GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.1 315 The zonation presented in this study was developed with the aim to be applied as a branch of the logic tree that will be used for the new Italian seismic hazard analysis (MPS16) according to the probabilistic approach originally proposed by Cornell (1968), using the Openquake software (Pagani et al. , 2014). The Cornell’s approach (1968) is based on two hypotheses: 1) earthquake occurrence intervals follow an exponential distribution (i.e., occurrences follow a Poisson process); 2) magnitude is distributed exponentially according to the Gutenberg – Richter (GR) relation. A third hypothesis is that the seismicity is considered to be uniformly distributed inside seismic sources. The Cornell (1968) method needs the following input data: seismic source geometry, earthquake potential (which is defined in terms of average number of earthquakes per magnitude class, andmaximummagnitude), and one or more groundmotion prediction equations (GMPEs). Uncertainty quantification (McGuire, 1977) represents a crucial point in probabilistic seismic hazard analysis (PSHA). In fact, PSHA results are characterized by two kinds of uncertainties: aleatory variability and epistemic uncertainty (McGuire and Shedlock, 1981; Toro et al. , 1997). The aleatory variability is related to the randomness of natural phenomena. The epistemic uncertainty is related to limited knowledge about the earthquake process. In practice, it is reflected in the choice of alternative mathematical models and parameter values used in the computations. The logic tree approach (Kulkarni et al. , 1984; Coppersmith and Young, 1986) was proposed to treat the epistemic uncertainties affecting the various assumptions of a PSHA. It consists of a series of nodes and branches that collect all alternative assumptions, each of which is assigned a weighting factor that is representative of the relative likelihood of that assumption being correct. The models considered in the logic tree must be exhaustive (representing the scientific community opinions) and mutually exclusive. The sum of the probabilities (weights) associated with each path in the logic tree must be 1. The final weight of the final results of every path is equivalent to the product of the weights associated with each branch along the same path. The new SZs have been 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; • 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 zone boundaries, particular attention was paid to 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 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). This new zonation is generally more detailed if compared to the current national one [ZS9: Meletti et al. (2008)]. When the difference between the new SZs and those of the ZS9 were