GNGTS 2013 - Atti del 32° Convegno Nazionale

Paleoseismological investigations along the San Demetrio ne’ Vestini fault (AQ) A.M. Blumetti, P. Di Manna, E. Vittori, V. Comerci, L. Guerrieri ISPRA, Geological Survey of Italy Introduction and geological framework. In the months following the April 6 th , 2009, Mw 6.3 L’Aquila earthquake, in the framework of a seismic microzonation, the Geological Survey of Italy (ISPRA) carried out a detailed geological and geomorphological survey of the San Demetrio ne’ Vestini territorial municipality. This village, severely damaged by the 2009 event, is located in the south-eastern part of the L’Aquila basin, a complex tectonic basin characterized by a system of northwest-southeast- trending tectonic depressions (total length about 30 kilometres) developed inside the inner sector of the Meso-Cenozoic orogenic belt of the Apennines, between the Gran Sasso and the Monti d’Ocre morphotectonic units (Fig. 1). Active crustal extension in this zone is demonstrated by GPS observations, which reveal velocities across the Central Apennines in the order of 3 mm/yr (D’Agostino et al. , 2011), and confirmed by the large historical and instrumental seismicity occurred in the region. The geological evidence of active faulting is also widely recognised (Bagnaia et al. , 1992; Blumetti, 1995; Boncio et al. , 2004; Galadini and Galli, 2000; Roberts and Michetti, 2004; Blumetti and Guerrieri 2007; Galli et al. , 2010; Giaccio et al. , 2012; Blumetti et al. , 2013). The San Demetrio area is located at the southern tip of the L’Aquila basin fault zone, which is characterized by the presence of en echelon and sub-parallel NW-SE trending fault segments. This is a typical structural setting at the transition between two primary fault zones (in this case between the L’Aquila Basin and Subequano Basin primary fault zones). Moreover in this area extension and faulting progressively migrated toward the inner part of the basin (Giaccio et al. , 2012; Blumetti et al. , 2013). The territory of the San Demetrio municipality is crossed by 4 NW-SE trending fault segments that have displaced also Quaternary deposits (Fig. 1B; Bosi and Bertini, 1993). One of them, crossing the historical settlement of the village, was pointed out as a capable fault by Working Group MS–AQ, (2010). This fault (named San Demetrio fault) already identified as a possibly active fault by Bagnaia et al. (1992), does not crop out, but is “inferred” at the base of an up to 25 meters high fault scarp that displaces the flat surface of a recent alluvial terrace (Figs. 1 and 2). This note summarizes the first results of paleoseismological investigations carried out by ISPRA along the San Demetrio fault, with the aim to i) characterize its capability in terms of seismic and surface faulting potential and ii) map the fault, and an appropriate setback area along it, where introduce specific land use restrictions, as requested by the Mayor of the San Demetrio ne’ Vestini. Paleoseismological analysis. In order to better locating the most suitable trenching site, a geophysical survey was performed, consisting of an ERT (Electrical Resistivity Tomography) profile and a seismic refraction tomography line. A previous ERT profile (Working Group MS– AQ, 2010), crossing the fault not far from the trench site, is reproduced in Fig. 2C. The site selected for trenching is located north-west of the historical centre of the village (Fig. 1), where morphotectonic observations and geophysical data were consistently indicating the occurrence of a fault cutting up to the surface. The detailed analysis of the stratigraphy exposed in the trench walls confirmed that the exposed fault is capable of producing surface ruptures (Fig. 3). In fact, a major fault zone, 5 meters wide, characterized by four main (e.g. offsets larger than the trench wall height) synthetic faults and an antithetic structure, was found in the trench walls. Analysing the fault zone exposed in the eastern wall, at least two colluvial wedges were identified, i.e. Levels 4 and 10 in Fig. 3B. Level 4 is about 20 cm thick and is bounded 29 GNGTS 2013 S essione 1.1