GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.2 GNGTS 2023 Fig. 2 shows the spatial distribution of the resonance frequency f 0 extracted from the NHV for the 110 sites. Areas with similar resonance characteristics can be identified, in particular a central area northeast of Riva del Garda characterized by f 0 < 1 Hz, and another area east of Monte Brione characterized by f 0 values between 1 and 3 Hz. In contrast, sites on the valley margins or on a slope show high f 0 , even above 10 Hz, or in some cases a flat curve, indicating thinner or absent sediment cover, as expected from the valley floor to the surrounding slopes. Comparison between the spectral ratios estimated by GIT with respect to the reference site and those obtained by the NHV method shows excellent agreement between the curves, especially for frequencies equal to or lower than f 0 . Fig. 3 summarizes the results of the three different analyses along transect T1, transversal to the valley. From top to bottom, the panels show the 2D representation of the amplitude values obtained by interpolating the NHV, GIT and duration curves along the transect. Two zones can be identified, both characterized by high amplification but different f0. The first, west of Monte Brione, is characterized by very low resonance frequencies (below 1 Hz) associated with high amplitudes (above 5) and by duration lengthening of about 20 seconds. The secondary valley, east of Monte Brione, on the other hand, is characterized by higher resonance frequencies (3-5 Hz). Since it requires the availability of earthquake recordings (and thus seismological stations acquiring for a given period), the site response was evaluated at a limited number of locations. To extend the seismic response to the entire study area, we relied on the methods proposed by Strollo et al. (2012) and Ullah et al. (2013). The approach assumes NHV curves of ambient seismic noise as a proxy for the GIT amplification functions. The method is performed in 2 steps: 1) cluster analysis and validation of the NHV proxy; 2) correlation analysis of NHV curves and zonation. The analysis allowed us to divide the area into three zones characterized by different features of amplification and duration. Acknowledgements This study was performed in the framework of the project ‘Studio riguardante la risposta sismica locale del tratto terminale della valle del fiume Sarca in prossimità del Lago di Garda’ co-funded by ‘Servizio Geologico della Provincia Autonoma di Trento’.