GNGTS 2013 - Atti del 32° Convegno Nazionale

features with eastern Sicily. Present study aims to carry out a more detailed analysis about the seismic features of different sectors of Mt. Etna volcano and for this purpose a seismic catalogue covering a longer span time has been adopted. Spatial Analysis of Seismicity. The IETs distributions within the main seismogenic depth intervals identified at Mt. Etna (Fig. 1b) have been computed, for the time interval 1976-2011, with the aim of obtain information about the seismic behaviour of different crustal volumes. The level z ≤ 5 km appears to be quite active from the seismogenic point of view and very well defined by the statistical parameters. The high values of skewness (S > 0), observed in this depth interval, point out that most of the IETs are characterized by short recurrence times and are concentrated on the left of the distribution, therefore implying the existence of correlated events. In particular, in the north-eastern and south-eastern sectors of the volcano (Pernicana and Timpe fault systems respectively) IET distributions are characterized by bimodal shape, while a peaked curve at low IET values and therefore correlated events is observed close to the summit area. As concerns the aperiodicity values, we observe α > 1.5 for the whole depth level that assume in particular values α > 3 in proximity of the summit area, therefore indicating a strongly clustered seismic activity. In our opinion, the seismicity of the seismogenic volume with z ≤ 5 km seems affected by both the regional stress field, which imply a background seis- micity and the local stress connected to the magma rising processes. These latter activities, in the summit area in particular, usually with a very low background seismicity, is characterized by swarms that point out the beginning of some eruptions. The shallower seismogenic level can therefore be defined as the more typically “volcanic” volume since it appears activated as a consequence of magma induced stresses. In the 5-12 km depth interval, negative values of the skewness are found. This denotes a seismicity with a moderate number of correlated events and IET distributions inclined to be unimodal with high recurrence times, as well. Moreover a different seismic behaviour characterizes the eastern and western sectors of the volcano. The former being characterized by high IET values (S ~ -1.5, k > 3), indicating the prevalence of independent events, the latter being characterized by an higher percentage of correlated events (S ~ -0.5, k ~ 2). Finally, in the z > 12 km depth interval, two sectors can be discriminated: the first one located in the NW part of the volcano, with S > 0, which is affected by the occur- rence in recent time (after the 2009) of several seismic swarms; the second, located towards the south-western portion of the summit area, with α ~ 1.5, indicating quite homogeneously distributed seismicity recurrence times. The cumulative number of earthquakes has been calculated for different sectors of the volcano at the afore-mentioned depth intervals. A different behaviour considering either z ≤ 5 or z > 5 km depth is observed. In the first case, alternating periods of high and modest seismic activity are found; these periods are so relevant that the global cumulative rate depicted in Fig. 1d is strongly affected by them. The time interval 1991-2001 shows in particular a reduced seismic activity. This could be related to the lack of lateral eruptions at Mt. Etna and the prevalent summit activities occurred after the end of the flank eruption of 1991-1993 (Allard et al. , 2006). Such behaviour is particularly evident close to the summit area, where usually the moderate background seismic activity is interrupted by sudden increments of the seismic occurrence rate that point out the beginning of lateral eruptions whereas, summit eruptions are preceded and accompanied by a gradual increase of the seismic activity. These findings further support the tight relationship between volcanic activities and seismic rate inside the first 5 km beneath the volcano. When the cumulative number of earthquakes is calculated for depth z > 5 km no significant sharp increments of the seismic activity are observed during the whole considered time interval (1976-2011). Only in a few instances slight increments of the seismic activity in the western sector of the volcano precede the onset of some flank eruptions. Such behaviour continues to be observed at depth greater than 12 km, although seldom seismic swarms without a clear correlation with eruptive episodes are detected. A detailed analysis of the number of intermediate (5 < z ≤ 12 km) and the deep (z > 12 km) earthquakes in the western 281 GNGTS 2013 S essione 1.3