A Comparison of NGA Ground-Motion-Prediction Equations to Italian Data

Ground-motion prediction equations (GMPEs) have recently been developed 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.