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

Abstract

S U M M A R YCoulomb 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 M w 6.4 mainshock of 1976 May 6, consists of 5 events of M w between 5.2 and 6.0 during the same year and one event of M w 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 (M w 5.1) and the events of 1976 September 11 (M w 5.2 and 5.6), 1976 September 15 (M w 6.0) and 1977 September 16 (M w 5.3). The Coulomb stress caused by co-seismic dislocations decreased in the zone where an M w 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 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.The Friuli seismic sequence which started on 1976 May 6 (at 20:00) with a mainshock of M w 6.4 was the most important that affected Northeastern Italy during the 20th century. The maximum intensity, measured using the Medvedev-Sponheuer-Karnik scale, was X. The sequence displayed some interesting peculiarities. The focal mechanisms of the strongest shocks (M w > 5.0) were similar and involved thrust processes (Fig. 1); a migration of seismicity from east to west was observed. The seismic moment release was not characterized by a uniform temporal decay. Major aftershocks occurred on May 9, of M w 5.1, and on May 11, of M w 5.0, and then the seismicity decreased through to August. On September 11, two strong shocks occurred at 16:31 (M w 5.2) and 16:35 (M w 5.6), followed by two further strong events on September 15, at 03:15 (M w 5.9) and 09:21 (M w 6.0). The seismic activity then decreased until September 16 1977, when a M w 5.3 occurred. The last relevant event of the sequence occurred on 1979 April 18, with M w 4.8.The seismic moment release and its time evolution do not follow a simple occurrence model of mainshock-aftershock sequence, that is, a dominant large earthquake...

Stress field of Italy — Mean stress orientation at different depths and wave-length of the stress pattern

“…This feature is ascribed to the progressive motion of the Adria microplate and its anticlockwise rotation with respect to the Eurasian plate (Anderson and Jackson, 1987;Bressan et al, 1998Bressan et al, , 2003Mantovani et al, 1996). The area is characterized by a compressional stress regime, derived from focal mechanisms (Bressan et al, 2003;Perniola et al, 2004;Slejko et al, 1999;Viganò et al, 2008) and by overthrusts with a southward movement towards the Adria plate. Presently, the pre-alpine Friuli sector is uplifting, while the frontal plane is subsiding (Zanferrari et al, 1982).…”

Static stress changes due to the 1998 and 2004 Krn Mountain (Slovenia) earthquakes and implications for future seismicity

“…3 for the relevant stress change map), but optimally oriented faults to regional compression in this area have been loaded. We also note that this area is located 30 km to the east of the 1976 Friuli events where positive Coulomb stresses of the order of 0.1-0.2 bar have been estimated by Perniola et al (2004). Therefore, the combined, post-2004 load in this area may reach 0.6 bar.…”

“…This region is characterised by moderate rates of seismicity (Poljak et al, 2000;Bressan et al, 2007) with three significant earthquakes recorded in the last 30 years: the 1976 Friuli M w =6.4 event across the Italian border (e.g. Perniola et al, 2004), the 1998 M w =5.6 Krn mountains earthquake Bajc et al, 2001) and the 2004 Krn mountains M w =5.2 event (Cecić et al, 2005;Aoudia et al, 2005). On 12 April 1998, Easter day, a moderate size earthquake (M w =5.6), occurred at the junction between the Alps and the Dinarides mountain chain (NW Slovenia).…”

Site effects and soil-structure resonance study in the Kobarid basin (NW Slovenia) using microtremors

“…In this area the M w =6.4 Friuli earthquake occurred in May 1976. The seismic sequence that followed (Table 1) consisted of 6 events of M w between 5.0 and 6.0 during the same year and one event of M w 5.3 that occurred in September 1997 (Perniola et al, 2004). The intensity of the main shock in Kobarid was VII EMS-98, but the cumulative intensity which includes also damage from aftershocks reached VIII EMS-98.…”

“…The strongest ground motion in the Friuli seismic sequence was recorded for the 15 September 1976 aftershock (M w =6.0, distance=37 km) as 0.138 g peak ground acceleration (Ambraseys et al, 2002). Perniola et al, 2004;Zupančič et al, 2001;Cecić et al, 2006 et al, 1991) was based on surface geological data, since no subsurface geological or geophysical information was available. It was prepared to be used together with the old seismic hazard map of Slovenia for a 500-year return period showing expected intensities on the MSK scale (Ribarič, 1987), in which Kobarid is estimated to have IX MSK intensity.…”