GNGTS 2019 - Atti del 38° Convegno Nazionale

614 GNGTS 2019 S essione 3.2 Passive seismic surveys. In order to improve the stratigraphic setting of the landslide body, microtremor measurements were carried out both in the landslide area and outside it (HVSR or H/V in Fig. 1), with a portable digital tomographer (Tromino®) equipped with three orthogonal velocimeters, recording in the 0.1 to 1024 Hz band. The noise was acquired at 128 Hz sampling frequency, amplified, digitized to 24-bit equivalents and recorded for 20 minutes at each of the measurement sites. Data processing was carried out by means of the software Grilla supplied by the manufacturer (Moho srl 2011) and fitted according to the 1-D layered soil theory (Castellaro and Mulargia, 2009). The HVSR spectra were obtained by a triangular smoothing function with a width equal to 5% of the central frequency. Datasets retrieved and processed correspond to: (i) the H/V curves and (ii) the velocity spectrum curves of the three directions of seismic wave motion (vertical, north-south, east- west), derived after the analysis of (i). Starting from the relationship between f , resonance frequency [Hz], Vs , shear waves velocity [m/s], and ( d ), strata depth [m], given by Eq.(1) f=Vs/4d (1) is possible to determine the thickness d , of the layers. Strata with Vs of the order of some hundreds m/s and frequency above 10 Hz are distinctive of a thickness of few meters; while strata below 0.1 Hz are related to strata deeper than 100 m. Spectral H/V ratios analysis. The H/V curves represent the value of the ratio between the average spectral amplitudes of the ambient vibrations (referred to the vertical and horizontal components of the motion) according to the frequency. Fig. 1 - Geological map of the area (top left); Geognostic surveys plan (top right); Geological cross-section (below) (from Barbero and Tissoni, in press, modified).