GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 The Role of Structural Complexity During the 1976 Friuli Seismicity (Eastern Southern Alps, NE-Italy) G. Patricelli 1 , M.E. Poli 1 , D. Cheloni 2 1 Dipartimento di Scienze Agroalimentari, Ambientali e Animali, Università degli Studi di Udine, Udine, Italy 2 Osservatorio Nazionale Terremoti, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy By integrating geological and seismological data we reconstructed the seismotectonic model of the northeastern Friuli Plain and Julian prealpine border region (NE Italy), which represents the epicentral area of the 1976 Friuli earthquake (Mw 6.5). Located in the northeasternmost portion of the Adria microplate, the study region represents the junction area between the SSE-verging polyphase thrust-front of eastern Southern Alps and the NW-SE trending dextral strike-slip fault systems of western Slovenia – eastern Friuli domain (Fig. 1). At present this area is subjected to a compressive regime with an about N-S oriented sigma1 and velocity vectors of the order of 2-3 mm/yr (Serpelloni et al., 2016). The seismicity of the area includes many destructive seismic events such as the 1511 (Mw 6.3) earthquake, which affected the Italian-Slovenian border region, and the 1976 May (Mw 6.5) and September (Mw 6.1) Friuli earthquakes (Rovida et al., 2022). With the aim to investigate the relationship between reverse and strike-slip faults and how their interaction affects seismicity, we elaborated the 3D structural model of the study area, deep up to 5 km, starting from the geological cross sections derived from the interpretation of ENI seismic lines. In a second step we integrated the elaborated model with the hypocentral distribution of the 1976-1977 seismicity (Rebez et al., 2018) in order to define the deep geometry of the tectonic structures and to investigate their involvement during the 1976 strongest events, in terms of activation and/or interaction. In addition, the comparison of the seismic volume activated during the 1976-1977 seismicity with the geological volumes reconstructed from the fault planes geometry represented a first attempt to test the volumetric approach for the seismogenic potential estimation proposed by Petricca et al. (2019). The elaborated seismotectonic model suggests for the 1976 earthquakes the activation of a seismogenic source model (corresponding to the Susans-Tricesimo Source) characterized by structural complexity derived from the polyphasic tectonic evolution. The interplay between the inherited thrusts and the NW-ward propagating dextral transpressive fault systems is highlighted, suggesting that the high angle tectonic structures controlled the stress release during the strongest