GNGTS 2019 - Atti del 38° Convegno Nazionale

342 GNGTS 2019 S essione 2.2 If one compares the characteristics of local topography with changes in amplitude and frequency of H/V peaks revealed at each station by HVIP analysis, information on lateral variations in velocity and thickness can be inferred. From YJG0 to YJG8, the lower resonance frequencies (2-4 Hz) show a decreasing trend, which is negatively correlated with the surface elevation (Fig. 2a). Thus, it can be assumed that the frequency variation is mainly linked to an increase of the thickness of the deposit, which is also consistent with the observed increase in the altitude of its surface. On the contrary, the corresponding H/V amplitudes show a tendential increase with the surface elevation along the profile (Fig. 2b).This can be due to an increase of the impedance contrast, possibly caused by a decrease of the surface layer velocity, or by an increase of the velocity of the substratum. A deeper interpretation of these results is supported by the outcomes of ERT survey, which was carried out according to a Wenner array along the profile of ambient noise measurements. The inversion of the resistivity data variation provided supplementary information about the structure and mechanical properties of the dam materials (Fig. 3), revealing a contrast between the twomain layers, a more resistive superficial layer overlying a less resistive one. By estimating the thickness of the upper layer, in correspondence of each station, a good correlation was found with the higher resonance frequencies (6-13 Hz) obtained from ambient noise analysis. Using the thicknesses derived from the ERT data inversion to estimate the Vs velocity of the surface layer from resonance frequencies (through the relation f 0 = ), we note good coherence with the H/V amplitude trend that reflects a variation in the impedance contrast. Indeed, the part of the profile characterized by a thicker and higher resistivity deposit, is also that where the impedance contrast appears lower; this possibly is the effect of a major content of stiffer material (rocky blocks) within the most recent 2008 slope failure deposit. Fig. 2 - Profiles showing the surface topography of the landslide dam deposit along the left (NE) river bank (see Fig. 1) and (a) the low resonance frequencies detected at each noise measurement station and (b) the corresponding amplitude of the Rayleigh wave ellipticity peaks.