Slip distribution of the 29 December 2020 Mw 6.4 Petrinja earthquake (Croatia) from dense geodetic benchmarks and optical image correlation measurements

The intracontinental Mw 6.4 Petrinja earthquake (Croatia) of December 29^th, 2020, is one of the strongest earthquakes recorded in slowly deforming Eastern Europe. In low strain contexts, sparse seismic monitoring and the rare occurrence of strong earthquakes often prevent the detailed analysis of coseismic rupture. Discontinuous right-lateral coseismic surface rupture and extensive damages reported in the area suggest a relatively shallow seismogenic source for the Petrinja earthquake. Here, we leverage dense near field measurements from optical image correlation and numerous geodetic benchmarks for cadastral and engineering purposes to model the surface and subsurface slip distribution of the Petrinja earthquake. Optical image correlation based on pre-event (7^th December 2017) WorldView and post-event (12^th and 20^th February 2021) Pleiades satellite images is used to refine the trace of the segmented surface rupture and derive coseismic displacements in the very near-field (< 1km from the fault). The ~13 km long imaged fault trace reveals an en échelon geometry in agreement with field observations, and a right-lateral slip reaching up to ~1 m. These results are consistent with the displacement field derived from the dense cadastral GNSS measurements. No additional conjugate fault is visible on the image correlation outcomes. The elastic inversion of these data shows that the coseismic slip was localized on a near-vertical strike-slip fault at shallow depth, < 10 km, and that significant slip reached the surface. It also suggests that the fault bending near Župić interfered with the rupture propagation as the largest slip, > 3 m, is localized on the northern section at depth < 5 km. In conclusion, this study not only provides new constrains on the seismogenic source of the Petrinja earthquake, it also underlines the potential of optical image correlation and cadastral GNSS measurements to retrieve a dense surface displacement field in the epicentral area of moderate intracontinental earthquakes.