Evidence for static stress interaction among earthquakes in the south-central Apennines (Italy)

Abstract

We have computed static stress changes caused by several earthquakes that occurred in the Apenninic chain (Italy). Static stress associated with fault slip has been computed using the Okada (1992) formulation. Static Coulomb stress changes associated with the three subevents forming the 1980 Irpinia MS = 6.9 main shock indicate that each subevent was consecutively triggered by stress changes produced by the previous ones. Furthermore, aftershocks of this complex faulting event are well correlated with zones of Coulomb stress increase. The interplay of regional stress and local stress changes due to the main shock produces an aftershock distribution wider than expected and a large variation in focal mechanism. The variation in focal mechanisms is consistent with a low level of background regional stress (less than 2 MPa). Moreover, static stress changes due to the Irpinia earthquake appear to have triggered a moderate‐magnitude (ML ≈ 5) seismic sequence in an adjacent tectonic area (close to the town of Potenza), with a delay of some years. The analysis of a further two seismic sequences in the central Apennines, which occurred in 1979 close to the town of Norcia (ML = 5.9) and in 1984 in the Abruzzo National Park (ML = 5.5), also show a clear correlation between aftershocks and the positive Coulomb stress changes generated by the main shocks. Aftershocks of the 1979 Norcia earthquake cluster at two lateral edges of the main fault, as expected for a moderate‐magnitude main shock in which the local stress change is considerably lower than the regional stress field. The static stress changes due to the 1984 main shock in Abruzzo are likely to have triggered a further main shock four days later (ML = 5.1) at the northern edge of the main fault, where the Coulomb stress change is maximum. This evidence indicates a strong correlation between the earthquakes in the Apenninic chain, through static stress changes, at several time‐ and space‐scales. Modelling of such effects is useful both for improving our knowledge of the earthquake dynamics, and for a better evaluation of seismic hazard.