A Comparison of NGA Ground-Motion Prediction Equations to Italian Data by Giuseppe Scasserra, Jonathan P. Stewart, Paolo Bazzurro, Giuseppe Lanzo, and Fabrizio Mollaioli Abstract Ground-motion prediction equations (GMPEs) have recently been devel- oped in the Next Generation Attenuation (NGA) project for application to shallow crustal earthquakes in tectonically active regions. We investigate the compatibility of those models with respect to magnitude scaling, distance scaling, and site scaling implied by Italian strong motion data. This is of interest because (1) the Italian data are principally from earthquakes in extensional regions that are poorly represented in the NGA dataset, and (2) past practice in Italy has been to use local GMPEs based on limited datasets that cannot resolve many significant source, path, and site effects. We find that the magnitude scaling implied by the Italian data is compatible with four NGA relations. However, the Italian data attenuate faster than implied by the four NGA GMPEs at short periods; the differences are statistically significant. Comparison with the fifth one was not possible because it was developed for rock conditions only. Three regression coefficients are reevaluated for the four NGA GMPEs to reflect the faster attenuation: a constant term, a term controlling the slope of distance attenuation, and a source fictitious depth term. The scaling of ground motion with respect to site shear wave velocity is consistent between the NGA models and Italian data. Moreover, the data are found to contain a nonlinear site effect that is generally compatible with NGA site terms. The intraevent scatter of Italian data is higher than in the NGA models, although interevent scatter is comparable to NGA recommendations when the faster distance attenuation is considered. On the basis of these findings, we recommend using the NGA relations, with the aforementioned minor modifications, to evaluate ground motions for seismic hazard analysis in Italy. Introduction The characterization of earthquake ground motions for engineering applications generally involves the use of em- pirical models referred to as ground-motion prediction equa- tions (GMPEs). Ground-motion prediction equations describe the variation of the median and lognormal standard deviation of intensity measures (such as peak acceleration, spectral ac- celeration, or duration) with magnitude, site-source distance, site condition, and other parameters. A review of GMPEs for peak acceleration and spectral acceleration published prior to 2006 is given by Douglas (2003, 2006). In recent years a number of GMPEs have been redefining the state of practice for probabilistic seismic hazard analysis (PSHA) in many earthquake-prone regions. For European ap- plications, Ambraseys et al. (2005) and Akkar and Bommer (2007a, b) have introduced GMPEs that are considerably more sophisticated than widely used previous relations such as Am- braseys et al. (1996) and Sabetta and Pugliese (1996). The Next Generation Attenuation (NGA) project developed a series of GMPEs intended for application to geographically diverse regions; the only constraint is that the region be tec- tonically active with earthquakes occurring in the shallow crust. The NGA GMPEs are presented by Abrahamson and Silva (2008), Boore and Atkinson (2008), Campbell and Bo- zorgnia (2008), Chiou and Youngs (2008), and Idriss (2008). An important issue for many practical applications is whether ground motions or GMPEs for one region can be applied to another. For example, this issue prompted consider- able study for the Senior Seismic Hazard Analysis Com- mittee Level 4 PSHA (Budnitz et al., 1997) performed for the PEGASOS project in Switzerland (Abrahamson et al., 2002). The subject region for the PEGASOS project had relatively few ground-motion recordings; hence, GMPEs were borrowed from other areas for use in PSHA. Cotton et al. (2006) describe how source characteristics, path effects related to geometric spreading and anelastic attenuation, and site effects can vary from region to region. Those underlying physics ideally should be manifest in how a GMPE represents the scaling of a particular ground-motion intensity measure (IM) with 2961 Bulletin of the Seismological Society of America, Vol. 99, No. 5, pp. 2961–2978, October 2009, doi: 10.1785/0120080133