Estimates of stress directions by inversion of earthquake fault-plane solutions in Sicily

The tectonic stress orientation is estimated in the lithosphere of northern Sicily, the southern Tyrrhenian sea and southern Calabria, and in the Wadati-Benioff zone below the Tyrrhenian, by inversion of fault-plane solutions of earthquakes covering a magnitude range from 2.5 to 7.1. Focal mechanisms of 97 earthquakes are taken from the literature, after a critical evaluation of their data quality, An average misfit of F = 13 degrees indicates that the set of all shallow (<50km) earthquakes is generated by a heterogeneous stress field. For three subsets, based on regional and magnitude separation, F was small enough (2.8 degrees less than or equal to F less than or equal to 5.9 degrees) to support the assumption of a homogeneous stress direction; for an additional subset, with F = 7.4 degrees, such a condition is close to being fulfilled even though some heterogeneity appears to be present. The number of earthquakes in these subsets ranged from nine to 22, and the uncertainties of the principal stress directions were generally of the order of 20 degrees at the 90 per cent confidence level. The earthquakes with M greater than or equal to 5 define a regional stress field with the greatest principal stress, sigma(1), dipping at a shallow angle to the south. In north-eastern Sicily and south-western Calabria the stress field estimated by earthquakes is extensional, with sigma(3) in a direction of WNW, and a near-vertical sigma(1), in agreement with the graben tectonics mapped geologically in this area. In western Sicily the sigma(1) direction is oriented WNW, but this result is judged less reliable than the others, based on the broader confidence limits of the solution and the average mis fit of 7.4 degrees, The earthquakes in the Wadati-Benioff zone define sigma(1) dipping at about 70 degrees to the NW, subparallel to the zone, with sigma(2) horizontal and striking parallel to the zone.