Seismic Disturbances and Geodetic Effects: Unraveling the Impact of the Mw = 6.2 Petrinja Earthquake on the Croatian Positioning System CROPOS

A destructive Mw = 6.2 earthquake struck NW Croatia on 29 December 2020, ranking among the most hazardous events in the country in the last century. This study assesses the impact of the earthquake on the permanent geodetic stations of the Croatian national positioning system. Using a novel approach, we combined positioning station data with satellite radar data for the first time in a geodynamic study across Croatia. The analysis involved collecting daily solutions data from 11 nearby network stations as well as multi-temporal satellite images before and after the earthquake. The results reveal a significant co-seismic shift, with the Sisak reference station experiencing the largest displacement of 5 cm to the southeast. Stations up to 100 km from the epicenter exhibited horizontal shifts between 1 and 2.5 cm and vertical shifts between 2 and 4 cm. Satellite images confirmed the co-seismic effects on the permanent stations. The study establishes a strong correlation between geodetic and geological findings, suggesting that vertical displacements result from superficial deposit consolidation in the river plains due to the earthquake shaking, while positional displacement reflects tectonic block movements along the activated strike-slip fault system. The late 2020 earthquake may have further relaxed the accumulated strain on the Mt. Medvednica thrust fault system, which had ruptured in early 2020 with a lower magnitude than expected.