GNGTS 2017 - 36° Convegno Nazionale

298 GNGTS 2017 S essione 2.1 negligible, in terms of geographical boundaries and seismotectonic characteristics, the ZS9 zones were retained; • some new SZs were introduced including areas not considered seismic before. The new zonation was defined taking into account the available information on: • epicentral distribution of earthquakes from the new historical catalogue CPTI15 (Rovida et al. , 2016) and regional instrumental bulletins (Scafidi et al. , 2015); • observed and/or estimated maximum magnitude (M max ); • 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 3.2.0 (DISS Working Group, 2015) and the available literature. In defining the SZ boundaries, particular attention has been paid to the regional seismotectonic settings and seismic history, in order to avoid excessive extrapolation of local features, which could lead to an underestimation of the hazard produced by more active local structures, and to an overestimation of the hazard related to less active sources. The seismotectonic conditions are considered homogeneous within each area. For each SZ, 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). For some SZs, more failure mechanisms were considered possible; in such cases, various estimates of the percentage of occurrence have been assigned (depending on the information available). To cover the border areas of national territory the SHARE zonation (Giardini et al. , 2013) was also adopted with relative rates and Mmax values. A logic tree of six branches was considered (Fig. 1a): three branches to account for the epistemic uncertainty (McGuire and Shedlock, 1981; Toro et al. , 1997) in the seismicity model and two branches were related to alternative Mmax values. Concerning the node relative to the seismicity model, one branch accounted for individual rates (I- R) with a weight of 0.4, while the remaining two used different approaches to compute the values of the Gutenberg-Richter (1956, GR) coefficients ( a and b -values): one used the Least Fig. 1 - Logic tree for the A1 zonation model: a) the 2016 version; b) the 2017 version.