Seismic hazard estimates for the Vittorio Veneto broader area (NE Italy)

The logic tree approach has been used to compute robust seismic hazard estimates for NE Italy (Friuli - Venezia Giulia and eastern Veneto regions). These hazard estimates were planned to be used for the expected damage assessment at a regional scale. The logic tree approach has been followed, to quantify the epistemic uncertainties. Our logic tree consists of 54 branches for rock and soft soil conditions for which three seismogenic zonations, representing various levels of seismotectonic knowledge, three methods for seismicity rate computation, three approaches for maximum magnitude estimation, and two PGA attenuation relations of different spatial relevance (Italian, European) were used. For stiff soil conditions, an additional attenuation relation of regional applicability was considered with an enlargement of the logic tree to 81 branches. The regional hazard assessment was made according to a standard probabilistic approach for several return periods: 189 runs were processed in total. The hazard estimates coming from all branches, contribute to the final aggregate seismic hazard map. Two areas (central Friuli and the area around Vittorio Veneto) show the highest hazard in these maps. All results were stored and elaborated by a GIS system, that allowed us to produce the final soil seismic hazard map. The computed PGA with a return period of 475 years in Vittorio Veneto (stiff soil conditions) is 0.38 g, considering the aleatory variability; it becomes 0.51 g when the epistemic uncertainties are added. For damage assessment purposes, an additional hazard map in terms of macroseismic intensity has been obtained transforming the PGA estimates into macroseismic intensity by a relation calibrated on the data of the 1976 Friuli earthquake. The intensity hazard map shows similar features as those of the hazard map in terms of PGA with the maximum values along the northern Tagliamento River valley.