Changes in failure stress and stress transfer during the 1976-77 Friuli earthquake sequence

Abstract

Coulomb stress changes related to the sequence of strong earthquakes that affected the Friuli area (Northeastern Italy) during the period 1976-77 have been investigated to explore the possibility of fault interaction and stress transfer mechanisms. The seismic sequence that followed the Mw 6.4 mainshock of 1976 May 6, consists of 5 events of Mw between 5.2 and 6.0 during the same year and one event of Mw 5.3 that occurred in 1977 September, all characterized by thrust focal mechanisms. The area is affected by a regional compressive stress, which tends to activate N-dipping thrust planes. The goal of this study is to analyse and verify the relationship between the stress changes induced by the mainshock and the occurrence of subsequent events. Our results show that the mainshock induced a variation in the stress field in the area where most of the seismic events occurred. Our modelling shows a positive correlation between the areas of increased Coulomb stress induced by the mainshock and the distribution of seismicity that followed. This evidence suggests stress interaction and the possibility that the mainshock promoted the rupture of those faults associated with the event of 1976 May 9 (Mw 5.1) and the events of 1976 September 11 (Mw 5.2 and 5.6), 1976 September 15 (Mw 6.0) and 1977 September 16 (Mw 5.3). The Coulomb stress caused by co-seismic dislocations decreased in the zone where an Mw 5.9 earthquake occurred on September 15 at 03:15. When we included the contribution of the regional tectonic loading in the calculation of Coulomb stress changes, we found a positive correlation between the hypocentral area of the September 15 at 03:15 event and the area of Coulomb stress increase. Aftershocks that occurred after the mainshock but before the September 15 03:15 event are also located in the area of Coulomb stress increase.