GNGTS 2013 - Atti del 32° Convegno Nazionale

The maximum intensity estimate by PSHA for the Emilia-Romagna, Toscana, Umbria and Marche Regions (Fig. 2) shows that exceeding 8° MCS in the next decades will be unlikely. This is at variance with information about past earthquakes, which suggests Imax has repeatedly reached 10° MCS in several areas of the above regions (e.g., Locati et al. , 2011). The discrepancy is due to the fact that the probability of next shocks producing an intensity exceeding the 8°MCS is considered low, because past similar events has been relatively rare. In other words, very damaging shocks would be characterized by a very long seismic cycle. However, this assumption does not take into account the shortness of the seismic catalogues and the fact that we do not known when a seismic cycle started for each major seismic source. Furthermore, the reliability of the seismic cycle approach is challenged by the interaction of seismic sources, which implies a reorganization of stress and strain regimes after major shocks (e.g., Viti et alii, 2012, 2013). As suggested above, the distribution of expected Imax (Fig. 2) is clearly controlled by the geometry of the adopted seismogenic zoning, in particular by the zones 915 and 919 of ZS9 model (Fig. 1). For Emilia-Romagna, the Imax expected in the Piacentino, Parmense and Reggiano (Fig. 2) are scarcely affected by the strong earthquakes occurred in AD 1117, whose importance has recently been highlighted by Mantovani et al. (2013). As discussed above, the PSHA assumes a very low probability that an event similar to the 1117 shock will occur in the next 50 years. For Tuscany, the zone with the largest expected Imax covers without interruptions the main Apennine watershed (Fig. 2). This not agrees with the spatial distribution of past earthquakes. Indeed, the seismicity of intramontane basins such as Lunigiana-Garfagnana, Mugello ed Upper Tiber Valley is far stronger than that of nearby zones, such as the Pistoiese Apennine and Casentino basins (e.g., Mantovani et al. , 2011, 2012a, 2013). Furthermore, the probabilistic map predicts an uniform value (7° MCS) for most of Tuscany, out of the above- mentioned watershed strip and part of the Tyrrhenian coast (6° MCS). This result apparently neglects the role of important seismic sources located in western Tuscany, such as the Florentine Chianti hills, Middle Valdarno, Leighorn-Pisan hills and the Amiata Mt., where strong earthquakes were recorded in the past (Mantovani et al. , 2011, 2012a, 2013). Similar shortcomings also affect both Marche and Umbria. The Imax predicted by PSHA for most of such regions is 8°MCS (Fig. 2), whereas the past and recent seismic history suggests that much higher seismic damage may be observed (e.g., Locati et al. , 2011). For instance, the recorded macroseismic intensity was 9° MCS for the 1997, Colfiorito event and presumably reached 10° MCS for the 1751, Gualdo Tadino and 1781, Cagli earthquakes. Conclusions. The probabilistic procedure currently adopted to estimate seismic hazard is affected by several shortcomings, concerning both the main assumptions underlying the procedure and its application to specific zones. The effects of the destructive earthquakes occurred in the last decade in Italy and in the rest of the world have pointed out many of the flaws of PSHA. The analysis of the estimate of macroseismic intensity, provided by the probabilistic procedure for four Italian regions, suggests that the maximum intensity values expected from the next strong shocks may be considerably underestimated. Moreover, several local features of the distribution of maximum intensity values (as deduced from macroseismic databases) are neglected in the probabilistic seismic hazard map. This fact casts doubt on the possibility that Italian public bodies (such as Regioni, Province and Comuni) can reliably use PSHA results in order to manage seismic risk. References Albarello D., D’Amico V.; 2008: Testing probabilistic seismic hazard estimates by comparison with observations: an example in Italy . Geophys. J. Int., 175 , 1088-1094. Algermissen S.T., Perkins D.M.; 1976: A probabilistic estimate of maximum acceleration in rock in the contiguous United States . U.S. Geological Survey Open-File Report 76-416 . Ambraseys, N.N., Simpson K.A., Bommer J.J., 1996; Prediction of horizontal response spectra in Europe , Earthq. Eng. Struct. D., 25 , 371-400. 478 GNGTS 2013 S essione 2.3