GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 75 As several building codes consider nowadays a more conservative probability level (i.e., 2% in 50 years corresponding to a 2475-year return period), an additional elaboration has faced this aspect. The new seismic hazard map showing the vertical PGA (Fig. 3a) puts in clear evidence the contribution to hazard given by the individual seismic sources as the combined contribution of the Gemona sources (ITIS119 and ITIS122) produce high vertical ground motions, that anyway, remain lower than 0.5 g. Considering for the whole region a different soil type [soft soil of the Eurocode 8 (CEN, 2004), i.e., 180≤v s 30 ≤360 m/s]), a notably higher vertical PGA is obtained and values exceeding 0.7 g cannot be neglected for a large portion of central Friuli (Fig. 3b). In all the previous maps only the mean value of the results from the logic tree (3 branches) has been considered but an advantage of the use of logic trees is that also the (epistemic) uncertainty related to the average estimate can be quantified. Consequently, in the last elaboration the ground motion has been computed in terms of mean values plus one standard deviation and values exceeding 1 g have been calculated on soft soil for a large part of central Friuli (Fig. 3c). To compare again our vertical estimates with those of the Eurocode 8 (CEN, 2004), the ground motion along the E-W oriented transect has been computed also considering the Fig. 2 – PGA with a 475-year return period for rocky sites in NE Italy considering the logic tree described in the text (mean values): a) vertical PGA; b) horizontal PGA; c) vertical PGA based on the sources of the FRI zonation; d) vertical PGA along an E-W oriented transect (see Fig. 1a for its location). The red, blue, green, violet, yellow, and light blue curves refer, respectively, to the Ambraseys and Simpson (1996), Cauzzi and Faccioli (2008), Bindi et al. (2011), GEMPs, average value, horizontal estimate, and vertical estimate according to EC8.