GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.2 GNGTS 2023 Site Response Analyses And Zonation: Application In The Lower Sarca Valley At North Of Lake Garda G. Laurenzano 1 , M. Garbin 1,2 , S. Parolai 1,3 , C. Barnaba 1 1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – OGS 2 Provincia Autonoma di Trento, Servizio Geologico 3 Università degli Studi di Trieste, Dipartimento di Matematica e Geoscienze In this study, we examined the site response of the lower Sarca Valley on the northern shore of Lake Garda. Although the area is characterized by moderate seismicity, the sedimentary deposits which compose the valley can significantly increase both the amplitude and duration of the seismic signal. For this reason, we estimated the seismic response not only in terms of spectral amplification, but also evaluated the increase in the duration of ground motion in relation to its frequency content. Finally, to better define the seismic hazard of the area, seismic response evaluated at selected sites was spatially extended. The study area is represented by the lower part of the Sarca Valley, which is about 5-6 km long and 3-4 km wide and is divided to the south by the ridge of Monte Brione, which extends for about 2.5 km from the coast of Lake Garda toward the NNE. The Sarca River flows southward into the narrower eastern part. The features of the sedimentary cover range from glacial, fluvial-glacial and glacial lake deposits below more recent deposits formed by the Sarca River. A reflection seismic survey oriented NW - SE and approximately perpendicular to the valley axis (Felber et al., 2000) shows discontinuity between rock and sediment fill at a depth of about 420 meters. First, we evaluated the site response using well-known methods such as the Horizontal-Vertical Spectral Ratio NHV (Nogoshi and Igarashi, 1970; Nakamura, 1989) and the Generalized Inversion Technique GIT (Andrews, 1986).We then applied the Joint Analysis of Sonogram and Standard Spectral Ratio methodology (Parolai and Bard, 2003), which combines the analysis of sonograms and the standard spectral ratios method (Borcherdt, 1970), to evaluate the frequency-dependent amplification and increase in duration of ground motion respect to a reference site The recordings of seismic noise of approximately 30-45 minutes in length were collected at 110 sites (Fig. 1). Continuous recordings of ground motion were collected at 19 sites over a period of about 8 months to 2 years to create a database of earthquake waveforms. The stations are oriented along three transects: two transverse and one longitudinal to the Sarca Valley.