Frequency- and Component-Dependent Topographic Effects in Alpine Valleys Using 3D Physics-Based Simulations

ABSTRACT We present three-dimensional (3D) spectral-element simulations of seismic wave propagation in the Swiss Rhône Valley to quantify how realistic Alpine topography affects ground motion across frequency bands. The simulations incorporate a digital elevation model–based free surface topography representation, a 3D crustal velocity model, and geotechnically constrained near-surface layers. The simulations are performed for three local earthquakes (Mw 3.2–4.5) recorded in two different configurations: topographic and flat-surface with basin geometry preserved. Topo/flat spectral ratios reveal a systematic component asymmetry in the topographic response. At mountain sites sampled by a spatially uniform synthetic grid, horizontal motion is deamplified across the dominant low-to-intermediate frequency bands (median 0.80–0.97× at 0.05–0.50 Hz), whereas vertical motion is consistently amplified (1.16–2.32×). The real seismic network with preferential sites confirms the strong vertical amplification, whereas the horizontal regime yields weak deamplification (0.65–1.00×). Basin sites exhibit mixed horizontal response and strong vertical amplification, consistent with coupled topographic scattering and basin-wave interactions as well as excitation of the Rhône basin’s fundamental frequencies. The observed asymmetry is consistent with topography-induced mode conversion and constructive interference associated with Alpine ridge geometry. These results demonstrate that realistic surface topography can fundamentally redistribute seismic energy between motion components and that sparse observational networks may incompletely capture this behavior. Physics-based simulations with realistic relief and spatially unbiased receiver sampling therefore provide a complementary framework for seismic hazard assessment in mountainous terrain.