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

Abstract Fault-plane solutions of 78 earthquakes with magnitude larger than 3.2, which occurred in Friuli between May 1976 and August 1979, have been used to estimate the orientations of the principal stress axes by an inversion technique. The analysis of the confidence limits and the average angular misfit of the solution found when inverting the entire data set revealed that faulting 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 reduced significantly misfit and confidence areas of the solution for some data sets. Therefore, we conclude that the stress directions changed as a function of time. A nearly uniform stress direction, compatible with tectonic models proposed for the study area (ca. N-S convergence of the Adriatic and South Alpine units), was found between the main event of 6 May 1976 (ML 6.4) and the strong aftershocks of mid-September 1976 (5.1 ≤ ML ≤ 6.1). The analysis of the individual misfit cumulative pattern reveals that some degree of stress heterogeneity appeared in July 1976, which is explained here in terms of heterogeneous local responses of the stress field to the seismic activity superimposed to the regional compressive factor. The degree of perturbation of the principal stress directions by local effects increased with time and showed, in particular, a clear variation after the strong earthquakes of mid-September 1976. The perturbations appear to be reflected by low-magnitude quakes, whereas the faulting in strong events is compatible with the regional stress field.