New paleoseismological evidence of Late Quaternary to historical activity along the outer thrust front of the eastern Southern Alps (Polcenigo, NE Italy)

New paleoseismological and geological investigations were carried out at the Pliocene-Quaternary front of the eastern Southern Alps (ESA, NE Italy) to better constraints the activity of thrust systems propagating toward the Friuli Plain. The study focused on the Polcenigo area, located at the western margin of the Carnic Prealps, in the outer sector of the ESA, a S-SE verging active thrust-and-fold belt, in evolution from the Middle Miocene to the Present (Castellarin & Cantelli, 2000). The Carnic Prealps are characterized by a system of arched SW-NE to WSW-ENE trending, S-verging thrusts that accommodate ongoing crustal shortening of about 2-3mm/yr, as estimated by geodetic data (Serpelloni et al., 2005).

The area is characterized by moderate to high seismic hazard and risk, and experienced some historical earthquakes with Mw ≥ 6, including the earthquake of Alpago of 29^th June 1873 (I_max=X MCS e M_w=6.3) and the one of Cansiglio of 18^th October 1936 (I_max=VIII MCS e M_w=6.1) (Rovida et al., 2022). However, the seismogenic sources responsible for these earthquakes remain debated.

Whitin the framework of the third level Caneva-Polcenigo Seismic Microzonation Project, some paleoseismological investigations were performed along the Cansiglio-Col Longone thrust system, whose Late Quaternary tectonic activity is documented by the displacement of thick pre-LGM conglomerates (Poli et al., 2015). A trench excavated at the boundary between the LGM slope deposits of the Coltura fan and the Upper Miocene-Pliocene Molasse of the Col del Cao hill, exposed a high angle tectonic contact between the sub-horizontal Lower Molasse (Cavanella Group, Lower-Middle Miocene) and the sub-vertical Upper Molasse (Conegliano Unit, Pliocene). This structure is consistent with the Col Longone fault, interpreted as the transpressive left-lateral closure of the Caneva-Cansiglio thrust-system (280/65 dipping). The fault affects not only molasse deposits but also upper trench units dated between the XVII and the XIX sec. AD, indicating that the co-seismic effect of an historical earthquake may have been recorded in the trench stratigraphy, coherently with paleoliquefaction evidences documented in the nearby lacustrine succession of the Palù di Livenza basin (Early Holocene) (Monegato et al., 2023).

These results provide new evidence for Late Quaternary to historical activity of the outer ESA thrust front and highlight the seismogenic potential of tectonic structures at the prealpine piedmont plain, with important implications for seismic hazard assessment in the Friuli Plain.

REFERENCE

Castellarin and Cantelli (2000). Neo-Alpine evolution of the Southern Eastern Alps. https://doi.org/10.1016/S0264-3707(99)00036-8

Monegato et al. (2023). LGM glacial and glaciofluvial environments in a tectonically active area (southeastern Alps). https://doi.org/10.3301/GFT.2023.07

Poli et al. (2015). Seismotectonic characterization of the western Carnic pre-alpine area between Caneva and Meduno (Ne Italy, Friuli). DPC-INGV-S1 Project.

Rovida et al. (2022). Catalogo Parametrico dei Terremoti Italiani (CPTI15), versione 4.0. Istituto Nazionale di Geofisica e Vulcanologia (INGV). https://doi.org/10.13127/CPTI/CPTI15.4

Serpelloni et al. (2005). Crustal velocity and strain-rate fields in Italy and surrounding regions: new results from the analysis of permanent and non-permanent GPS networks. https://dx.doi.org/10.1111/j.1365-246X.2005.02618.x