GNGTS 2013 - Atti del 32° Convegno Nazionale

results of PSHA can be used as a solution of that problem, since such maps may be affected by unknown uncertainty, as argued earlier. Thus, what is presented as a reasonable compromise could instead be an unjustified bad defence from the real seismic risk. Conclusions. We argue that the present seismic hazard maps for Italy, being derived by an approach (PSHA) whose basic assumptions cannot be proved, may be affected by a not well define bias. Thus, the use of such maps could significantly underestimate seismic risk. For instance, in the two Regions here considered (Tuscany and Emilia-Romagna) the PSHA map provides that an event with intensity greater than VIII MCS is unlikely (probability of excedence in 50 years lower than 10%), in spite of the fact that these Regions have been hit by several earthquakes with higher intensity (IX, X MCS). Since the quantification of that probability could be not reliable, it seems quite imprudent to build our strategy of defence from earthquakes on the arbitrary hypothesis that events of intensity IX and X are unlikely in the next 50 years. Fig. 2 – Seismogenic zones identified for the Tuscany and Emilia-Romagna territories ( Mantovani et al. , 2012, 2013). In each zone, a value of I max is tentatively defined, taking into account the intensities of shocks located inside the zone. Then, it is assumed that such value holds throughout the zone involved. This choice may change the value of I max previously assigned to the communes of the two Regions considered. The I max map finally resulting from the series of operations described above is given in Fig. 3. 425 GNGTS 2013 S essione 2.3