Assessing Regional and Site-Dependent Variability of Ground Motions for ShakeMap Implementation in Italy

This study investigates the theoretical improvement of the ground-motion models used for ShakeMap implementation in Italy, obtainable by accounting for regional differences of wave propagation and for site effects. The analysis considers ;922 recordings of 116 earthquakes with local magnitude M(L) in the range 2.7-4.5, collected from 137 stations at epicentral distances up to 100 km. Alternative partitions of the available stations divided into separated zones are considered. For each zone in a partition, an empirical ground-motion model is estimated, which includes station correction terms. A genetic algorithm is used in order to minimize the standard deviation of the residuals log(10) (observed) - log(10) (predicted) over the set of the possible partitions. For the logarithm of the peak ground acceleration, it is estimated that site effects contribute to about 30% of the overall standard deviation, while regional differences contribute with no more than 4%. Furthermore, the estimated station correction terms are weakly correlated with the amplification factors based on shear-wave velocity in the upper 30 m (V(S30)) used in ShakeMap to model site effects. Similar results apply to peak ground velocity and spectral acceleration response at 0.3, 1, and 3 sec. These results suggest that parametrizations of site effects alternative to those based on V(S30) should be considered, while the contribution of any regionalization of the ground-motion models is marginal.