Unraveling the complex rupture of the 2020 Mw 6.4 Petrinja Earthquake (Croatia): insights from joint inversion of geodetic benchmarks, InSAR and optical correlation data.

The Mw 6.4 Petrinja earthquake, which struck Croatia on December 29, 2020, is among the most powerful earthquakes recorded in the slowly deforming region of Eastern Europe. In areas of low tectonic strain, limited seismic monitoring and the sporadic occurrence of strong earthquakes often hinder detailed analyses of coseismic ruptures preventing the scientific community to fully understand the processes governing these moderate and destructive events. In particular, it's not clear whether those continental earthquakes follow the same scaling laws than the ones occurring on mature faults, hence the need to better understand the source of these events.

Seismic source inversions and InSAR-based models from multiple studies indicate that the coseismic rupture of the Petrinja earthquake results from a single patch of right-lateral slip. On the other hand, we showed in a previous study that discontinuous surface ruptures and slip inversions of near-field geodetic benchmarks suggest rather along-strike complexities of the fault slip (Henriquet et al., 2023). To better constrain the slip distribution of the Petrinja earthquake, we leverage dense near field measurements from optical image correlation and numerous geodetic benchmarks together with InSAR data. We first assess the sensitivity of the model to each dataset to show that slip patterns are overall consistent to first order, although significant differences appear along dip, mainly depending on the distance between the fault trace and the considered measurements. We then jointly invert all the displacement data to provide a robust solution of the coseismic slip. The results confirm that the coseismic slip occurred on a near-vertical strike-slip fault at shallow depths, less than 10 km, with significant slip extending to the surface locally. It also indicates that fault bending near Kriz influenced the rupture propagation, as the largest slip, exceeding 3 meters, was concentrated in the northwestern section at depths of less than 5 km and that a deeper slip of smaller amplitude is required by the data to the southeast. This along-strike variation in slip depth and amplitude also correlates with changes in aftershocks rate and average depth (Herak et al., 2023), which confirms that the Petrinja fault is not a straight, mature fault system. This complexity in the slip distribution is in agreement with the large stress drop values obtained by seismological studies (Lončar et al., 2024). In conclusion, this study offers new insights into the seismogenic source of the Petrinja earthquake and highlights the value of combined displacement fields in improving source models of moderate intracontinental earthquakes.