Aftershocks’ effect on structural design actions in Italy

Although earthquakes generally form clusters in both space and time, only mainshocks, usually the largest magnitude events within clusters, are considered by probabilistic seismic hazard analysis (PSHA; Cornell, 1968). On the other hand, aftershock PSHA (APSHA), based on the modified Omori law, allows the quantification of the aftershock threat (Yeo and Cornell, 2009). Classical PSHA often describes event occurrence via a homogeneous Poisson process, whereas APSHA describes occurrence of aftershocks via cluster-specific nonhomogeneous Poisson processes, the rate of which is a function of the mainshock magnitude. It is easy to recognize that clusters, each of which is made of the mainshock and the following aftershocks, occur at the same rate of mainshocks. This recently allowed the generalization of the hazard integral to account for aftershocks in PSHA (i.e., Iervolino et al., 2014), which resulted in the formulation of the so-called sequence-based PSHA (SPSHA). In the present study, SPSHA is applied to Italy countrywide, using the same source model (Stucchi et al., 2011) that lies at the base of the official PSHA used for structural design, to quantitatively assess the increase in seismic design actions for structures when accounting for the aftershocks.