A Possible Seismic Gap within a Highly Seismogenic Belt Crossing Calabria and Eastern Sicily, Italy

Abstract

A review of the seismic, geologic, and geodynamic information available for the Calabro-Sicilian region of southern Italy leads us to suggest a unifying view of the larger ( M ≥ 7) earthquakes that have occurred there in the last millennium coincidentally concentrated in the last 3.7 centuries. The seismicity coincides with a narrow curvilinear extensional belt that passes through western Calabria and eastern Sicily (wces belt) and which includes a nearly continuous north–south succession of primarily east-dipping normal faults. In our reconstruction of the seismotectonic process the faulting is activated by west-northwest–east-southeast extension induced by residual rollback of the Ionian subduction slab. Our analysis of the space-time distribution of strong earthquakes indicates a zone of conspicuous aseismicity ( M > 4.5 since 1700, M ≥ 7 since 1000) along the belt, corresponding to the 30-km-long Scaletta-Fiumefreddo segment of the Messina–Fiumefreddo fault in eastern Sicily. Moreover, because estimated recurrence times are on the order of a millennium for M 7 earthquakes along different parts of the wces belt, and because historical data for destructive earthquakes in the first millennium a.d. are not detailed enough to allow reliable identification of the source zones, we cannot definitely state whether or not there is a late-stage seismic gap for a large earthquake in eastern Sicily. By applying standard relationships, the potential for a magnitude 7 earthquake can be estimated for the Scaletta–Fiumefreddo 30-km-long normal fault segment. The level of seismic activity has been low in the possible gap area in the past two decades when the upgraded local network has detected tens of events above magnitude 2.5 with values up to 3.7. Hypocenter locations of these events seem to delineate the deep geometry of two faults reported in the surface geologic maps, one of which is the Scaletta–Fiumefreddo silent fault. Coulomb stress changes not larger than +0.6 bar produced on the silent fault by the most recent M 7 regional earthquake (1908), and the substantially nil Coulomb stress change on the same fault by M 7 earthquakes of 1693 and 1783, imply relatively small perturbation by previous earthquakes to the silent fault compared with the Coulomb stress perturbation of 2–2.5 bars estimated by other investigators on the 1908 earthquake source caused by major earthquakes of the previous centuries.