GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 19 Geological and seismotectonic background. The Po Plain represents the foreland of two mountain belts: the NNE-verging northern Apennines and the S-verging central Southern Alps. This architecture has been basically due to the convergence between the African and European plates (Carminati et al ., 2012). Since at least the recent 5 Ma, the Po Plain area underwent a continuous subsidence, which allowed the sedimentation of huge thicknesses of Plio- Quaternary foredeep terrigenous units (Fantoni and Franciosi, 2010; Consiglio Nazionale delle Ricerche, 1992). These units range from few hundred meters on top of the shallowest buried anticlines to several thousand meters (greater than 8000 m) in the depocenters between the main thrust fronts (Fig. 2). Since the 1950s of the last century those areas have been extensively investigated for hydrocarbons exploration purposes, with several oil and gas fields still actives (Bertello et al. , 2010; Casero, 2004). Thus, a great amount of underground data is present (mostly seismic profiles, well logs and structural maps, made available by the Italian Ministry of Economic Development -UNMIG- under the VIDEPI project, www.videpi.com ). These data set allowed a refined interpretation of underground structures, which are mostly represented by blind ramp anticlines and anticlinal stacks belonging to the northern Apennines arcs, currently buried below the Plio–Quaternary sequence (Fig. 2). Several detailed stratigraphical analyses, based on geophysical well logs, were also conducted in order to deeply characterize the whole sedimentary cover down to the pre-foredeep basement units. The reconstruction of the sedimentary sequence, starting from the base of Pliocene sediments within the Po Plain, has been the historical target of geological literature (Pieri and Groppi, 1981; Cassano et al. , 1986; Casero et al. , 1990; Consiglio Nazionale delle Ricerche, 1992; RER and ENI-Agip, 1998; Fantoni e Franciosi, 2010; Vannoli et al. , 2014 and reference therein; Maesano et al. , 2015 and reference therein). Even if the large dataset show a complex geological variability, the general stratigraphy can be summarized in several group of units which have been coherently observed throughout thewhole Po Plain basin (Montone andMariucci, 2015 and reference therein; Bertello et al. , 2010 and reference therein). Those units are, from the top to the bottom: i) a Quaternary sequence made up by alluvial deposits, clays, silts, and sands (e.g., alluvial sediments, Sabbie Fig. 1 – Left side: Station Distribution of National Seismic Network (RSN) in an area of about 100 x 100 km 2 (green box): velocimeters (red triangles), accelerometers (blue triangles) and six channels stations (yellow triangles). The center of the area, used to select the stations of the RSN, corresponds to surface projection of the reservoir (magenta line). The blue box underlines the extended domain of detection (DE) (cfr. MiSE-DGRME (2014) guidelines). Right side: Inner domain (red line) and the extended domain of detection (blue line). (cfr. MiSE-DGRME (2014) guidelines) with some of the main source of anthropic noise: highway (orange line) and railway line (black dotted line). The microseismic network composed of 5 temporary stations and the station FIU of RSN, is also shown.