Spatial variations of seismogenic stress orientations in Sicily, south Italy

Seismogenic stress orientations are estimated in the lithosphere of Sicily by inversion of 131 local earthquake focal mechanisms (FMs) selected from the literature. An average misfit F = 14.5° between stress tensor and FMs indicates that the entire set of earthquakes is generated by a highly heterogeneous stress field. Detailed analysis of stress tensors and related earthquake misfits obtained for tens of subsets based on spatial separation of data allowed us to identify two main stress domains in the study region: (i) a compressional domain, including Etna, western Sicily and the southern Tyrrhenian sea offshore Sicily, where the σ1 orientation roughly changes from NW-SE in the Etna area and western Sicily to NNE-SSW at the northeastern edge of the domain (Eolian Islands) and (ii) an extensional domain in northeastern Sicily between the Eolian Islands and Etna, where σ3 is oriented analogously to previously estimated in the easterly confining extensional area of the Messina Straits. General agreement is found between this stress pattern and the GPS crustal displacement vectors reported in the most recent literature for the study region. Moreover, stress inversion enables us to locate the extensional domain more accurately than the presently available GPS data. Finally, the stress orientations estimated in the southern and western sectors of the study area (Etna and western Sicily on and offshore) match well with the displacement fields predicted for Sicily by large-scale models of plate motion, conversely a mismatch is observed in the northeastern sector (Eolian Islands and northeastern Sicily). We suggest that the joint action of Africa-Eurasia convergence and Ionian subduction trench retreat (rollback of the subducting slab) may explain the stress pattern detected in Sicily in the present study. Work for computation of a finite-element regional geodynamic model based on geophysical and geological data collected over the last few years has recently started with the purpose of quantitatively checking this hypothesis.