When time and faults matter: towards a time-dependent probabilistic SHA in Calabria, Italy

In this study, we attempt to improve the standards in Probabilistic Seismic Hazard Assessment (PSHA) towards a time-dependent hazard assessment by using the most advanced methods and new databases for the Calabria region, Italy. In this perspective we improve the knowledge of the seismotectonic framework of the Calabrian region using geologic, tectonic, paleoseismological, and macroseismic information available in the literature. We built up a PSHA model based on the long-term recurrence behavior of seismogenic faults, together with the spatial distribution of historical earthquakes. We derive the characteristic earthquake model for those sources capable of rupturing the entire fault segment (full-rupture) independently with a single event of maximum magnitude. We apply the floating rupture model to those earthquakes whose location is not known sufficiently constrained. We thus associate these events with longer fault systems, assuming that any such earthquake can rupture anywhere within the particular fault system (floating partial-rupture) with uniform probability. We use a Brownian Passage Time (BPT) model characterized by mean recurrence, aperiodicity, or uncertainty in the recurrence distribution and elapsed time since the last characteristic earthquake. The purpose of this BPT model is to express the time-dependence of the seismic processes to predict the future ground motions in the region. Besides, we consider the influence on the probability of earthquake occurrence controlled by the change in static Coulomb stress (ΔCFF) due to fault interaction; to pursue this, we adopt a model built on the fusion of BPT model (BPT + ΔCFF). We present our results for both time-dependent (renewal) and time-independent (Poisson) models in terms of Peak Ground Acceleration (PGA) maps for 10% probability of exceedance in 50 years. The hazard may increase by more than 20% or decrease by as much as 50% depending on the different occurrence model. Seismic hazard in terms of PGA decreases about 20% in the Messina Strait, where a recent major earthquake took place, with respect to traditional time-independent estimates. PGA near the city of Cosenza reaches ~ 0.36 g for the time-independent model and 0.40 g for the case of the time-dependent one (i.e. a 15% increase). Both the time-dependent and time-independent models for the period of 2015–2065 demonstrate that the city of Cosenza and surrounding areas bear the highest seismic hazard in Calabria.