Physics-based modelling of ground motion in alpine valleys including strong surface topography and 3-D basin structure: A case study of the Rhône valley (CH)

Physics-based numerical simulations of seismic ground motion are crucial for advancing our understanding of regional earthquake hazard and risk. Complex geometries in sedimentary basins, coupled with strong surface topography, can cause significant variations in ground motion. To model these effects, a 3D representation of surface topography and available 3D models of the sedimentary basin velocity structure are used in numerical simulations. Using the spectral-element method, we synthesise displacement, velocity and acceleration waveforms in the Rhône valley, Switzerland. This region is characterized by complex topography, morphology, and significant seismic hazard.

We perform spectral-element waveform simulations with a maximum resolvable frequency of 5 Hz to investigate the joint effects of 3D basin structure and surface topography on ground shaking. Moderate-magnitude earthquakes that have been recorded in the area are used as point sources. Additionally, we compute waveforms for scenario earthquakes taken from the disaggregation of the current Swiss hazard model, SUIhaz2015 (Wiemer et al., 2016).

Our goal is to assess how these factors affect amplification patterns in different basin parts and topographic areas. We do so by comparing ground motion peak values at different altitudes (on mountains and valley floors with soft sediment conditions). Additionally, we calculate engineering-relevant ground motion parameters, such as cumulative velocity and significant duration up to the resolved frequencies, that help improve hazard estimations in the Rhône valley.

With our study, we show that the joint effects of topography and basin structure lead to larger amplification variations within the basin, and in the surrounding reliefs. We conclude that physics-based simulations have the potential to provide an adequate alternative for input ground motion in seismic hazard analysis. This is particularly relevant for modelling hypothesized earthquakes, which are essential for the assessment of seismic hazard in areas facilitating crucial infrastructure.