Seismotectonic re-evaluation of the 1976 Friuli, Italy, seismic sequence

The 1976 Friuli seismic sequence is known for its complexity, with several unusually large aftershocks occurring in the epicentral area. The source region, where the northeastern part of the Alpine chain meets the northern Dinaric chain, is characterized by a complex compressional tectonic regime. Previous studies have not clearly identified which of the two main tectonic systems was activated, in part due to the limited precision of derived earthquake parameters, such as hypocentral coordinates and focal mechanisms. We review the locations for the 10 largest events of the sequence, including the September 16, 1977 and April 18, 1979 earthquakes and we compute their centroid moment tensors. Source parameters are calculated using intermediate period surface waves and the modification of the Harvard centroid-moment tensor algorithm proposed by Arvidsson and Ekstrom (1998). A summary of all available geological, geodetic and seismological data show that most of the earthquakes may be associated with the Periadriatic overthrust and other related thrust faults. Based on their locations and focal mechanisms only two of the early aftershocks (May 7, 1976 and May 9, 1976) appear instead to have occurred on Dinaric structures.