GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 90 COSEISMIC SURFACE EFFECTS RELATED WITH THE 29 DECEMBER 2020, MW 6.4, PETRINJA EARTHQUAKE (CENTRAL CROATIA) E. Tondi 1,6 , A. M. Blumetti 2 , M. Čičak 3 , P. Di Manna 2 , P. Galli 4 , C. Invernizzi 1 , S. Mazzoli 1 , L. Piccardi 5 , G. Valentini 1 , E. Vittori 5 , T. Volatili 1 1 School of Science and Technology, Geology Division, University of Camerino, Italy 2 Italian Institute for Environmental Protection and Research - Geological Survey of Italy, Roma, Italy 3 Water Management Department for Middle and Lower Sava Flood Protection Service, Croatian Waters 4 Civil Protection Department, Presidency of the Council of Ministers, Roma, Italy 5 Institute of Geosciences and Earth Resources, National Research Council, Firenze, Italy 6 National Institute of Geophysics and Volcanology, Roma, Italy The Mw 6.4 Petrinja earthquake that struck central Croatia on 29 December 2020, causing 7 casualties and thousands homeless, was one of the strongest seismic events that occurred in Croatia. Here, we report a first-hand description of the coseismic surface effects related with the Petrinja earthquake. Since surface effects may be erased by degradation of fault scarps or by road/infrastructure repair, as well as overprinted by postseismic afterslip, we began surveying soon after the earth- quake. Field surveys in the epicentral area allowed us to observe and map primary coseismic ef- fects, including geometry and kinematics of surface faulting, as well as secondary effects, such as liquefaction, sinkholes and landslides. The description of surface coseismic effects is necessary in earthquake geology as it provides a unique opportunity to observe short-term time scale defor- mation. These observations allow a much more robust interpretation of the long-term time scale geological features for seismic and surface faulting hazard evaluation purposes. We reported a total of 222 georeferenced observation points, including records related to the mapped coseis- mic effects, divided in the following categories: i. shear fractures, ii. open fractures, iii. landslides, iv. sand boils, v. sinkholes (see Tondi et al ., 2021 for further details). The earthquake caused surface faulting defining a ‘conjugate’ fault pattern that comprise two sets of coseismic shears (striking NW–SE and NE-SW) within a ca. 10 kmwide (across strike), NW– SE striking right-lateral strike-slip shear zone (i.e., the Petrinja Fault Zone, PFZ; Fig. 1a). The NW– SE-trending structure, here named Župić Fault, represents a Y shear with right-lateral strike-slip displacement of up to 36 cm, on the other hand, the NE-SW-trending structure, here named Kupa Fault, represents a X shear with left-lateral displacement of up to 10 cm. Both coseismic shears include tension cracks (T fractures), and mole tracks (P shears; Fig. 1b, c). The InSAR imaging of the deformation field is in good agreement with the field observations, both in terms of concen- tration of effects, which are most evident where the InSAR shows the highest displacement, and in terms of type of displacement expected for a dominantly dextral-slip event. Our findings suggest that the coseismic ‘conjugate’ Y and X faulting is mainly controlled by the pre-existing, active PFZ within the framework of the ongoing northward ‘push’ of the Adria Plate along the margins of the Pannonian Basin. The regional geodynamic setting of partitioned transpres- sion results in active thrusting in the outer Dinarides and dominant strike-slip faulting in the interior of the belt, as it occurs in the epicentral area of the 29 December 2020, Mw 6.4, Petrinja earthquake. The mapped pattern of coseismic fault ruptures is relevant for improving the assessment of the seismic and surface faulting hazard of this region, beside the danger related to landslides, liquefaction and sinkholes. More in general, the prompt, accurate mapping of the coseismic ruptures associated with this moderate magnitude earthquake contributes to improve our un- derstanding of earthquake faulting processes and to better forecast the impact of the more en- ergetic earthquakes expected in the Alpine-Dinarides-Albanides orogen, where the knowledge regarding such phenomena is still modest. References