Issues in near-source probabilistic seismic hazard analysis of the Messina Strait (Italy): preliminary investigation

One challenge for construction projects of strategic importance near known faults, is incorporating near-source (NS) effects in probabilistic seismic hazard analysis (PSHA) to define design actions. The most prominent NS effect with engineering relevance is forward rupture directivity, which can cause ground velocity pulses to manifest at sites pseudo-aligned with the direction of rupture propagation, leading to peculiar spectral amplitudes and inelastic-to-elastic ratios. Another issue is mainshock occurrence process on specific faults, which may depart from the homogeneous Poisson process (HPP), typically assumed for large-scale seismic source zones. This work offers a preliminary examination of such NS issues for the Messina Strait, where the 1908 M7.1 earthquake occurred. First, a NS-PSHA – conditional to event occurrence on the fault – is performed for a coastal site, accounting for impulsive ground motion occurrence. Second, two alternative earthquake occurrence models, accounting for the seismic history of the source are considered, the Brownian passage time (BPT) and the slip-predictable model (SPM), calibrated against available information about the fault. It is found that due to pulse-like effects, the relative increase in spectral pseudo-acceleration caused by a magnitude M7±0.3 earthquake occurring on the fault exceeds 50% for vibration periods between 3s and 10s, yet this corresponds to absolute differences lower than 0.08g. Moreover, both BPT and SPM yield lower probabilities of at least one earthquake occurring within the next two-hundred years, compared to the corresponding result from HPP. These probabilities are respectively 0.060 and 0.134 for BPT and SPM, compared to 0.237 from HPP.