Rarity, proximity, and design actions: mapping strong earthquakes in Italy

At the state-of-the-art of structural codes, seismic design actions are based on probabilistic seismic hazard analysis (PSHA). In the performance-based earthquake engineering framework, the return period of exceedance of the reference ground motion is established based on the desired performance of the structure. It is easy to show and recognize that exceedance of elastic spectra, for the most common return periods considered for design, is very likely for some earthquakes if they occur close to the site of interest, and that this does not necessarily contradict the results of PSHA. Therefore, it might be relevant to gather insights about: (i) the probability that the site is in proximity of earthquakes of magnitude that can imply exceedance; (ii) the probability that earthquakes occurring close cause exceedance of design actions; (iii) the minimum magnitude of close-by events that are likely to cause exceedance of design actions, which are then referred to as the strong earthquakes; (iv) the accelerations that structures could be exposed to in the case of exceedance of design spectra. These results, which are produced for Italy in this study, may be considered by-products of PSHA, and are helpful in determining what to expect in terms of elastic actions for code-conforming structures in countries where probabilistic seismic hazard lies at the basis of structural design.