GNGTS 2014 - Atti del 33° Convegno Nazionale

76 GNGTS 2014 S essione 2.1 estimates referred to the 2475-year return period (Fig. 3d). Again, the vertical PGA estimated according to the Eurocode 8 (CEN, 2004) standards results conservative with respect to the result of the present study. Anyway, the results evidence that the vertical component in the areas nearest to the faults could be meaningful in NE Italy. Final comments . Summing up all the aspects cited above, it is reasonable to state that it is very difficult to quantify the real uncertainty associated to any result of a PSHA but surely this uncertainty is quite large when the site under investigation is located in the proximity of seismogenic sources. Consequently, large vertical PGAs cannot be neglected also in regions characterized by earthquakes of medium magnitude (less than 7) in unfavourable soil conditions. Finally, it is worth noting that an effective seismic design in the near field (where an important vertical PGA can be expected) requires the simultaneous evaluation of both horizontal and vertical seismic ground motions, especially for the design of friction-based structures. Therefore, the calculated hazard maps can be considered as a preliminary application aimed at defining an additional layer over the horizontal hazard map identifying the macro PEACH area where the vertical acceleration is quantified so to be considered for seismic design purposes. Fig. 3 – PGA with a 2475-year return period for NE Italy considering the logic tree described in the text: a) mean vertical PGA on rocky sites; b) mean vertical PGA on soft soil sites; c) mean values plus one standard deviation vertical PGAon soft soil sites; d) vertical PGAalong 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.