The 1976-1977 seismic sequence in Friuli: new seismotectonic aspects

Most of the seismological data recorded during the first year of the seismic sequence that struck Friuli in 1976 have been collected and elaborated with a standard approach. A good quality data set of 291 hypocentral locations and 101 fault plane solutions has been obtained and used for checking the seismogenic interpretation, and some vertical cross-sections representing a quantity, the hypocentral probability, which takes into account the errors in the locations have been constructed. These sections show two high probability vol- umes: one is gently north-dipping and the second is steeply south- dipping. The fault plane solutions have been projected, with the cor- rect space rotation, onto the sections, and their nodal planes are in agreement with both elongations of the hypocentral probability vol- ume. These elaborations put under a different light the traditional seismotectonic interpretation of the seismic sequence. On the basis of new geological data, a 2D structural model for the frontal sector of the Southalpine chain has been constructed. According to the classical tectonic setting suggested for the Eastern Southalpine chain, the cross-section shows a S-vergent thrust-belt arranged in an embricate fan geometry. Nevertheless, at the Gemona latitude, at a depth of 5-8 km, a N-vergent steep backthrusting system becomes active producing a local thickening of rigid carbonatic rocks: part of the cracks of the sequence seems to concentrate here.