Combining a large, nationwide ambient noise database with morphometric analyses to map 2D resonance effects in sedimentary basins in Switzerland

Deeply incised valleys or sedimentary basins often exhibit complex resonance patterns that diverge from the commonly assumed one-dimensional (1D) behaviour. In such cases, the soil resonance fundamental frequency f₀ is not determined by the local depth-to-bedrock; instead, f₀ is constant across the central portion of the basin section, reflecting the overall geometry and material properties of the sedimentary infill. These 2D (or even 3D) resonance regimes are challenging to identify and are generally overlooked in building codes. This study, funded by the Swiss Federal Office for the Environment, seeks to characterize 2D resonance phenomena across Switzerland by leveraging over 6000 ambient noise measurements and a large-scale morphometric dataset.

The primary dataset comprises ~4000 ambient vibration measurements acquired across Switzerland since the late 1990s, archived in the Swiss Seismological Service (SED) site characterization database. The recordings were processed using the horizontal-to-vertical spectral ratio (H/V) technique and soil resonance frequencies were identified following the best practice criteria. This database has been further enhanced by recent high-resolution ambient noise campaigns conducted by SED in key sedimentary basins: the Swiss Rhône Valley, the Lucerne and Horw basins in Central Switzerland, and the High Rhine Valley near Basel. These campaigns, with spatial resolutions ranging from 100 to 400 m, contribute approximately 2000 additional measurements with their f₀ for the areas of interest.

This sizeable ambient noise database is paired with a collation of various geological/geophysical models: the backbone model by the Swiss Federal Office of Topography is complemented by regional models for the Alpine and High Rhine valleys, the Geneva Basin, the Grisons, and the Basel area. The collation of such models maps the depth of the sediments-to-bedrock interface over most of Switzerland. Based on this information, we performed morphometric analyses, which allowed extracting key geometrical parameters (shape, width, maximum depth) of the sedimentary infill along 4500 transects – spaced by 250 m and spanning all large sedimentary basins.

Cross-referencing the soil resonance frequencies with the morphometric characteristics of the sedimentary basins, we observed patterns consistent with those predicted by numerical studies from the literature. Our analysis distinguishes valleys with 1D resonance behaviour from those with 2D resonance regimes. Furthermore, as a valley's shape ratio (half-width over maximum depth) increases, resonance frequencies converge towards specific 2D vibration modes, particularly fundamental SH- and SV-modes and their higher harmonics. We also examined whether these ambient vibration resonance modes reflect into the (directional) ground motion local response at seismic stations.

The results of this study are synthesized into a national-scale map identifying basins and valley bottoms with 1D or 2D resonance behaviours and their corresponding resonance frequencies. Our study will contribute to the decision of whether the Swiss national building code should adopt tailored elastic response spectra for alpine valleys prone to 2D resonance patterns.