Introducing epistemic uncertainties into seismic hazard assessment for the broader Vittorio Veneto area (N.E. Italy)

The logic tree approach has been used to compute robust seismic hazard estimates for the border area between the Veneto and Friuli - Venezia Giulia regions (N.E. Italy). In order to quantify the epistemic uncertainties, different options for the seismogenic zonation, maximum magnitude, and PGA attenuation relations have been considered. Three seismogenic zonations, representing different levels of our seismotectonic knowledge, were used. Moreover, the maximum magnitude was estimated in three different ways. Since no specific PGA attenuation relations are available for N.E. Italy, a European and an Italian relation were considered. The final logic tree has 16 branches: 3 zonations, 3 maximum magnitude estimates, 2 attenuation relations, 2 branches being empty. The regional hazard assessment was done according to a standard probabilistic approach for a 475-year return period. The results obtained for the different branches show remarkable differences that highlight, moreover, the influence of the seismogenic zonation. The results, evenly weighted, coming from all branches, contribute to the final aggregate seismic hazard map. Two areas (central Friuli and the area N.E. of Vittorio Veneto) show the highest hazard in these maps. One specific analysis interested the Vittorio Veneto test site, where the influence of the different characteristics of the branches in the logic tree have been investigated. In general, the results are rather similar and depend on the combination of the different choices. The average PGA value (mean of the median values) is 0.30 g taking into account the aleatory uncertainty of the attenuation relations, and it becomes 0.33 g, when the epistemic uncertainty is also added.