GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 76 THE EFFECTS OF THE 1976 FRIULI EARTHQUAKES IN THE CAVAZZO LAKE BASIN A. Polonia 1 , L.G. Bellucci 1 , C. Bonetti 1,2 , J. Bonetti 1,2 , G. Giorgetti 1 , S. Giuliani 1 , M. López Correa 1,3 , C. Mayr 4,5 , L. Peruzza 6 , G. Stanghellini 1 , L. Gasperini 1 1 CNR-ISMAR, Bologna, Italy 2 Laboratório de Oceanografia Costeira, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil 3 GeoZentrum Nordbayern, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany 4 Institute of Geography, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany 5 Dept. Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Munich, Germany 6 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Sgonico (TS), Italy The Lake of Cavazzo is a natural lacustrine basin in the Friuli-Venezia Giulia region (NE Italy), which occupies a fluvio-glacial suspended valley of the Tagliamento river, at the southern front of the Alpine chain. The lake formed after the melting of major glacial tongues at the end of the Last Glacial Maximum, and has been affected in recent years by natural and anthropogenic events. It is located in the seismically active region of the Southern Alps within the Eurasia-Adria collision zone undergoing about 2 mm yr -1 of crustal shortening (Anderlini et al ., 2020). Predominant tec- tonic structures are seismogenic faults, which are considered sources of strong earthquakes. The lake’s surroundings are characterized by a complex structural setting, with intense deformation and shortening (Zanferrari et al ., 2013), under the interference between the west-verging NW-SE Paleogene External Dinarides thrust belt and Neogene-Quaternary Alpine tectonics. Historical earthquake catalogues include reports of the most destructive events in the region (https://emidius.mi.ingv.it/CPTI15-DBMI15/ ). On May 6, 1976 a M L 6.4 earthquake struck central Friuli region, destroying villages and causing 977 casualties (Slejko et al ., 1999). Focal mecha- nisms show thrust solutions, on planes gently dipping towards the North. The main shock was followed by a renewal of the sequence in September (the strongest ML=5.8-6.1 on September 15, 1976). The main 1976 shocks are proposed to be associated with blind thrusts buried below the Friuli plain (Peruzza et al ., 2002; Poli et al ., 2002; Cheloni et al ., 2012). Seismicity was located eastwards of the Tagliamento river valley (Figure 1), and it migrated towards the NW in Septem- ber (Aoudia, 2000; Pondrelli et al ., 2001). The final phase of seismic activity started on September 16, 1977, with a moderate shock (M L = 5.2) and its aftershocks located SW of the Cavazzo lake. The most relevant surface effects observed after the May 1976 earthquakes were located at the same latitude of the main event epicenter (e.g. Martinis and Cavallin, 1978) and the lake of Cavazzo might represent a key area for investigating the effects of co-seismic deformation and resedimentation processes since the earthquakes caused rock falls and landslides in the lake’s valley. Lacustrine paleoseismology is a well-established methodology in determining the fre- quency of large-magnitude earthquakes (Moernaut, 2020, and references therein). The potential contribution of lacustrine seismo-stratigraphy and paleoseismology, successfully applied in the Southern Alps, has never been explored in the Friuli region. In this work, based on a densely- spaced grid of high-resolution seismic reflection profiles complemented by a multiproxy analysis of sediment cores, we analysed the uppermost lake stratigraphy, highlighting major changes oc- curring at a decadal scale. Our main purpose is to verify whether and how the lake sediments record the effects of multiple natural impacts including the 1976-77 Friuli earthquake sequence. The results of our analysis suggest varying environmental conditions of the lake after the 1950s indicated by changing sediment mineralogy, increased deposition of allochthonous clas- tic sediments, and recurrent episodes of anoxic conditions at the lake’s floor, likely triggered by hyperpycnal flows. The effects of the 1976-77 seismic sequence are recorded in the lake as resedimented levels, likely due to shaking, in situ deformation, landslides, and turbidity currents (Polonia et al ., 2021a). Anomalous sedimentary processes were, in fact, capable of producing