Space variations of stress along the Tyrrhenian Wadati-Benioff zone

GEPHART and FORSYTH's (1984) algorithm for stress inversion of earthquake f ault-plane solutions has been applied to a set of ninety intermediate and d eep events occurring in the southern Tyrrhenian region between 1976 and 199 5. P- and S-wave data from local seismic networks in southern Italy, the It alian National Network and international bulletins, have been used for hypo center and focal mechanism computations. Stress inversion runs performed af ter accurate selection and weighting of fault-plane solutions have allowed us to identify stress space variations at a higher level of detail than ava ilable from all previous investigations carried out in the study area. The maximum compressive stress has been shown to follow the depth-decreasing di p of the Wadati-Benioff zone, along the entire zone from a depth of 90 km, to the depth of the deepest events (about 500 km). Variations to such a str ess pattern have been found, possibly related to mantle dynamics and the co mplex composition of the subducting structure. The diffused state of down-d ip compression suggests that the Tyrrhenian subduction has already evolved to the point where the lower end of the slab has reached high-strength mant le materials, the load of the excess mass is entirely supported from below and most of the subducted slab is under compression. In agreement with the lack of large, shallow thrusting events in the immersion zone, the findings of the present study appear to agree well with geodynamic models assuming a passive subduction process with eastward roll-back of the Ionian lithosph ere in the study area. In this context, the depth-decrease of the slab dip may also find a reasonable explanation.