New seismotectonic evidence from the analysis of the 1976-1977 and 1977-1999 seismicity in Friuli (NE Italy)

The seismicity collected during the sequence that struck Friuli in 1976 and 1977 has been compared with that collected by the regional seismometric network in the following years (1977-1999). A standard approach was used to elaborate the seismological data recorded during the first year of the seismic sequence. A good quality data set of 479 hypocentral locations and 123 fault-plane solutions was obtained and used to check the seismogenic interpretation. Some vertical cross-sections representing the hypocentral probability, that takes into account the errors in the locations, have been constructed. These sections show two high-probability volumes: one is gently north-dipping and the second is steeply south-dipping. The fault-plane solutions have been projected onto the sections, and their nodal planes are in agreement with both elongations of the hypocentral probability volume. A similar elaboration was also carried out for the subsequent seismicity confirming the previous pieces of evidence. Thus, an improvement was obtained for the seismotectonic interpretation of the 1976-1977 seismic sequence as well as that of the present Friuli seismicity. On the basis of new geological data, a 2D structural model for the Southalpine chain has been constructed. According to the classical tectonic setting suggested for the Eastern Southalpine chain, the cross-section shows a south-verging thrust-belt arranged in an embricate fan geometry. At the Gemona latitude, at a depth of 5-8 km, a north-verging steep back-thrusting system becomes active: there seems to be a concentration of cracks in the carbonatic rocks. On the contrary, the 1977-1999 seismicity appears distributed in a larger crustal volume, with an evident westward shift of the maximum seismic activity. The relatively low level of the HP in the 1976 source zone is interpreted as a temporary and partial equilibrium reached by this crustal volume after the main ruptures. The stress redistribution caused by the 1976 sequence possibly produced a transfer of the deformation to the western sector.