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The 29th December 2020, Mw 6.4 Petrinja Earthquake (Croatia): Geological Framework and Observed Coseismic Deformation Features

On 29th December 2020, at 11:19:54 (UTC), the Mw 6.4 earthquake struck Petrinja, about 50 km southeast of the Croatian capital Zagreb. The day before the main event, two foreshocks (Mw 5.2 and 4.7, at 6:28 and 7:49, respectively) were recorded in the area, causing no significant damage. The largest aftershock (Mw 4.9) happened at 17:01 UTC, 6th January 2021, and was recorded on the temporary network installed just a day earlier. The 2020 Petrinja earthquake is one of the largest earthquakes recorded in Croatia since the instrumental recording began. The main event caused seven fatalities, thousands of homeless, and many heavily damaged or destroyed buildings and other objects. Following the Mw 6.4 event, geoscientists conducted geological, geophysical, seismological, geodetic, and hydrogeological investigations. The Geophysical Department of the University of Zagreb has organized a post-event support survey collaborating with the Seismological Research Center of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale. Mitigation encompassed the installation of five seismic stations at the locations surrounding the epicentral area. For precise mapping of the Mw 6.4 Petrinja earthquake surface fault rupture and measure of coseismic deformation, we used the Differential Interferometric Synthetic Aperture Radar (D-InSAR) on the Sentinel-1 images. The coseismic interferograms show a dextral strike-slip deformation pattern with maximum surface displacements of 39 cm. For four months, geological fieldwork included data collecting and mapping the coseismic deformation features in the Petrinja fault zone. Field observations identified numerous geological features, e.g., liquefaction near major streams, gravitational slides, rotated gravestones, and seismically-induced opening of cover-collapse sinkholes in the villages of Mečenčani and Borojevići. Groundwater levels measured immediately after the mainshock was very high, probably significantly contributing to liquefaction and formation of cover-collapse sinkholes. Fieldwork also shows that a total of 136 cover- collapse sinkholes was mapped by remote sensing and field mapping in the 4 km2 area around Mečenčani and Borojevići villages, of which 91 opened due to the Petrinja earthquake series. Other sinkholes were opened before the earthquake since the area is historically known to be naturally prone to collapse of thick soil into underlying heavily karstified limestones. Since dozens exist near houses, geoelectric tomography was conducted to define geological settings up to 40 m depth and delineate safety from high-risk areas. The Petrinja earthquake series is still ongoing, but most earthquakes occurred following the mainshock's first weeks. During the first two months (December 28, 2020–February 22, 2021), more than 4300 earthquakes have been recorded.

Petrinja Earthquake ; Cover-collapse sinkholes ; Coseismic Deformation Features

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