GNGTS 2013 - Atti del 32° Convegno Nazionale

seismic behaviour of different crustal volumes, a space-time analysis of the IETs has been performed by sub-grouping the earthquakes into three depth classes: 1) from the surface to 5 km b.s.l., 2) between 5 km and 12 km b.s.l. and 3) below 12 km b.s.l. We then have calculated the IET distributions and some associated statistical parameters (aperiodicity α, skewness S, kurtosis k) for all the earthquakes located within 3.5 km around each node, in a grid map having inter-node distance of 2 km. The statistical parameters allow quantifying the shape features of each IET distributions (Sicali et al. , 2012). The kurtosis k and the skewness S, provide a measure of the distribution “peakedness” and an indication of the peaks position with respect to high or low IET values. The aperiodicity α (α = σ/µ, where σ is the standard deviation and µ is the mean) represents a way to estimate the regularity of earthquake occurrence over time. In general the α value defines: (i) periodic regime, if α ~ 0, σ << µ; (ii) stationary regime, if α ~ 1, σ ~ µ; (iii) clusters regime, if α ≥ 2, σ >> µ. The skewness is particularly suitable for our purposes since it gives information about the position of the dominant peak in the IET distribution (high or low IET values, in relation to negative or positive skewness). Finally, in order to verify a possible correlation between the occurrence of eruptive phenomena and the activation of particular seismogenic volumes, following the results of IET analysis, we have analyzed the seismic rate - time variations of the seismic sectors characterized by different earthquake recurrence time. Data analysis and discussion. In a recent study, Sicali et al. (2012) framed, through the analysis of spatial variation of IET distributions, the seismicity related to volcanic processes in the regional seismotectonic context highlighting the presence of sectors showing specific seismic behaviours. The statistical parameters suggest that the Mt. Etna is located at the boundary between two different regional domains. The IET distribution pattern, evaluated for depth z > 5 km, set indeed into evidence that the western sector of the volcano and north- western Sicily share similar recurrence times, whereas eastern sector of Mt. Etna shares similar Fig. 2 – Cumulative number of earthquakes during 1976-2011 in the western flank of Mt. Etna. Black curve refers to earthquakes occurring at depth z > 12 km green curve refers to earthquakes occurring at depth 5 < z ≤ 12 km. The grey bands indicate the periods during which the increase of deeper crustal volume seismicity precede the shallower ones. Dashed lines indicate the beginning of either flank F or summit S main eruptions. 280 GNGTS 2013 S essione 1.3