A database for the empirical observation and characterization of earthquake site effects in alpine valleys

The alpine valleys are peculiar geological environments, as the thickness of their sedimentary infill increases from few meters at the valley borders to several hundred meters at its centre. This setting determines distinctive earthquake response effects. A number of works have examined such effects, which can be summarized as: i) 2D/3D resonance phenomena; ii) edge-generated surface waves; iii) trapped seismic waves (i.e. waves remaining trapped in the valleys’ basin due to the impedance contrast with the surrounding bedrock). Although several studies have tackled one or more of such effects, only recently attempts have been made to systematically identify – in numerical modelling studies – which subsurface parameters control these phenomena and to quantify their effect on local site response.

In the framework of the “Alpine valleys” project, we have compiled a database to empirically observe and systematically characterize earthquake site effects in alpine valleys at the national scale of Switzerland; this work builds on the data and results of the national amplification model developed in the framework of the project Earthquake Risk Model for Switzerland. The “Alpine valleys” database comprises earthquake observations, geophysical measurements and morphological, topographic parameters. In particular, the database includes:

• A dataset of empirical amplification functions estimated (by means of spectral modelling technique) at about 275 (urban) free-field seismic stations of the Swiss networks. These instrumented sites cover a variety of geological and morphological settings (alpine valley beds and flanks, as well as sites in the plains of the Swiss Plateau for comparison with the former).
• A dataset of waveforms recorded by the seismic stations mentioned above, collecting local and regional earthquake records from the period 2000-2022.
• A dataset of measured S-wave velocity profiles, derived from geophysical measurements performed at a portion (~120 sites) of the set of considered seismic stations.
• A dataset of 1750 single-station noise recordings, processed in terms of horizontal-to-vertical spectral ratio. 160 of such measurements coincide with a seismic station with measured site amplification function.
• A map of the unconsolidated sediments-bedrock interface, covering all Switzerland. The map allows to quantify the geometry of the valleys’ sedimentary infill (e.g. depth, width, shape ratio), one of the key elements determining the peculiar site effects observed in valley beds.
• Maps of multi-scale topographical parameters. Maps of planform, profile and standard curvature were derived for the entire Swiss territory, at 7 spatial scales between 75 and 7800 m.
• A classified map of the Swiss alpine valley beds, obtained with a GIS algorithm combining topographical and morphological data.

The ambition of our work is cross-referencing the empirical earthquake observations (waveforms, local amplification functions) with geophysical measurements and geological, topographic and morphological parameters, in order to single out the site effects proper of deeply incised valleys, identify the frequencies of ground motions where these appear, and map the morphological and geological settings where they become significant. The implications for the soil classification of the Swiss seismic building code will also be assessed.