GNGTS 2015 - Atti del 34° Convegno Nazionale

extensional environment, surface primary ruptures (i.e. principal faulting, sensu Youngs et al. , 2004) are expected to occur mainly in the hanging wall of the master fault, we have considered an asymmetric zone located mainly on the downthrown block, and width proportional to the maximum surface offset. For example, for capable faults with maximum magnitude equal to 6.0 and maximum offsets lower than 5 cm, the hazard zone width is at least 50 m in the hanging wall but not more than 30 m in the footwall. Indeed, for capable faults with M max > 7.0 and maximum offsets larger than two meters (e.g. Calabria) the hazard zone width is in the order of 400 m (100 m in the footwall and 300 m in the hanging wall). Conversely, a clear documentation of surface faulting pattern occurred in compressive and strike-slip environment is quite scarce: only for the reverse 1976 event in Friuli some local surface ruptures have been interpreted by some authors as surface faulting (cfr. Bosi et al. , 1976; Aoudia and Suhadolc, 2000 based on Martinis and Cavallin, 1976). Moreover, a paleoseismic evidence in a compressive environment (secondary surface faulting) has been clearly documented at Monte Netto (south of Brescia) (Livio et al., 2012). Anyway, considering these cases but also the available cases of documented reverse surface faulting events around the world (cfr. Lettis et al. , 1997), it is clear that in a compressive environment surface faulting features typically occur not only in correspondence to the main thrust but also at the hinge of the growing anticline, even with normal displacement ((e.g., 1980 El Asnam M 7.3 earthquake; Shan and Bertero, 1980; Meghraoui et al. , 1988; Meyer et al. , 1990). This zone may be located in the hanging wall of the main thrust at a variable distance from the thrust front (up to some km). During the 1980 El Asman event, extensive fissures and normal faulting occurred on the upthrown block of the primary thrust as far as 2 km from the trace of the primary thrust producing, in most places, most pronounced scarps than those along the primary thrust fault (Shan and Bertero, 1980). Thus, the hazard zone width in the hanging wall of reverse faults has to be significantly larger than in the hanging-wall of normal faults, at least twice the corresponding width for normal faults. Of course, secondary ruptures cannot be excluded even at larger distance (up to some km from the thrust front) in case of growing near-surface anticlines. For strike-slip fault systems (e.g. in the Gargano area, Puglia), a relevant role is played by surface ruptures occurring along faults located off the principal fault trace and in response to an earthquake along the principal faults (concept of distributed faulting; cfr. Petersen et al. , 2011). Such faults may be linked in depth to the major structure (e.g. flower structures, pull-a-part basins, etc.), but might be also located at a distance of several hundreds of meters up to several km. Therefore, each fault segment has been managed as an independent source of FDH, locally with normal or reverse component. Fault Displacement Hazard in Italian urban areas. We have applied the illustrated model on the administrative territory of 73 Italian cities with population > 60,000 inhabitants, and in particular on the 45 cities that, according to ITHACA, are crossed by capable faults (Fig. 3 left). Taking into account the maximum expected offsets, it is possible to state that Fault Displacement Hazard is very relevant • in four cities (Reggio Calabria, Messina Catanzaro and Cosenza), where offsets even larger than one meter are expected (class 5); • in six cities (Siracusa, L’Aquila, Ragusa, Benevento, Catania and Potenza), where maximum offsets are in the order of several decimeters up to one meter (class 4); • in five cities (Trieste, Udine, Perugia, Treviso andVenezia) where maximum displacements can reach 50 cm (class 3); In the other 30 cities, Fault Displacement Hazard does exist but is much less relevant, as maximum displacements are in the order of some centimeters (19 cities) or lower (the remaining 11 cities). The total area exposed to FDH in the 45 cities is equal to about 2.5 % of the study area. In particular, the total area of Fault Class 5 is about 52 km 2 , with a large contribution of those GNGTS 2015 S essione 2.2 193