Stress directions and shear wave anisotropy: observations from local earthquakes in Southeastern Sicily (Italy)

The spatial distribution of 414 earthquakes (1.0 ML 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-northwest– 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 predominance 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-southeast to northwest–southeast compressional stress regime. Moreover, anisotropy analysis 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.