The seismic sequence of Friuli 76: 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 volumes: one is gently north-dipping and the second is steeply southdipping. The fault plane solutions have been projected, with the correct space rotation, onto the sections, and their nodal planes are in agreement with both elongations of the hypocentral probability volume. 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.