GNGTS 2013 - Atti del 32° Convegno Nazionale

IDP = C (c’) S (s) P (p) where S, C and P are functional forms of s, c’ and p respectively); a(T) and b(T) are regression parameters; f M and f R are tapering functions depending on magnitude (M) and distance (R), respectively. Moreover, the authors provide an analysis of the inter-event residuals against several independent variables to resolve the most relevant parameters for the corrective fac- tor. They reported no significant correlation of inter-event residuals with rupture velocity to shear velocity for all events; as a consequence they indicated 0.8 as an appropriate number. No significant correlation is found for magnitude, S-wave velocity in the first 30 meters (VS 30 ), fault dip or rake, which therefore can be introduced independently in the proposed strategy. The hazard model implementation. In this study we have implemented and modified the USGS seismic hazard computer code adding the SC2008 factor to one of the existing 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) needs ad- hoc calibrations. Note that the SC2008 is effective from T≥0.5 up to T=10 s. The highest is the period the highest the efficacy of the correction. As a first test we focused on a single fault segment treated as an extended fault. The entire fault extension was explored by a “moving sub-fault” with magnitude-dependent dimension. GMPEs used in the PSHA standard practice do not account for the nucleation position whereas the SC2008, which is based on rupture propagation from the hypocenter to the closest point to the site, strongly depends on it. Because each point of the sub-fault (the grid spacing is 1 km) has the same probability of nucleating an event (it is assumed an uniform distribution for the nucleation position), and each nucleation point would produce a specific corrective model, the PSHA computation would end in a number of corrected models equals to the number of the nucleation point considered times the number of each sub-fault. Our choice, in this preliminary analysis, was to keep the nucleation position which gave the maximum of the SC2008 correction within each sub-fault. An alternative for further implementation is either to mediate for the effect of a uniform distribution of nucleation points, or assigning a proper probability density function for the distribution of the hypocenters (Spagnuolo et al. , 2012). Results. In oder to test the impact of the corrective factor on seismic hazard (10%probability in 50 yrs) we first considered its effect on a normal fault (dip=45°, M=6.6) and on a strike slip a b Fig. 1 – Representation of the ratio between seismic hazard spectral acceleration at 3 s, SA-3s (%10 in 50 yrs) with a) directivity and null-directivity models calculated using a normal (NF: 45°) and b) a strike slip (SS) M w =6.6 fault, respectively. The correction was applied to the ground motion predictive equation of Boore and Atkinson (2008) at 3 s spectral acceleration. In red: positive amplifications. In blue: negative amplifications. 143 GNGTS 2013 S essione 2.1