ALBERT

Notes: 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° 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°〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:0956540X:GJI857:les" location="les.gif"/〉F〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:0956540X:GJI857:les" location="les.gif"/〉 5.9°) to support the assumption of a homogeneous stress direction; for an additional subset, with F= 7.4°, 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° at the 90 per cent confidence level. The earthquakes with M〈inlineGraphic alt="geqslant R: gt-or-equal, slanted" extraInfo="nonStandardEntity" href="urn:x-wiley:0956540X:GJI857:ges" location="ges.gif"/〉 5 define a regional stress field with the greatest principal stress, σ1, 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 σ3 in a direction of WNW, and a near-vertical σ1, in agreement with the graben tectonics mapped geologically in this area. In western Sicily the σ1 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 misfit of 7.4°. The earthquakes in the Wadati-Benioff zone define σ1 dipping at about 70° to the NW, subparallel to the zone, with σ2 horizontal and striking parallel to the zone.

Notes: A one-station method was developed to invert simultaneously for seismic moment, focal depth, and the orientations of the nodal planes. This method was tested on seismograms recorded at the broadband Gréfenberg array (GRF) for earthquakes in the central to eastern Hellenic arc and southern Turkey. The complete P and S body-wave responses, including all near source surface reflections, were synthesized using the reflectivity method. The minimum misfit between the observed and theoretical seismograms in the WWSSN-LP band was determined by cross-correlation, searching the whole parameter space of strike, dip and rake of a pair of orthogonal nodal planes, and adjusting the source depth and seismic moment. The initial searching step of 20° was reduced to 5° in the vicinity of the minimum misfit position, for a final search. The best fitting solution was then compared to the Harvard (HRVD) moment tensor solution best double couple which was derived using long-period data and to the P-wave first motion polarities reported by the ISC. Most solutions based on GRF data agree well with those found by HRVD, but in some cases some features of the observed GRF signals cannot be matched by the HRVD solution or solutions similar to it.The nine earthquakes we analysed in detail had magnitudes between 5.2 and 5.8. Our depth determinations ranged from 35 to 155 km. The deepest events (80–155 km) were located near Rhodes and to the east of it. One earthquake located in southern Turkey was confirmed to have a focus at about 125 km depth. The focal mechanisms of the intermediate depth earthquakes in the eastern Hellenic arc show P-axes approximately parallel to the strike of the deep slab.