GNGTS 2013 - Atti del 32° Convegno Nazionale

fault (M=6.6). Seismic hazard is given in terms of SA at 3 s. Fig. 1 reports the percentage ratio between the seismic hazard computed with the SC2008 (a directivity model) and the seismic hazard resulting from the standard practice (using a null-directivity model) normalized to the latter. It is presented as a relative difference in percentage, computed as: [PSHA corrected – PSHA standard] / PSHA standard * 100. (2) The corrective factor affects mainly the spatial distribution. A positive contribution (which increases the PSHA up to 35%) was achieved at sites in a (namely) directive direction: for a normal fault, the up-dip rupture direction along the surface exposure of the fault; for a strike slip fault, the along strike direction. A decrease (up to 20%) was achieved in the backward direction. Part of this effect is modulated by the nodes and lobes of the radiation pattern (Spagnuolo et al. , 2012). Beside, the correction proposed by SC2008 reduces the uncertainty sigma (the record-to-record standard deviation) of a value estimated around 4%. This reduction itself affects the PSHA computation even in regions where the corrective factor is negligible. We have applied this corrective factor to the DISS (Basili et al. , 2008) faults in the Southern Apennines. The percentage change between seismic hazard spectral acceleration at 1 s and 3 s is presented in Fig. 2 (left panel and right panel respectively). At 3 s the increment of the seismic hazard estimation was up to 25% and it was achieved along the entire system of faults. A significant reduction (up to 10%) was achieved in the region north to Naples (Campania), and in the Gargano national park (Puglia). At 1 s the effectiveness of the corrective factor is lower; the increase is up to 10% and the decrease is up to 5%. The areas affected by the modifications were approximately the same. Conclusions. In this study we improved the seismic hazard maps in the near source region incorporating rupture complexity (i.e. directivity) in the ground motion prediction. We have implemented and modified the USGS seismic hazard computer code adding the SC2008 factor to one of the NGA equations, the Boore and Atkinson (2008). The application to other GMPEs of the package (Akkar and Bommer, 2010; Bindi et al. , 2012; Cauzzi and Faccioli, 2008) is also feasible but needs ad-hoc calibrations which are planed for a future activity. a b Fig. 2– Representation of the ratio between seismic hazard at a) 1 s and b) 3 s SA (%10 in 50 yrs) with directivity and null-directivity models calculated using the database DISS of individual fault segments, respectively. The correction was applied to the ground motion predictive equation of Boore and Atkinson (2008, BA08), to the spectral acceleration (SA) at 1 s and 3 s. In red: positive amplifications. In blue: negative amplifications. 144 GNGTS 2013 S essione 2.1