Abstract

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, σ1. The eastern sector displays a nearly horizontal σ1 trending northwest–southeast; the central sector is affected by a low-dip north-northwest–south-southeast σ1; whereas, in the western sector, a north-northwest–south-southeast–oriented σ1 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.

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