GNGTS 2016 - Atti del 35° Convegno Nazionale

308 GNGTS 2016 S essione 2.1 in September 2013 we acquired 5 active/passive multichannel seismic surveys based on surface waves (SoilSpy system (MoHo s.r.l.)) and 25 single-station passive seismic surveys (Tromino® (MoHo s.r.l.)) to assess the resonance frequencies of the inspected sites. The surveys showed that the soft layers (weathered rock, laterite soil, etc.) had thickness generally lower than 10 m and intermediate Vs (usually higher than 200 m/s). At a very few sites the bedrock was practically outcropping. A few meters of weathered/fractured rock were often present under the soft cover and could be recognized from intermediate Vs values (300- 500 m/s). Sites on quartzite showed higher Vs at shallower depths (> 800 m/s) compared to sites on gneiss and syenite. Following the described results, two Vs profile models were selected as representative of the whole area (Fig. 2): the first model (black in Fig. 2) was applied to sites on non- outcropping (> 5 m cover) quartzite, while the second model (red in Fig. 2) was applied to sites on gneiss/ syenite. By using 7 earthquake recordings characterized by a PGA = 0.09 g we performed a numerical modelling of site response with a linear equivalent code (Bardet et al. , 2000). Conclusions. The expected ground motion obtained from the 4 branches of the considered logic tree shows different levels according to the various branches but it remains low in all cases, with values below 0.12 g. The weighted mean expected PGA for a 712-year RP is 0.089 g (0.115 g with standard deviation SD is added) for rock. The seismic motion expected at the bedrock level from the PSHA was then propagated to the surface through a site-specific analysis. Two Fig. 3 – Subsoil amplification map.