Earthquakes and faults at Mt. Etna (Southern Italy): problems and perspectives for a time-dependent probabilistic seismic hazard assessment in a volcanic region

Abstract

We investigated the seismic potential of a given set of faults in the Etna region, by analysing the inter-event times of major earthquakes as given by the earthquake catalogue. Among the active structures of the volcano, the Timpe fault system in the eastern flank is responsible for the largest earthquakes occurring in historical time, with long-term behaviour characterised by earthquake rates of ~ 20 years for severe/destructive events (epicentral intensity I0 ≥ VIII EMS). By means of coseismic effect analyses and thanks to the peculiarity of earthquake source in this volcanic district, we associated the seismic events to the individual seismogenic sources, obtaining the seismic history of each fault. Mean recurrence time of major events referred to a specific fault can therefore be defined. Then, we calculated the probabilities of occurrence of destructive events both with Poisson and Brownian Passage Time (BPT) models. A time-dependent BPT distribution function has been used to calculate the conditional occurrence probability for each structure of the Timpe seismogenic zone. In a memoryless perspective, the probability of having a major earthquake on individual faults is about 7% in 5 years, while it changes from fault to fault if the probability is conditioned to the time elapsed since the last event. As a result, impending earthquakes are expected on the S. Tecla fault (11%), and on Moscarello and Fiandaca faults (~ 6-9%), all involved in the complex dynamics of the eastern flank of Mt. Etna. These results are consistent with those independently obtained through the site approach, calculated by the SASHA code.