Do earthquake storms repeat in the Apennines of Italy?

Abstract

Series of multiple main shocks that develop on adjacent faults is a typical way in which active extension is accommodated in the Apennines of Italy. This behaviour is explained by fault interaction that occurs at a scale ranging from seconds to days, yielding a space–time clustering of earthquakes, termed as earthquake storms. We show that the seismic energy released by historical earthquakes in central Apennines is clustered into two main small time periods, around 600 and 300 years ago, during which a great portion of the normal faulting belt failed. We favour the hypothesis that clustering results from sudden input of deep fluids into the brittle upper crust. The roughly 300 years periodicity and the 3–4 mm year−1 of tectonic extension suggest that earthquake storms need to be taken into account in seismic hazard scenarios.