GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 Figure 2. T1/L1 frequency ratios as a function of the aspect ratio and of the basin type. The gray lines refer to the Adda basin case described in the following part of the text.x When the basin geometry is known, the provided relations offer a prediction of the expected modal sequence, thus helping in the design of future field surveys to better characterize the basin fill. We test the results of our models on some experimental cases from previous literature and collected by us, to also comment the uncertainties. Among the others we consider, for example, a section across the basin beneath the Alpine Adda valley, in the proximity of Teglio (Sondrio, northern Italy), where the glacially formed valley is 1700 m wide. Here, a migrated reflection seismic section and a borehole reaching 200 m depth are available (De Franco et al., 2009). Along this section we recorded ambient vibrations at 8 sites. The spectral analysis of the recorded signals shows clear peaks (Figure 3) of varying amplitude along the section, definitely stable over time. The modal shape reconstructed from the spectral amplitudes allow to associate identify the main modal frequencies in each direction. The T1/L1 experimental frequency ratio, measured at the basin under study, are given by the horizontal gray dashed lines (average value and uncertainty) in Figure 2. The intersection between the experimental frequency ratios and the theoretical curves provides the expected aspect ratio for each valley type. In this case, h/w ≈ 0.5 on average, which stands for a maximum basin depth of 425 ± 80 m which agrees with the maximum bedrock depth indicated by the seismic reflection profile (500 m). Finally, we discuss how relaxing our idealized assumptions influences the results.