Paleoearthquakes modelling and effects of uncertainties on probability assessment of next fault ruptures: the case of Central Italy surface faulting earthquakes

SUMMARY Using the published paleoseismological trenching data for 16 faults in Central Italy, we compile a new data base of surface faulting earthquakes, having a quite stationary temporal distribution since 6000 BCE. By applying a probabilistic aggregation method, we correlate the event ages from distinct trenches on each fault, to construct all possible individual fault rupture scenarios, consistent with geological constraints. These inferred fault time histories are the basis for both individual fault and regional seismic hazard evaluation. We found that the mean recurrence time of each fault goes from about 1 to 4 thousand years for individual faults, whereas the value at regional scale is close to 120 yr. The small size of individual fault data samples does not allow us to infer straightforward information on the fault temporal behaviour, but only to evaluate the reliability of a chosen occurrence model for each fault. Therefore, hazard assessment is carried out by including the uncertainties related to both ages and probability distribution of the interevent times. We find that both these have a large impact on the probabilities of next rupture for individual faults: these depend on basic features of the temporal model and on the relation between the elapsed time and the mean interevent time. At a regional scale, we cannot exclude the simplest possible model, that is, the Poissonian behaviour, that provides quite stable probabilities of future events, close to 27 per cent in the next 50 yr.