Structural Complexity and Seismogenesis: The Role of the Transpressive Structures in the 1976 Friuli Earthquakes (Eastern Southern Alps, NE Italy)

Abstract

We reconstructed the seismotectonic setting of the area comprising the northeastern Friuli Plain and the Julian pre-Alpine border (NE Italy) by integrating geological and seismological data. The study area represents the junction between the SSE-verging polyphase thrust-front of the south-Alpine Chain and the NW–SE-trending strike-slip faults of the eastern Friuli–western Slovenia domain. Following a multidisciplinary approach, the 3D geometry of the Susans–Tricesimo thrust system was reconstructed through the elaboration of four geological cross sections derived from the interpretation of ENI industrial seismic lines. In a second step, the seismogenic volume of the central-eastern Friuli area was investigated through hypocentral distribution analysis: the seismic events of the latest 50 years (1976–1977 and 1978–2019 time intervals) were plotted on four NE-SW-oriented seriated sections together with the fault plane’s geometry. Through this procedure, we were able to investigate the relationship between the NW-SE-striking high-angle faults, which characterize the northern Julian pre-Alps, and the WSW-verging medium-angle reverse fronts located at the piedmont of the Friuli plain, which experienced NW-SE- to NNW-SSE-oriented compression starting at least from the Pliocene. In detail, we examined the involvement of these structures during the seismic sequences of May and September 1976, in terms of activation and/or interaction. The resulting seismotectonic model highlights the interplay between transpressive/strike-slip and reverse planes. In particular, this study suggests that Predjama and Maniaglia transpressive faults strongly control the stress release and likely played a fundamental role both during the 6 May (Mw 6.5) and 15 September (Mw 6.0) Friuli earthquakes.

LGM glacial and glaciofluvial environments in a tectonically active area (southeastern Alps)

“…At present, Venetian and Carnic Prealpine areas are characterised by a dominant dip-slip motion accommodated by the SE verging, NE-SW striking fold and thrust belt (Castellarin and Cantelli, 2000). At the transition between these two structural domains, the Alpine and Prealpine Julian regions define a compressional domain with a strong oblique component (Falcucci et al, 2018;Poli and Zanferrari, 2018;Patricelli and Poli, 2022). After a long history of rifting and subsidence during the Mesozoic (Masetti et al, 2012), ESA experienced compressive tectonics during the latest Mesozoic and Cenozoic, when they were affected by two main tectono-sedimentary phases: the Late Cretaceous-late Eocene (Mesoalpine) event and the Middle Miocene-Quaternary (Neoalpine) event (Doglioni and Bosellini, 1987;Doglioni, 1992).…”

Present-Day Surface Deformation in North-East Italy Using InSAR and GNSS Data

“…Together with the distributed seismicity, these signals are consistent with the activity of the tectonic structures in the area, which was also hit by the destructive seismic sequence in 1976-1977 (mainshock Mw 6.4) [45][46][47]85]. According to recent studies, the event was attributed to the activation of the Susans-Tricesimo thrust with the potential contribution of transpressive faults (i.e., Predjama fault), which played an essential role during the 1976 activity (characterized by the two events of 6 May 1976; Mw 6.4 and 15 September 1976; Mw 6.0) [87,120]. The evidence suggests a complex interaction between the two tectonic systems, indicating the need to further investigate regions characterized by a high seismogenic potential [12,15].…”

Surface deformation analysis in Northeast Italy by using PS-InSAR and GNSS data 

“…According to recent studies, the event was attributed to the activation of the Susan-Tricesimo thrust with the potential contribution of the transpressive faults (i.e., Predjama fault), which played an essential role during the mainshock (6th May 1976; Mw 6.5) and the foreshock (15th September 1976; Mw 6.0) (Poli and Zanferrari, 2018;Patricelli et al, 2022). The evidence suggests a complex interaction between the two tectonic systems, indicating the need for further investigations in these regions characterized by a high seismogenic potential (Cheloni et al, 2014;Serpelloni et al, 2016).…”