GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 615 The peaks of the H/V curve at a given frequency may be more or less evident and interpretable, depending on the magnitude of the seismic impedance contrast between the rigid substrate (Lugagnano Clay) and the most surficial layers (reworked clays of landslide body). The modelling of the H/V curves was carried out through an reverse procedure that compares the single spectra and the H/V ratios measured with synthetic ones. The optimal interpretation was the one with the best match between measured data and synthetic ones. The seismostratigraphic reconstructions were obtained by assuming for all investigated sites shear-wave velocities at the top of about 100-150 m s -1 , as representative of the boundary between the agricultural soil and the new landslide body. Seismic layers correspond to facies in which the greatest contrasts of impedance were observed. The spectra H/V ratios interpretation allowed us to recognize a set of discontinuities in the landslide body (Fig. 2). The peaks between 37-58 Hz can be related to the colluvial cover with a depth of about 1 meter. In the HVSR2 site 15 and 25 Hz peaks can be related to the overlapping of the recent landslide body on the older one. In the HVSR1 and H/V2 the peaks between 9-10 Hz can be associated to the older landslide body. HVSR peaks at 3,5-4,5 Hz can be related to transitional facies. Frequencies lower than 3Hz don’t show clear contrasts of impedance and thus they can be associated to the clay substrate. In the H/V3, located outside the older landslide body, the plateau between 10 and 13 Hz is interpretable as coalescence of minor peaks corresponding to minor discontinuities due to anthropic reworked landslide body. Fig. 2 - Spectral H/V ratios recorded at H/V1 site (above) and H/V site (below). The red line is the average H/V, the black lines are the 95% confidence interval. The interpretation of HVSR3 spectra allowed to estimate in 449 m/s the value of the Vs eq according to the NTC2018. The transition from Lugagnano Clay to marly clay (transitional facies) is underlined by a clear discontinuity in the velocity profile (Fig. 3). Conclusions. The use of the passive seismic surveys allowed us to improve the geological model of the landslide area, highlighting the presence of new stratigraphic layers, interpretable as slides surfaces due to the triggering of the older landslide.