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S U M M A R YThe 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 (<50 km) earthquakes is generated by a heterogeneous stress field. For three subsets, based on regional and magnitude separation, F was small enough ( 2 . 8 " l F s 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 A425 define a regional stress field with the greatest principal stress, ul, 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 o3 in a direction of WNW, and a near-vertical ol, in agreement with the graben tectonics mapped geologically in this area. In western Sicily the c1 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 ol dipping at about 70" to the NW, subparallel to the zone, with o2 horizontal and striking parallel to the zone.

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A statistical model of the microearthquake occurrence in the Messina area, assuming periodic transitions among several Generalized Poisson states, is proposed. Four states are defined, of different duration, cluster mean rate and earthquake interdependence level. Their sequence is cyclically repeated with annual period. The findings of this work can be seen as a generalization of the approach of Bender (1984) who considered transition phenomena of the seismic temporal pattern among distinct states of the Simple Poisson type. Further, the investigation aims at identifying possible factors leading to the observed periodic changes in the earthquake statistical parameters and to the main 'deviations' of experimental data from the model. The rainfall and regional tectonic stress patterns appear to be reasonable candidates for explaining the microearthquake periodicity and the most relevant anomaly of the observed seismic temporal pattern with respect to the model, respectively.

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