Stress Directions and Shear-Wave Anisotropy: Observations from Local

Abstract

The spatial distribution of 414 earthquakes (1.0 ≤ ML ≤ 4.6), recorded from 1994 to 2002 in Southeastern Sicily (Italy), has been analyzed; it generally coincides with mapped Plio-Quaternary faults, including the NNW-SSE offshore fault system which is the most important tectonic structure of the area. For the best located events, we computed 70 focal mechanisms by combining P-wave polarities with S-wave polarizations. A predominance of strike-slip and normal faults was observed. Focal mechanisms were then inverted for stress tensor parameters using the algorithm of Gephart and Forsyth. The results highlighted a region governed mainly by a compressional stress regime. Moreover, anisotropy analysis of shear-waves showed a polarization of fast S-waves prevalently aligned in the NNW-SSE to NW-SE direction over the whole area. A finer analysis of stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis, 1. The eastern sector displays a nearly horizontal 1 trending NW-SE; the central sector is affected by a low dip NNW-SSE 1; whereas in the western sector a 1 NNW-SSE oriented, with a higher dip angle, was detected. Finally, the comparison of the spatial distribution of seismicity occurring during 1994-2002, with locations of previous instrumental earthquakes and larger (M ≥ 5.0) historical events showed that the seismic patterns are persistent.