Oliva, G.

Stress inversion of the twenty best-quality earthquake fault-plane solutions available in the area of the 1908 Messina earthquake showed a nearly uniform extensional regime with σmin constrained between N284°E and N312°E, coinciding with the direction of extension derived from geostructural data. The misfits of earthquake nodal planes and related slip vectors to the stress tensor allowed us to identify the fault planes of thirteen of the earthquakes used for inversion. In particular, the fault plane of 1908 earthquake was found in the north-trending east-dipping nodal plane of the focal mechanism. These findings and strain tensor estimates performed with the same dataset lead us to propose that in spite of stress uniformity detected over the study area the seismic strain orientations change significantly in the crustal volume under investigation due to different fault orientations in the different sectors. However, when comparing strong earthquakes with background seismicity in a given sector the strain orientations are found to be similar.

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.