Abstract
Since 1989 a new digital permanent seismic network, run by Istituto Internazionale di Vulcanologia (IIV) — CNR, has improved the monitoring of Mt. Etna volcano allowing detailed studies of the two major eruptive episodes of September–October 1989 and December 1991–March 1993. In this study, we attempt to analyze the seismic pattern and focal mechanisms in the 1988–1991 period by use of the IIV data bank integrated with other information from temporary arrays operating in the area, and neighboring and national permanent networks. Problems related to classic fault plane solution (FPS) computation (polarity method) in volcanic areas were faced and analysis was performed following quality criteria in order to obtain a reliable data set.The investigation was conducted on earthquakes with Md ≥2.7 (126 shocks) and several tests were done to choose the better velocity model among those available at Mt. Etna, and 105 very reliable hypocentral locations were obtained. After the adoption of severe selection criteria 64 FPS were selected on the basis of the number of polarity, their azimuthal distribution and nodal planes errors.Seismic pattern provides indications for a spatial cluster of the foci in the central part of the volcano with prevalent alignment in the NNW–SSE direction.A kinematic interpretation is furnished by the FPS, and P and T axes distribution analysis. The former shows prevalent strike-slip mechanisms, but both direct and reverse dip-slip ruptures are also observed. The latter supports the existence of two different depth seismic strain domains. The deeper one (Z >10km) is coherent with regional stress field (P axes oriented in the NNW–SSE and NW–SE directions), while a rotation in the P and T pattern distribution at depth less than 10km is observed. Results obtained support the hypothesis that uprising and intruded magma modify (or overcome), the regional stress pattern giving rise to the rotation of the P and T axes.
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