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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.

The ambient vibration analysis is a non-invasive and low-cost technique used in site characterization studies to reconstruct the subsurface velocity structure. Depending on the goal of the research, the investigated depth ranges from tens to hundreds of meters. In this work, we aimed at investigating the deeper contrasts within the crust and in particular down to the sedimentary-rock basement transition located at thousands of meters of depth. To achieve this goal, three seismic arrays with minimum and maximum interstation distances of 7.9 m and 26.8 km were deployed around the village of Schafisheim. Schafisheim is located in the Swiss Molasse Basin, a sedimentary basin stretching from Lake Constance to Lake Geneva with a thickness ranging from 800 to 900 m in the north to 5 km in the south. To compute the multimodal dispersion curves for Rayleigh and Love waves and the Rayleigh wave ellipticity angles, the data were processed using two single-station and three array processing techniques. A preliminary analysis of the inversion results pointed out a good agreement with the fundamental modes of Rayleigh and Love waves used in the inversion and a quite strong disagreement with the higher modes. The impossibility to explain at the same time most of the dispersion curves was interpreted as the co-existence, within the investigated area, of portions of the subsurface with different geophysical properties. The hypothesis was confirmed by the Horizontal-to-Vertical spectral analysis (H/V) which indicated the presence of two distinguished areas. The observation allowed a new interpretation and the identification of the Rayleigh and Love wave fundamental modes and of the S-wave velocity profiles to be reconstructed for each investigated zone. It results in two S-wave velocity profiles with similar velocities down to 15 km deferring only in their shallow portions due to the occurrence of a low velocity zone at a depth of 50–150 m at the centre of the investigated area., Pure and Applied Geophysics, 177 (9), ISSN:0033-4553, ISSN:1420-9136, ISSN:1557-7368

The difficulty and the high cost to assess the subsurface properties led to the development of several geophysical techniques. Generally, the focus of a site study is the reconstruction of the S-wave velocity profile down to few tens to hundreds of metres (e.g. 30–300 m), but not the investigation of deeper structures, such as the transition to the crystalline basement. However, such deeper structures are of interest when seismic hazard products have to relate to a reference rock-velocity profile, for example in regional seismic hazard assessment and microzonation studies. To estimate the S-wave velocity profiles down to several kilometres, we study the potential of Rayleigh and Love waves at low (down to 0.1 Hz) and high (up to 20 Hz) frequencies using two seismic arrays of increasing size. The small array, with a maximum inter-station distance of 900 m and a recording time of 3 hr, was aimed at constraining the shallow subsurface down to about 350–400 m, while the big one, with a maximum inter-station distance of more than 29 km and 23 hr of recording had the goal to constrain the deeper structure. The arrays were deployed in northern Switzerland (east of the village of Herdern) within the Swiss Molasse basin, a sedimentary basin north of the Alps stretching from Lake Constance to Lake Geneva; its thickness increases from 800 to 900 m in the northeast to more than 5 km in the southwest. The seismic data recorded by the two arrays were analysed using the techniques developed for the analysis of small-aperture arrays. The results were inverted for the S-wave velocity profile in two steps: first, the Rayleigh and Love wave phase dispersion curves were inverted together. Secondly, the previous dispersion curves were jointly inverted with the measured Rayleigh wave ellipticity angle. The resulting S-wave velocity profiles are similar and show agreement with the available geological and geophysical data, confirming the potential of surface waves to investigate deep structures. Moreover, our analysis proves the feasibility of site characterization techniques to large arrays and the possibility to estimate the P- and S-wave velocity profiles down to 5 km, deeper than the contrast between Molasse basin and crystalline rock at around 2.1 km., Geophysical Journal International, 222 (1), ISSN:0956-540X, ISSN:1365-246X