SOURCE PARAMETERS OF SEISMIC EVENTS AT MOUNT ETNA VOLCANO, ITALY, DURING THE OUTBURST OF THE 1991-1993 ERUPTION

A volcanic eruption lasting 472 days occurred at Etna, Italy, between December 1991 and March 1993 on the upper eastern flank of the volcano. This activity produced a lava flow of more than 7 km in length which stopped at an elevation of about 750 m above sea-level, causing some damage at the periphery of the village of Zafferana. We have estimated the source parameters of seismic events which preceded and accompanied the outburst of the eruptive activity. Source parameters have been determined from the SH-displacement spectra using the source model of Brune. We have also calculated the relations between the equivalent Wood-Anderson magnitude (M(WAeq)) of shocks, the local coda-duration magnitude (M(D)), the seismic moment (M(0)), the stress drop (Delta sigma), and the radius (r) of the seismic sources. Finally, we compared our results with those obtained in a reference extensional region (Swabian Jura, Southwest Germany) to speculate on the theological properties of the Etnean crust with respect to that of a well-studied non-volcanic area with similar structural and tectonic features. The comparison suggests that at Etna regional stress affects a highly fractured medium, where the reactivation, the coalescence and the propagation of pre-existing fractures and faults are the dominant processes in the formation of new dislocation structures. This hypothesis is strengthened by the repetitive occurrence of volcanic eruptions along the same structural trends, as is deduced from the very recent eruptive history of Etna. Frequent temporal changes in the orientation of the main stress axes occur, as is suggested by the azimuthal dispersion of the main structural lines and the superimposition with time of extensional and shear mechanisms along the same tectonic alignment.