GNGTS 2013 - Atti del 32° Convegno Nazionale

since it can help to assess the maximum expected hazard level and we cannot know when such intensity may again occur. This choice is criticized by supporters of the PSHA approach (e.g., Marzocchi, 2013), who believe that a correct estimate of seismic hazard must also take into account other aspects of seismic activity, such as the return period of the considered intensity value. However, this objection is based on the arbitrary assumption that seismicity parameters can be estimated with satisfactory precision. To help the discussion about this problem, we can mention a significant example, concerning the very strong earthquake that occurred in 1117 in the Po Valley (e.g., Galli et al. , 2005; Guidoboni et al. , 2005). In spite of the fact that such event is the most intense ever occurred in the Padanian zone, its well documented damage has very poorly influenced the present PSHA map (Fig. 1), since the occurrence of a similar shock in the next 50 years results is considered unlikely. In line with the above concepts, we have adopted a procedure composed by the following steps: • A preliminary identification of the maximum intensity value (I max ) expected for each commune of the Region involved is first deduced by the official catalogue of macroseismic effects (DBMI11, Locati et al. , 2011). • Since the historical documents cited by this catalogue do not cover the entire set of macroseismic effects produced by the known earthquakes, the above data set is integrated by computed macroseismic data, obtained by using the catalogue of hypocentral parameters (CPTI11 database, Rovida et al. , 2011) and the intensity versus distance attenuation empirical models now available for the study area (Berardi et al. , 1993; Gomez Capera, 2006; Pasolini et al. , 2006). However, one must be aware that the available information on past damages could not inform about the effects of other past large earthquakes that, for some reasons (long return periods, destruction of important archives etc), are not cited by historical documents. In order to mitigate this problem, we have defined a numbers of seismogenetic zones (Fig. 2), whose seismogenic potential is tentatively assumed as homogeneous, in line with the suggestion of other authors (Swafford and Stein, 2007). The I max value assigned to each zone is chosen on the basis of the strongest shocks occurred within the contours adopted and the available information on seismogenic fault systems and tectonic features. The final I max map for the communes of two Regions involved (Fig. 3) has been then defined by taking into account the effects of the adopted seismogenic zones. A comparison between the map here proposed (Fig. 3) and the one produced by the PSHA procedure (Fig. 1) suggests some considerations. For a number of communes in the 2 Regions the Imax here proposed is higher, even significantly, with respect to the PSHA maps. This is clearly related to our choice of taking into account the maximum damages reported by historical documents. In the Po Valley, the effects of the major 1117 earthquake in the Verona area cause a generalized increase of the expected Imax. A similar increase of Imax values can be noted in the inner Tuscany (Pisa zone), as an effect of the strong event occurred in 1846 near Orciano Pisano. In this case, the difference is even greater, the Imax provided by PSHA (VII) changes to X (in the commune mainly involved). Another major change with respect to PSHA, concerns some zones of the northernmost sector of Tuscany (Appennino Pistoiese and Casentino). In this case the variation involves a slight decrease of the expected Imax. In our opinion this change can avoid the effects of the unreliable geometry assumed by PSHA for some seismogenetic zones in this area (see the comments and the figure given by Viti et al. in this volume). The supporters of PSHA argue that the results of our approach involve an exaggerated level of prevention with respect to a reasonable compromise between the most probable damage (estimated by statistical analysis) and socio-economic considerations, claiming that the PSHA maps can better fulfil the above requirement. We obviously agree that non seismological criteria must be taken into account in hazard assessment, but we do not share the idea that the 424 GNGTS 2013 S essione 2.3