Fast deformation processes and eruptive activity at Mount Etna (Italy)

The seismic and deformation patterns observed on Mount Etna before and during the 1991-1993 eruption, the third largest since the seventeenth century in terms of lava volume, are consistent with the regional tectonic framework of eastern Sicily. The pattern of the stress field acting on the intermediate and lower crust was defined at the local scale by focal mechanisms of microearthquakes occurring at depths between 10 and 25 km beneath the volcano, They provide evidence for a strike-slip compressional stress regime with the maximum compressive component acting approximately N-S. The fault plane solutions and the spatial and temporal distribution of seismicity indicate that usually sinistral shear ruptures occur along approximately NE-SW trending fault zones, while dip-slip ruptures affect approximately NNW-SSE trending fault zones. The latter include the avenues along which magma ascended during the 1991-1993 eruption. Seismic observations indicate a local inversion of the stress field acting on the upper crust (depth < similar to 10 km) underlying Mount Etna, which was initiated less than 2 months before the eruptive event and disappeared with its end. This is consistent with a local tensile regime that favored the magma ascent through the shallow crust. The events preceding the eruption and accompanying its onset (tilt anomalies and seismic swarms) and those occurring shortly after its beginning (a mainshock-aftershock seismic sequence and associated coseismic tilts) provide important evidence for understanding the dynamics of the two main volcano-tectonic structures (NE-SW and NNW-SSE trending fault zones) and associated intrusive mechanisms on Etna. The shape and location of the eruption-feeding dike have been modeled from ground deformation data. This approximately NNW-SSE modeled dike, the seismicity, and the position of the fractures are consistent with the regional stress field characterized by sigma(1) oriented approximately N-S. The geophysical data presented and analyzed in the present paper strongly suggest an overall regional tectonic control as well as an active role for the intruding magma in the dynamics of the volcano.