Soil characterization and seismic hazard maps for the Friuli Venezia Giulia region (NE Italy)

Seismic hazard maps that account for site amplification (soil seismic hazard maps) are very useful because they represent the expected ground motion at the Earth's surface, but need much more information and elaboration than the usual rock hazard maps. The regional soil hazard map has been developed for the Friuli Venezia Giulia region, in northeastern Italy, by considering the most updated approach. In fact, the structure of the seismic hazard analysis presented here is based on the logic tree approach to achieve a robust statistical computation taking into account, in addition to the aleatory variability, also the epistemic uncertainties. The logic tree adopted for rock and soft soil conditions consists of 54 branches: three seismogenic zonations, representing various levels of our seismotectonic knowledge, three methods for the seismicity rate computation, three statistical approaches for the maximum magnitude estimation, and two PGA attenuation models of different spatial relevance (European and Italian). An additional regional attenuation model was considered only for stiff soil conditions, increasing the number of branches of the logic tree to 81. A consolidated expeditive procedure, widely adopted in the United States, has been used to compute the soil ground motion, properly modified on the basis of specific calibrations based on the local geological conditions and the results of geotechnical soundings. The final result of this study is represented by the map of the expected ground motion in the Friuli Venezia Giulia region, computed considering the different litho-stratigraphic and morphological conditions existing in the area. This map clearly shows the contribution given by the soft sediments along the Alpine valleys and by the steep formation of the moraine amphitheatre in central Friuli. A comparison of these new results with those obtained by applying the amplification factors provided by the most popular seismic codes points out that the actual ground shaking could be notably larger than that obtained by the application of the seismic codes, suggesting a possible future implementation in the regional building code, so far not taken into account.