GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 45 LATE PLEISTOCENE REVERSE SURFACE FAULTING AT THE PECETTO DI VALENZA SITE (AL - NORTHERN ITALY): PRELIMINARY RESULTS F. Livio 1 , M.F. Ferrario 1 , C. Frigerio 1 , A.M. Michetti 1 , A. Zerboni 2 , L. Bonadeo 1 , G. Fioraso 3 , A. Irace 3 , F. Brunamonte 1 1 Dipartimento di Scienza ed Alta Tecnologia, Università dell’Insubria, Como, Italy 2 Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Italy 3 Istituto di Geoscienze e Georisorse (IGG), CNR, Torino, Italy Introduction. Notwithstanding the considerable investigation and research efforts produced almost 40 years ago for the Italian Nuclear Program, later published in the framework of the “Progetto Finalizzato Geodinamica” (e.g., Carraro, 1976; GSQP, 1976; Enel, 1984, 1985; CNR, 1992; Carraro et al. , 1995), the assessment of ground motion and surface faulting hazard in the Monferrato Arc is still poorly constrained. Since the western Po Plain shows subdued historical and instrumental seismicity, it is commonly regarded as a region characterized by a low seismic hazard (http://zonesismiche.mi.ingv.it/ ), and has been overlooked in the last 30 years from the paleoseismological point of view, while active tectonics studies flourished more to the east, basically due to the available record of strong seismic events during the Middle Ages and later (e.g., Magri and Molin, 1986; Serva, 1990; Benedetti et al., 2000, 2003; Maesano et al., 2001; Burrato et al., 2003; Galadini et al., 2005; Galli, 2005; Livio et al., 2009, 2014). However, several lines of evidence strongly suggest that the seismotectonic potential of the study area is similar to the one known for the central and eastern part of the Po Valley (e.g., Serva, 1990; Michetti et al. , 2012; Bonadeo, 2014 and references therein; Turrini et al. , 2015). The recent seismic crisis in Emilia clearly illustrates the need for revised seismic hazard assessment in northern Italy. In this line, the characterization of the maximum earthquake magnitude through paleoseismic analyses might represent a vital tool for mitigating seismic risks, in a region that seems unprepared to cope with the effects of strong earthquakes. In fact, high population density, clustering of industrial facilities, and the great deficit of seismic safety accumulated due to the lack of a proper seismic code (Stucchi et al. , 2012), make today the Po Plain one of the regions most exposed to seismic risk of the whole Italian peninsula. This is particularly true for the Monferrato Arc, which, as noted above, is characterized by quitemarginal seismicity according to the Italian Seismic Catalog. In this line, it is very important to remark that recent literature data describe geological features suggesting significant seismic potential in the western Po Plain. New observations, based on recently exposed stratigraphic sections at Cereseto and Ozzano Monferrato (Sassone et al. , 2015; Fig. 1b), reveal relevant evidence of Late Quaternary reverse faulting. Moreover, Giraudi (2014, 2015), based on fluvial terrace analyses and the revision of subsurface shallow stratigraphic data, identify previously unmapped Late Quaternary faults in the Vercelli Plain and Casale Monferrato plateau . In the following, we focus on the newly identified Pecetto di Valenza site, where recent paleoseismic surface faulting has been observed for the first time in the study area. We base our interpretations on data coming from an integrated and multidisciplinary analysis, including paleoseismology, pedostratigraphy and geomorphology, as already successfully applied in another case study in the Po Plain area (Livio et al., 2014; Zerboni et al., 2015). Geological and geomorphological setting. The Monferrato Arc is the westernmost of the 3 major salients enclosing the most external structural fronts of the northern Apennines (Fig. 1). The tectonic activity of the Po Plain is closely related to the recent evolution of these structural belts, mainly buried below the Plio-Pleistocene infilling of the basin but emerging or very shallow along the Monferrato Arc (e.g., Mosca, 2006). Study site (Pecetto di Valenza) is located in the easternmost Monferrato area, in a hilly area bordered by the Po and Tanaro River and encircled by the Valenza and Alessandria fluvial terraces, to the north and to the south of the site respectively. Miocene marls and conglomerates,