GNGTS 2013 - Atti del 32° Convegno Nazionale

Concluding remarks. The space-time IET distributions and the variation of the seismic rate, performed using a dataset collecting 25 years of instrumental earthquake records (1976- 2011), allows us to draw the following considerations: • The seismicity at Mt. Etna is not randomly located in time and space, but the hypocentral distribution of the earthquakes defines seismogenic volumes characterized by particular seismic patterns. Similar finding were obtained, through other methodologies (Cardaci et al. , 1993; Lombardo and Cardaci, 1994; Privitera et al. , 2001; Alparone et al. , 2010) that describe the relationships between different stages in the volcanic activity of Mt. Etna and the characteristics of recorded seismic activity. • A detailed analysis of different sectors of the volcano indicates that seismogenic volumes at depth shallower than 5 km show a quite stationary background seismic activity that sometimes is interrupted by sudden increments of the seismic rate linked to eruptive phenomena. This result was already observed by Sicali et al. (2012) although the used dataset was concerned a shorter time interval. • The temporal IET analysis highlights that during the periods with strong flank activities of the volcano, the occurrence of seismic swarms is mainly restricted within the seismogenic level at depth z ≤ 5 km which in other words means that a great number of correlated events take place in this seismogenic volume. On the contrary, during periods without flank activi- ties, only a moderate background seismicity is observed testified by unimodal IET patterns with uncorrelated events. • The seismogenic volumes at depth z > 5 km do not show significant temporal variations keeping steady the observed differences between the eastern and western sectors of the volcano. • Changes of the seismic rate underlines a tendency towards the migration of hypocenters from seismic volumes deeper than 12 km to shallower depth, especially in the western sector of the volcano, before the occurrence of the main flank eruptions. This result gives further support to similar observations of Lombardo and Cardaci (1994), Privitera et al. (2001), Bonaccorso et al. (2004) and Sicali et al. (2011) that describe the occurrence of shallow seismic swarms preceding the opening of eruptive fractures. These swarms follow sequences of deeper earthquakes which are interpreted as responsible for the refilling of the volcano feeding system. Such finding, at the same time, shows the soundness of present statistical approach. Finally, we have to affirm that present analysis validate the results obtained by Sicali et al. (2012) adding significant details as a consequence of the larger number of data used. Moreover, we can after all assert that the time variation of statistical parameters, evaluated by splitting the dataset in several periods, emphasize different seismic behaviours linked to different ty- pologies of the volcanic activity. Lateral eruptions imply the most evident variations in the space-time features of the seismicity, as confirmed by the activation of specific seismogenic volumes. On the other hand, summit eruptions follow and occur simultaneously to a regular increasing of the seismic rate. References Allard P., Behncke B., D’Amico S., Neri M. and Gambino S.; 2006: Mount Etna 1993-2005: Anatomy of an evolving eruptive cycle. Earth Science Review, 78 , 85-114. Alparone S., D’Amico S., Maiolino V. and Ursino A.; 2010: Sismicità all’Etna dal 1989 al 2010: evidenze sull’evoluzione spazio-temporale della attività sismica. Proc. 29° GNGTS. Alparone S., Barberi G., Bonforte A, Maiolino V. and Ursino A.; 2011: Evidence of multiple strain fields beneath the eastern flank of Mt. Etna volcano (Sicily, Italy) deduced from seismic and geodetic data during 2003-2004. Bull. Volcanol., 73 (7), 869-885. Andronico D., Branca S., Calvari S., Burton M., Caltabiano T., Corsaro R.A., Del Carlo P., Garfì G., Lodato L., Miraglia L., Murè F., Neri M., Pecora E., Pompilio M., Salerno G. and Spampinato L.; 2005: A multi - disciplinary study of the 2002 - 03 Etna eruption : insights into a complex plumbing system . Bull. Volcanol., 67 , 314-330. 284 GNGTS 2013 S essione 1.3