GNGTS 2013 - Atti del 32° Convegno Nazionale

sites characterised by unlike lithologic and geomorphologic features in addition to sensible variations in the level of suffered damage (Fig. 1). As concerns the superficial geology of the area, the outcropping materials are represented by alluvial deposits of three different depositional facies: 1) the recent fluvial deposits of the Po River plain outcropping to the NW of the outskirt of the city centre 2) sand and clay deposits of the Po River delta plain covering most of the historical city centre, where sands are present mainly in the southern and south-western portions and the rest of the city centre is covered by clay and silt, and 3) sand and clay sediments belonging to the Apennines rivers which outcrop in the SW sector of the outskirt of the city (Fig. 1). These superficial sediments overlay two distinct horizons known as the Lower Po Synthem and the Upper Po Synthem which were identified from the integration of available borehole information, AGIP wells and seismic reflection sections. The formation of these discontinuities is surely of tectonic origin (Amorosi and Pavesi, 2010). In fact, the simplified lito-hydrogeological section depicted in Fig. 2 shows a quasi-symmetrical wedging of the two marker horizons moving towards Ferrara structural high near Casaglia village (NW of Ferrara). Farther east toward Ferrara and generally toward the Adriatic Sea the depth to the base of the two Quaternary horizons increases reaching some 100-130 m and around 200 m respectively (Amorosi and Pavesi, 2010). Recently one long seismic reflection profile was carried out to investigate the shallow structure of the area located between Copparo, about 15 km NE of Ferrara, and the Po River (Accaino and Baradello, 2013). The survey was conducted by the OGS (Trieste) seismic reflection team. The final seismic section (personal communication with the authors) evidenced the presence of three main reflectors located at ca. 100 m, ca. 170-200 m and 350-450 m. As concerning the Vp velocities, the authors estimated values ranging between 800 m/s and 1200 m/s for the shallowest layer and between 1500 m/s and 2300 m/s in the deepest layer. The estimated average Vp velocities enclosed between the earth’s surface and the base of the third layer were found to increase from 1400 m/s (nearby Copparo) and 2000 m/s approaching the Po River. Further information about the P-wave velocity distribution, at some sites located in the immediate vicinity of the historical city centre, as obtained from the analysis of a number of old reflection profiles carried out by AGIP, indicated that the P-wave velocities are always less than 2000 m/s for depth ranges from ca. 400 m and 500 m. Additional analysis of the seismic velocities indicate the presence of a nearly continuous seismic discontinuity at about 70-80 m to the north of Ferrara that jumps to ca. 160 m toward east-southeast of Ferrara where sensible vertical jumps between 75 and 160 m are present. These variations are most probably of tectonic origin. These facts, although sporadic, indicate that the simplified lithostratigraphic model supposed to exist beneath Ferrara is coherent with the tectonic model of the area. This structure represents the frontal most sector of the Northern Apennines, composed of a series of folds-and-thrust active belts northward verging. Fig. 2 - Hydro-geologic sections running in the WNW-ESE direction, through a) the Casaglia structure (Ferrara) and b) the Mirandola structure (Modena) (modified after [Regione Emilia-Romagna and ENI-AGIP, 1998]). Both sections show the main aquifer bodies corresponding to Middle-Upper Quaternary lithostratigraphic units mainly consisting of alluvial and shallow water marine deposits progressively infilling the Po River foreland basin (modified after the geologic and seismic service of the Emilia-Romagna Region). 168 GNGTS 2013 S essione 2.2