GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 79 Meletti et al. , 2008), reference document for seismic hazard analyses, various studies and new data have been published (e.g., Boccaletti et al. , 2004, 2005, 2011; Basili et al. , 2008; Sani et al. , 2009; Fantoni and Franciosi, 2010; Locati et al. , 2011; Martelli, 2011; Mantovani et al. , 2011, 2013; Rogledi, 2013; Vannoli et al. , 2014), also as a result of specific investigations on significant seismic sequences that affected central and northern Italy and relocation of historical earthquakes. These new data and studies highlight the possibility of a better definition of potentially SZs in the northern Apennines and the central Po Plain. In the context of an agreement between OGS, Emilia-Romagna Region and ReLUIS (DGR, 2014), aimed at updating the seismic hazard and risk maps of Emilia-Romagna at a regional scale, a review of the seismotectonic knowledge, still in progress, has been carried out. This review led to the proposal, still in a preliminary state, of an alternative seismogenic zonation of the northern Apennines and surrounding areas (see Martelli et al. , 2014). The new SZs have been defined taking into account the available information on: • epicentral distribution of earthquakes from macroseismic and instrumental data, in particular those with M >3 [from the catalogues CPTI11 (Rovida et al. , 2011), ISIDe (ISIDe Working Group, 2011) and other data of the Istituto Nazionale di Geofisica e Vulcanologia (INGV)]; • observed and/or estimated maximum magnitude; • focal mechanisms [from European-Mediterranean RCMT catalogue by Pondrelli et al. (2011)]; • hypocentral depth; • geometry, type and kinematics of potentially active or recent (Quaternary) structures, identified on the basis of morphological and structural data and integrated with the sources from the database of the Italian seismogenic sources DISS 3.3 (Basili et al. , 2008) and the available literature. In defining the geographical boundaries, particular attention has been paid to the seismotectonic conditions and seismic history, in order to avoid excessive extrapolation of local characteristics that could lead to a “mediation” of the hazard inside, with underestimation of the hazard of more active structures and overestimation of the less active ones. In summary, the areas differ mainly for geometry and type of the observed structures, hypothesized focal mechanisms, depth of hypocentres, number and magnitude of the observed events. Within each area, the seismotectonic conditions are considered homogeneous. For each zone it was proposed a failure mechanism defined by: • geometry of the failure plane (strike and dip), • fault kinematics (normal, reverse, strike-slip, or mixed), • hypothesized hypocentral depth (range), • maximum expected magnitude, which coincides with the maximum magnitude estimated from historical data. When the difference between the new SZs and those of the ZS9 zonation has resulted minimal, in terms of geographical boundaries and internal seismotectonic characteristics, we have adopted the same boundaries and definitions of the ZS9 zones. For some zones, more failure mechanisms are considered possible; in such cases, various estimates on the percentage of occurrence have been assigned (when the information has allowed it). The main novelties of the proposed zonation are: • a division of some very large ZS9 zones (e.g., 912, 915, 916, 921), which, in the opinion of the authors, include seismogenic structures with different geometry and failure mechanisms; • the introduction of new SZs including areas until now not considered seismogenic, such as some sectors of the central Po Plain and the Tyrrhenian coast; • the introduction of transversal SZs motivated by the presence of Quaternary structures