Response Spectra for the Deep Sediment-Filled Rhone Valley in the Swiss Alps

Abstract eng:
We present numerical 2D modeling of earthquake ground motion for various profiles across the Rhône Valley in the Swiss Alps and characterize the seismic response in terms of spectral acceleration. Two methods were chosen to compute response spectra in the sedimentary basin: reflecting the epistemic uncertainty of a site response analysis. The first method uses recorded ground motions on rock representing the relevant distancemagnitude range of the probabilistic seismic hazard in the Valais. The signals were used as input in the 2D models of the sites. Even when we applied a generic rock profile to reduce possible effects of soft surface layers, the procedure remained uncertain. It is very likely that site effects affect the frequency content of the input signals used. Moreover, at larger periods the signals lack energy and the resulting response spectra from the limited number of waveforms used do not account for large earthquakes at larger distances. In the second method, the rock hazard spectrum is multiplied with the modeled amplification function at the specific sites. Since we do not know the characteristics of the reference rock condition for the hazard spectrum, it is impossible to estimate such differences in the modeling. At larger periods above 2s, the shape of the hazard spectrum was extrapolated and therefore remains uncertain. We show how the selected method influences the response spectra of the valley sites. Particular attention is paid to how the internal sediment structure and the often weakly constrained Q-factor shape the seismic response. Results obtained for the different profiles are compared with observed earthquake ground motion, and with reference spectra (Swiss building code and Eurocode 8).

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