GNGTS 2014 - Atti del 33° Convegno Nazionale

50 GNGTS 2014 S essione 2.1 1981; Toro et al. , 1997) is the scientific uncertainty in the process model, and it is caused by limited data and knowledge: it is taken into account in PSHA using alternative models in the logical tree approach (Kulkarni et al. , 1984; Coppersmith and Young, 1986). Each node of the tree structure represents a specific component of the computation (seismogenic zonation, seismicity model, attenuation relation, etc.) and collects a series of alternative models, which should be independent of each other and exhaustive. The final overall result (mean value and standard deviation) is obtained by weighting the appropriate individual results from the different branches (Rebez and Slejko, 2004). Seismic source characterization . In the PSHA, seismic sources are modelled either as fault planes (faults) or as large seismogenic zones (SZs), where earthquakes can randomly occur. In the present study, and in order to compare the results with those provided by the Italian seismic hazard map [MPS04 by Stucchi et al. (2011)], where the sources are modelled as large horizontal SZs, two alternative source models have been considered; one is based on composite sources adopted by DISS (DISS Working Group, 2010), the other model mainly on faults proposed in literature but not retained by DISS. Sources can collect more than one individual fault as segmentation is usually not clearly known. Around these sources, large “background areas” have been considered, where the seismicity external to the sources was collected. The DISS source model . In the source model derived from the DISS database, nine composite sources have been considered (Figs. 1a and 1b). In addition to the named sources, three broad “background zones” have been introduced: the western zone, which collects the earthquakes of the Tyrrhenian Sea, the central zone, along the Apennines chain, and one that collects the earthquakes in the eastern Ionian Sea. Fig. 1 – Seismogenic source models considered in the present study: a) 2D view of the DISS model; b) 3D view aimed at emphasizing the same fault geometry; c) 2D view of the OGS model; d) 3D view aimed at emphasizing the same fault geometry. The blue and red boxes show the DISS and OGS, respectively, sources (solid line for the top). The yellow boxes identifity the background zones. Solid triangles mark the dam locations.