GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 8 SURFACE FAULTING DURING THE 29 DECEMBER 2020 MW 6.4 PETRINJA EARTHQUAKE (CROATIA) P. Boncio 1 , S. Amoroso 1,2 , J. Atanackov 3 , S. Baize 4 , J. Barbača 5 , M. Bavec 3 , N. Belić 5 , L. Benedetti 6 , R. Brajkovič 3 , V. Brčić 5 , M. Budić 5 , M. Caciagli 2 , B. Celarc 3 , R. Civico 2 , F.R. Cinti 2 , P.M. De Martini 2 , R. Filjak 5 , M. Henriquet 6 , B. Kordić 5 , T. Kurečić 5 , F. Iezzi 1 , L. Minarelli 2 , A. Moulin 6 , R. Nappi 2 , A. Novak 3,7 , M. Novak 3 , B. Pace 1 , D. Palenik 5 , D. Pantosti 2 , S. Pucci 2 , T. Ricci 2 , P.J. Rupnik 3 , M. Špelić 5 , A. Testa 1 , S. Valkaniotis 8 , M. Vukovski 5 1 University of Chieti - Pescara, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Italy 3 Geological Survey of Slovenia, Dimičeva 14, 1000 Ljubljana, Slovenia 4 Institut de Radioprotection et Sûreté Nucléaire, Bureau des risques sismiques pour le Sûreté des Installations, 92260 Fontenay-aux-Roses, France 5 Croatian Geological Survey (HGI-CGS), Zagreb, Croatia 6 Aix Marseille Université, CNRS, IRD, Collège de France, CEREGE, Aix-en-Provence, France 7 University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geology, Aškerčeva 12, 1000 Ljubljana, Slovenia 8 Consultant geologist, 42131 Trikala, Greece The 29 December 2020, Mw 6.4 Petrinja earthquake nucleated at a depth of ~10 km in the Sisak-Moslavina County in northern Croatia, ~6 kmWSW of the Petrinja town. Focal mechanisms, aftershocks distribution, and preliminary Sentinel-1 InSAR interferogram suggest that the NW-SE right-lateral strike-slip Pokupsko-Petrinja fault was the source of this event. The Croatian Geological Survey, joined by a European team of earthquake geologists from France, Slovenia and Italy, performed a prompt systematic survey of the area to map the surface effects of the earthquake. The field survey was guided by geological maps, preliminary morpho- tectonic mapping based on 1:5,000 topographical maps and InSAR interferogram. Locally, field mapping was aided by drone survey. We mapped unambiguous evidence of surface faulting at several sites between the area of Međurače to the NW and Hrastovica to the SE, in the central part of the Pokupsko-Petrinja fault, for a total length of ~13 km. Surface faulting was observed on anthropic features (roads, walls) and on Quaternary sediments (soft colluvium and alluvium) and Miocene bedrock (calcarenites). The observed ruptures strike mostly NW-SE, with evidences of strike-slip right-lateral displace- ment and zones of extension (opening) or contraction (small pressure ridges, moletracks) at local bends of the rupture trace. Those ruptures are interpreted as evidences of coseismic surface faulting (primary effects) as they affect the morphology independently from the slope direction. Ground failures due to gravitational sliding and liquefaction occurrences were also observed, mapped and interpreted as secondary effects. SE of Križ, the rupture broke a water pipeline with a right-lateral offset of several centimetres. Measured right-lateral net displacement varies from a few centimetres up to ~38 cm. A portion of the maximummeasured displacement could be due to afterlisp, as it was mapped several days after the main shock. Hybrid surface ruptures (shear plus opening and liquefaction), striking SW-NE, with cm-size left-lateral strike-slip offsets were mapped on the northern side of the Petrinja town, ~3 km NE of the main fault. Overall, the rupture zone appears discontinuous. Several factors might be inferred to explain this pattern such as incomplete mapping of the rupture (the fault crosses some forbidden uncle- ared minefields), inherited structural discontinuities within the Pokupsko-Petrinja fault system, or specific mechanical properties of the Neogene-Quaternary strata. Corresponding author: paolo.boncio@unich.it