GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 415 intensity measures may provide, especially for long return periods, very (and in some cases unrealistically) conservative results (Brune 1999). On the other hand, if a sufficiently large number of rupture scenarios is considered in order to provide a realistic description of the earthquake ground motion variability, PBSs can be a helpful tool do identify the proper upper limits for each site. These considerations lead to an alternative use of PBSs into PSHA, in this work called footprint- based PSHA approach, consisting on the direct use of PBSs by taking all the realizations of the scenario earthquake within a logic-tree framework (Kulkarni et al. 1984), as multiple branches each with the same weight (Fig. 1,b). The scope of this work is, on one hand, to assess the seismic hazard at urban scale according the two Physics-based PSHA approaches (i.e. GAF-based and footprint -based) previously presented and compare the results with the traditional GMMs-based and, on the other, to carry out a preliminary losses estimate by considering an artificial portfolio of buildings. To this end the city of Istanbul has been selected as pilot case study because: (i) the proximity of the strike-slip North Anatolian Fault (NAF) hosting a seismic gap capable to trigger severe earthquakes (up to Mw 7.4) and (ii) a good geotechnical characterization of the soil is available. The PBSs have been carried out through the certified high-performance open source code SPEED, Spectral Elements in Elastodynamics with Discontinuous Galerkin, (http://speed. mox.polimi.it/ , Mazzieri et al. 2013) while the 3D numerical model has been numerically reconstructed combining three set of input data: i) the topography and bathymetry model, ii) the seismotectonic model, and iii) the 3D velocity model. The resulting numerical model (Fig. 1) extends over an area of 165km x 100km up to a depth of 30km. A total of 66 scenarios earthquakes have been simulated for three sets of magnitude: 25 scenarios of Mw7.0, 21 Mw7.2 and 20 Mw7.0. Since the results are reliable up to 1Hz, to obtain broadband (BB) waveforms, i.e. beyond the frequency limit of the numerical model, required by many engineering Fig. 2 - Top: PGV hazard maps for Tr: 475y and Tr: 975y. Bottom: PGV hazard curves for three selected sites in Istanbul.