Volcanological inferences from seismic-strain tensor computations at Mt. Etna Volcano, Sicily

Kostrov's (1974) algorithm for seismic-strain tensor computations, in the v ersion implemented by Wyss et al. (1992a) for error estimates, has been app lied to shear-type earthquakes occurring beneath the Etna volcano during 19 90-1996. Space-time variations of strain orientations and amplitudes have b een examined jointly with ground-deformation and gravimetric data collected in the same period and reported in the literature. Taking also into accoun t the information available from volcanological observations and structural geology, we propose a model assuming that hydraulic pressure by magma empl aced in nearly north-south vertical structures produces the E-W orientation of the maximum compressive strain found in the upper 10 km beneath the cra ter area. In contrast, regional tectonics deriving from the slow, north-sou th convergence between the African and European plates appear to play a dom inant role in the generation of stress and strain fields at crustal depths deeper than 10 km below the volcano. According to our interpretation, the p rogressive ascent of magma through the upper crust prior to eruption produc es the observed gravity changes, cone inflation and unusual seismic strain rate in the upper 10 km associated with a more sharply defined seismic defo rmation regime (i.e. very small confidence limits of the epsilon (1) orient ation). In agreement with this model, deflation revealed by ground-deformat ion data during the course of the major 1991-1993 eruption was accompanied by a practically nil level of shallow seismicity.