Characterization of seismicity at Mt. Etna volcano (Italy) by inter-event time distribution

The space–time inter-event time (IET) distributions of earthquakes occurring from 1988 to 2011 at Mt. Etna are analysed in order to identify the periodicity or stationary behaviour of seismicity, and to correlate it with the volcano-tectonic features of the region. The comparison between IET distributions at Etna with those obtained both for Sicily and Italy, shows that IETs at a larger scale are well-modelled by a gamma distribution, whereas at Etna local scale they are characterised by a bimodal curve, in which the two peaks are related to: (i) the contribution of local seismic swarms with very short inter-event times, and (ii) the background regional stationary seismicity. IET analysis is an important tool to investigate the behaviour of seismicity at different crustal levels in the Etna region, distinguishing sectors that are influenced by volcano dynamics or regional tectonics. Indeed, the spatial variation of IET distributions, obtained by analysing different Etna crustal sectors, shows that seismicity shallower than 5 km is almost entirely characterised by short inter-event times and is mainly confined to the summit area. For earthquakes deeper than 5 km occurring in the eastern flank of the volcano, as well as in eastern Sicily, IET distributions are characterised by independent events which suggest that both areas are influenced by the same extensional regional regime. By contrast, IET distributions obtained for the western flank and northwestern Sicily are marked by two peaks, indicating that the compressional stress is acting in both areas.