Stress Directions and Shear-Wave Anisotropy: Observations from Local Earthquakes in Southeastern Sicily, Italy

Abstract

The spatial distribution of 414 earthquakes (1.0 M L 4.6), recorded from 1994 to 2002 in southeastern Sicily (Italy), has been analyzed. The seismicity generally coincides with mapped Plio-Quaternary faults, including the north-north- west-south-southeast striking 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 predom- inance of strike slip and normal faults was observed. Focal mechanisms were then inverted for stress tensor parameters by using the algorithm of Gephart and Forsyth. The results highlighted a region governed mainly by a north-northwest-south-south- east to northwest-southeast compressional stress regime. Moreover, anisotropy anal- ysis of shear waves showed a polarization of fast S waves coherently aligned with this stress direction. A finer-scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis, r1. The eastern sector displays a nearly horizontal r1 trending northwest-southeast; the central sector is affected by a low-dip north-northwest-south-southeast r1; whereas, in the western sector, a north-northwest-south-southeast-oriented r1 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 seismicity patterns are persistent.