Abstract
Earthquakes are usually clustered in both time and space and, within each cluster, the event of highest magnitude is conventionally identified as the mainshock, while the foreshocks and the aftershocks are the events that occur before and after it, respectively. Mainshocks are the earthquakes considered in the classical formulation of the probabilistic seismic hazard analysis (PSHA), where the contribution of foreshocks and aftershocks is usually neglected. In fact, it has been shown that it is possible to rigorously, within the hypotheses of the model, account for the effect of mainshock-aftershocks sequences by means of the sequence-based PSHA (i.e., SPSHA). SPSHA extends the usability of the homogeneous Poisson process, adopted for mainshocks within PSHA, to also describe the occurrence of clusters maintaining the same input data of PSHA; i.e., the seismic rates derived by a declustered catalog. The aftershocks’ occurrences are accounted for by means of conditional non-homogeneous Poisson processes based on the modified Omori law. The seismic source model for Italy has been recently investigated, and the objective of the study herein presented is to include and evaluate the effect of aftershocks, by means of SPSHA, based on a new grid model. In the paper, the results of PSHA and SPSHA are compared, considering the spectral and return periods that are of typical interest for earthquake engineering. Finally, a comparison with the SPSHA map based on a well- established source model for Italy is also provided.
Highlights
Probabilistic seismic hazard analysis [probabilistic seismic hazard analysis (PSHA), Cornell, 1968; Mc Guire, 2004], in its classical formulation, describes the earthquakes’ occurrence through a homogeneous Poisson process (HPP)
While earthquakes are typically clustered in time and space, PSHA only models the occurrence of mainshocks, which are usually defined as the largest magnitude event in a cluster, and the effect of foreshocks and aftershocks, that is, the earthquakes of the cluster occurring before and after the mainshock, respectively, is neglected
The effect of aftershocks, evaluated by means of SPSHA, on that source model has been discussed by Iervolino et al [2018] showing that, for the studied spectral periods and return periods, hazard increase due to aftershocks is up to 30% and it is, generally, more significant within or around areas exposed to comparatively higher hazard according to classical PSHA
Summary
Probabilistic seismic hazard analysis [PSHA, Cornell, 1968; Mc Guire, 2004], in its classical formulation, describes the earthquakes’ occurrence through a homogeneous Poisson process (HPP). The objective of the study presented is to develop SPSHA using the cited grid-seismicity model, so as to evaluate the effect of aftershocks on the hazard in terms of the ground motion corresponding to exceedance return periods of typical interest for structural engineering. To this aim, in the remainder paper, after presenting the average seismicity model, the basics of PSHA and SPSHA are briefly recalled. As discussed in the following, in the analyses carried out in this paper, the earthquake rates with magnitude larger than 6.9 are associated to the 6.9 magnitude bin in order to not extrapolate the ground motion prediction equation
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