A comparison of observed and simulated site response in the Rhône valley

SUMMARY Site effects in the city of Sion in the Rhone valley are analysed from weak motion signals recorded on a dense temporary array. We simulate the recorded events with a 3-D finite differ- ence method for frequencies up to 4 Hz using a recently developed velocity model of the Sion basin. Site-to-reference Fourier spectral ratios are computed from 16 local and regional events. All sites exhibit amplification factors of up to 12 between 0.5 and 0.6 Hz, which can be reproduced by the numerical simulations. By rotating the weak motion to directions parallel and perpendicular to the valley axis, we show that this low-frequency amplification is caused by the SH00 and SV0 fundamental modes of 2-D resonance. Additional peaks of amplification can be observed at higher frequencies, with amplification factors of up to 20 at some sites. Application of the high-resolution frequency-wavenumber and the multiple signal char- acterization method to the vertical component of recorded and simulated signals show that edge-generated surface waves arriving from almost all directions dominate the wavefield at 1.25 and 2.50 Hz. Peak ground velocities computed from the simulated ground motion show interference patterns that depend strongly on the incidence direction, and the computed amplification of peak ground velocities are generally in agreement with the observations. We conclude that the complex 3-D geometry of the basin needs to be considered to evaluate site effects up to at least 2.5 Hz.

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