GNGTS 2013 - Atti del 32° Convegno Nazionale

earthquakes, the probabilistic expected PGA may well be smaller than the recorded ground motion, due to significant site effects (e.g., Celebi et al. , 2010; Meletti et al. , 2012). As discussed earlier, the first phase of PSHA is the definition of a set of seismogenic zones where past and future seismicity does occur. For the Italian region, the rather detailed ZS4 model (80 zones) has been replaced by the much simpler ZS9 model (36 zone only). This choice has favoured the statistical analysis, since in average each new, larger zone includes more earthquakes. On the other hand, the new seismic zoning is less realistic with respect to the seismotectonics of Italy. For instance, the Apennine chain is currently subdivided in few, very long belts (as shown in figure 1 for the central and northern Apennines), which heavily affect the resulting hazard estimates. However, the distribution of historical seismicity (Fig. 1) shows that the real seismic sources are not uniformly distributed within the adopted seismogenic zones (Mantovani et al. , 2011, 2012a, 2013). Thus, the seismic classification of the communes of the administrative regions located in the northern Apennines (Emilia-Romagna, Toscana, Umbria and Marche) are significantly affected by the above problems, since the classification is based by law on the PGA level predicted by PSHA (Ordinanza PCM/3519, 2006). Shortcomings of the PSHA for some Italian regions. The problems discussed in the previous section are better focused when specific sectors of the Italian region are considered. Moreover, the discrepancies between the results provided by PSHA and the information coming from seismic history become more evident when the probabilistic assessment is given in terms of maximum expected macroseimic intensity (Imax, Gomez Capera et al. , 2010 and references therein). Fig. 2 – Detail of the seismic hazard map obtained from PSHA, related to the four Italian region here considered (Emilia-Romagna, Toscana, Umbria and Marche). The map shows the maximum value expected for the Mercalli- Cancani-Sieberg (MCS) macroseismic intensity, which has a 10% exceedence probability within the next 50 years (modified after Gomez Capera et al. , 2010). Regional and provincial borders are indicated by thick and thin blue lines, respectively. 477 GNGTS 2013 S essione 2.3