GNGTS 2022 - Atti del 40° Convegno Nazionale

30 GNGTS 2022 Sessione 1.1 ASSESSING THE CAPABILITY OF THE PESCOPAGANO FAULT, PART OF THE ANTITHETIC FAULT OF THE 1980, Mw 6.9 IRPINIA EARTHQUAKE (SOUTHERN ITALY) L. Ferranti 1 , F. Iezzi 1 , A. Bacchiani 1 , S. Buoninfante 1 , F. Menna 1 , M. Sposato 1 , V. Di Fiore 2 , D. Bellini 3 , A. Citterio 3 , R. Calabrò 3 , C. Pasqua 4 1 DiSTAR - Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse Università di Napoli “Federico II” 2 CNR-ISPC - Istituto di Scienze del Patrimonio Culturale 3 Edison SpA 4 FrosioNext The ~5 km long Pescopagano fault (PF) is part of the ~22 km long, NW-SE striking and SW-dipping Conza della Campania fault (CCF), which lies above the SW-dipping “antithetic fault” that failed 40 sec after and ~10 km NE of the mainshock rupture of the 1980 Mw 6.9 Campania-Basilicata earthquake (Bernard and Zollo, 1989; Pingue and De Natale, 1993; Pantosti and Valensise, 1990). According to seismicity data, the antithetic fault is considered a blind atructure (with possible reactivation of an inherited thrust fault), with un upper tip at 1.5-1.8 km depth (Amato and Selvaggi, 1993; Amoruso et al. , 2005). Alternatively, the 1980 antithetic fault was considered to have caused surface ruptures on some strands of the CCF, including the PF (Blumetti et al. , 2002; Bello et al. , 2020). The PF is comprised in the ITHACA Catalogue of the Active and Capable Faults (http://sgi2.isprambiente.it/ithacaweb/viewer/ ) and is assigned a mainly pre-historic (3-9 ka) activity, slip rate of 0.3 mm/yr and an association with the 1980 event. The latter scenario has major implications for the local seismic hazard and territorial management, but robust kinematic and paleoseismological constraints are lacking. We report here the results emerging from a structural and paleoseismological study carried for the purpose of a major infrastructure planning and aimed at assessing the capability and, in case of positive answer, the magnitude of past surface ruptures. The WNW-ESE striking, SSW-dipping PF cuts across Upper Cretaceous-Oligocene rocks of the Flysch Rosso formation, part of the Lagonegro basin succession, which forms one of the main litho-tectonic units of the Southern Apennines fold and thrust belt. The fault is divided in three strands. We studied in detail the central (Croce dello Staccato, CS) and eastern (Madonna di M. Mauro, MM) strands, of broadly similar length (~1 km). These two strands juxtapose massive to bedded marly calcilutites and fine-medium calciturbidites in the footwall (member FYRc of the Flysch Rosso Fm) against thinly bedded reddish-greenish claystones and cherty argillites in the hanging-wall (younger member FYRa). These relations indicate the PF is predominantly a normal fault. Borehole data suggest that the PF has a 60°-70° dip and a ~100 m max throw. The fault scarp is rather degraded and only at one place along the eastern strand a limited (0.5 m high x 5 m wide) hanging-wall synthetic fault plane exposure is present. Altered tectogrooves with a ~90° pitch were observed on the 55° southward dipping fault plane, providing a N-S extension axis. The finding of a fresh ~8 cm high ribbon at its base, however, induced further investigations to prove or dismiss involvement of the fault during the 1980 or preceding earthquakes. High-resolution seismic reflection profiles and seismic tomographies provided images down to ~20 m depth that revealed the existence of three seismic units in the Vp model. These units have locally sharp lateral contacts, suggesting the possible presence of faults. Notwithstanding, three paleoseismological trenches co-linear with the seismic profiles documented that the fault scarp is sealed by several sedimentary units. The uppermost unit (U1) is composed by recent soil and colluvial deposits and was dated back to ~6 ka BP. Unit U2 includes laterally variable thickness of alluvial sands and gravels, which are often channelized in the upper part, and passes downward to alluvial to slope gravel wedges (U3). Units U2 and U3 rest with sharp erosive contacts on at least two generations of marshy clays (U4) developed at the expense of the FYR-a hanging-wall bedrock. Although radiometric age constraints are