SEISMIC ACTIVITY ACCOMPANYING THE OUTBREAK OF THE 1991-1993 ERUPTION OF MT-ETNA (ITALY)

The character and location of seismic activity accompanying the onset of the 1991-1993 eruption at Mt. Etna are compatible with the surface evidence of the volcanic pile rupture. Both the epicentral distributio n and the focal mechanisms of a swarm that occurred on December 14, 19 91, agree with magma ascent occurring along the main NNW-SSE-trending structure of the volcano and the consequent opening of a system of eff usive fissures with the same trend. A typical mainshock-aftershock seq uence, recorded the day after and indicating transcurrent displacement occurring along the second-principal structure of Etna (NE-SW), depic ts the tectonic response of the volcanic pile and the underlying basem ent to major stresses applied by the magma push.