part of the valley, the 2D predicted amplification value is about 2 times higher than in the 1D case, whereas the fundamental resonant frequency value is higher by a factor of 1.7. Moreover, 2D modeling results show that this valley presents a 2D resonance due to its shape ratio and shear-wave velocity contrast. The main characteristic is the unchanged position of the fundamental resonance frequency, regardless of the surface site along the cross section under consideration and the incidence angle. However, the asymmetric cross section induces a specific spectral amplification pattern at the surface site and along the cross section, which is also a function of the direction of incidence of the seismic wave. Such results show the importance of considering the morphology of a deep and narrow valley when investigating the local seismic response.