Stress field in Friuli (NE Italy) from fault plane solutions of activity following the 1976 main shock

Fault-plane solutions of 78 earthquakes with magnitude larger than 3.2, whi ch occurred in Friuli between May 1976 and August 1979, have been used to e stimate the orientations of the principal stress axes by an inversion techn ique. The analysis of the confidence limits and the average angular misfit of the solution found when inverting the entire data set revealed that faul ting was not produced by a uniform stress distribution, Subdivision of the study area into smaller volumes did not lead to significant improvements in terms of stress uniformity, whereas temporal subdivision of the sample red uced significantly misfit and confidence areas of the solution for some dat a sets. Therefore, we conclude that the stress directions changed as a func tion of time. A nearly uniform stress direction, compatible with tectonic m odels proposed for the study area (ca. NS convergence of the Adriatic and S outh Alpine units), was found between the main event of 6 May 1976 (M-L 6.4 ) and the strong aftershocks of mid-September 1976 (5.1 less than or equal to M-L less than or equal to 6.1). The analysis of the individual misfit cu mulative pattern reveals that some degree of stress heterogeneity appeared in July 1976, which is explained here in terms of heterogeneous local respo nses of the stress field to the seismic activity superimposed to the region al compressive factor. The degree of perturbation of the principal stress d irections by local effects increased with time and showed, in particular, a clear variation after the strong earthquakes of mid-September 1976. The pe rturbations appear to be reflected by low-magnitude quakes, whereas the fau lting in strong events is compatible with the regional stress field.