THE ROLE OF REGIONAL TECTONICS, MAGMA PRESSURE AND GRAVITATIONAL SPREADING IN EARTHQUAKES OF THE EASTERN SECTOR OF MT-ETNA VOLCANO (ITALY)

Data concerning M greater-than-or-equal-to 3.3 earthquakes that occurred in the eastern sector of Mt. Etna volcano (Sicily, Italy) in the period 1984-1989 are here presented and discussed. Only events with reliable focal mechanisms and detailed macroseismic investigations have been considered. Instrumental information come from local seismic networks, both permanent and temporary, run by the University of Catania (UCT), the Seismological Observatory of Acireale (SOACR), the CNRS (Grenoble, France) and the Institut de Physique du Globe de Paris (IPG, France). Observations regarding the macroseismic effects have been collected by means of field recognitions mainly carried out by the authors. All available data are analyzed in light of the more recent interpretations on the kinematic behaviour of the shallowest structures of Etna, with particular reference to its eastern flank which, according to Borgia et al. (1992), is likely affected by a seaward gravitational migration, driven by the sliding of this sector of the volcanic apparatus over its clay-rich substratum. The results obtained support the hypothesis that the shallow seismicity of the eastern flank of Mt. Etna is related to a complex stress field due to the combined effects of: (a) the tectonics associated with the interaction between the African and Eurasian plates; (b) the rise of magma into the crust; and (c) the gravitative seaward sliding of the eastern sector of the volcano. In particular, we hypothesize that: (1) tectonic forces caused the end of the 1984 eruption, by means of a ''locking mechanism''; (2) the increment of magma pressure at depth is periodically responsible for local re-orientation of the stress field acting on the volcano, which also produces earthquakes of high intensity; (3) shallow earthquakes (z < = 5 km), showing eastward compression mechanism, could be related to the gravitational sliding of unbuttressed sectors of the volcanic body accompanying the emplacement of intrusive dykes.