Shear-wave polarization alignment on the eastern flank of Mt. Etna volcano (Sicily, Italy)

Abstract

Recently, with the improvement of three-component seismic networks, studies revealing anisotropic characteristics in different regions have assumed great interest. In a complex volcanic area like Mt. Etna (Sicily, Italy), the existence of both iso-oriented fault systems and intrusive bodies consisting of olivine and clinopyroxene suggest the presence of anisotropic structures. In order to investigate this we analyzed the physical phenomenon of shear-wave splitting since under certain constraints, shear waves are less sensitive to local heterogeneity. The aims of this paper are: 1) to evaluate if in a structural complex situation like that at Mt. Etna the signal crossing an anisotropic volume could be enhanced in spite of effects due to undirectional properties along the source-receiver path; 2) to investigate the correlations, if any, between polarization direction of the leading shear wave and the patterns of compressive stress acting on the investigated area. Therefore we measured time-delays between the S-onsets on the horizontal components of 3D seismograms to reveal the possible seismic anisotropy in the Etnean region; moreover, we analyzed the polarization vector of shear-waves seismic data recorded during a survey carried out in the spring-summer 1988. We found clear evidence of splitting that we attributed to the presence of an anisotropic volume not homogeneously distributed on the eastern slope of Mt. Etna volcano.