GNGTS 2016 - Atti del 35° Convegno Nazionale

402 GNGTS 2016 S essione 2.2 (Madonna della Strada Synthem, Nocentini, 2016) onto the Meso-Cenozoic carbonate bedrock at 200-300 m below ground level. f 1 is caused by shallower impedance contrast due to 3-20 m thick of Red silt onto calcareous L’Aquila breccia (Colle Macchione-L’Aquila Synthem of Upper Middle Pleistocene, Marine Isotope Stages – MIS - 6-8) (Nocentini, 2016; Tallini et al ., 2016a). The Red silt corresponds to a colluvium and karst deposits made of reddish to dark brown clayey silts with sparse sub-angular clasts. These deposits show thickness up to 20 m and are generally interpreted as re-worked and colluviated paleosol (Alfisol) mantling the epikarst developed onto the L’Aquila breccia in a wet and warm interglacial stage (Last Interglacial, Eemian, MIS 5e) (Nocentini et al., 2016). The L’Aquila breccia is composed by clast- to matrix-supported, massive or stratified, both well cemented and incoherent debris flow and rock avalanche breccias and megabreccias, with highly heterometric (up to m3), poorly sorted, angular to sub-angular carbonate blocks in a whitish calcareous sandy silty matrix. Whitish to greyish calcareous clayey silts levels and lenses are interlayered at different elevations (Nocentini, 2016; Nocentini et al. , 2016). While the seismic significance of f 0 was already analyzed in detail (Bordoni et al. , 2014; Del Monaco et al. , 2013; Gruppo di Lavoro MS–AQ, 2010), then our study was focused on the relationship between the shallow subsoil model, the medium resonance frequency (f 1 ) and the buildings damage distribution due to the recent April 6, 2009 (Mw: 6.1) and the February 2, 1703 (Mw: 6.7) earthquakes. Results and discussion. We elaborated via ArcGis the map of the bottom surface of the Red silt, the maps of the f 1 contour lines and the building damage distribution due to the above mentioned earthquakes (Clementi and Piroddi, 1986; Tertulliani et al. , 2011). The contour line pattern of Red silt matches enough with that of f 1 (Figs. 1 and 2). To analyze deeply the relationship between the Red silt thickness and f 1 we select about ten sites which the subsoil model is well known (Tab. 1). We apply for the sites reported in Fig. 1 and Tab. 1 the following equation: f 1 =Vs/4H cal Fig. 1 – Comparison of the Red silt thickness and the damaged buildings due to the earthquake of February 2, 1703, Mw 6.7 (A) and April 6, 2009, Mw 6.1 (B). Blue and magenta colors refer to the heavy buildings damage or buildings collapse, respectively; the letters refer to the sites of Tab. 1.