Unraveling the Kinematics and Morphotectonics of the Petrinja Fault (Croatia), Source of the 2020 M 6.4 Earthquake

The 2020 Mw 6.4 Petrinja earthquake in central Croatia is one of the European strongest continental earthquakes in recent decades. This event shed light on the poorly investigated Petrinja-Pokupsko Fault (PPKF) zone, a right-lateral fault system accommodating a fraction of the shortening between the Adria and European plates. Through field observations and high-resolution Lidar-derived digital elevation models of the central section of the PPKF, we precisely mapped the fault trace, revealing a discontinuous geometry with a main fault strand and left-stepping segments in agreement with the position of the 2020 surface ruptures. To the north, the accumulated relief from this transpressive fault system decreases, and the fault trace becomes less distinct, suggesting a northward propagation of the deformation. Cumulative offsets of 4 to 24 meters along the main fault strands in the central and southern sections of the fault attest to its Quaternary activity. Dating results suggest offset markers at the Marine Isotopic Stage 4 (MIS 4), the Late Glacial Maximum (LGM) or the Early Holocene periods, allowing for the first direct estimation of the fault slip rates. Preliminary estimates indicate that, assuming a common MIS 4 or LGM age for the investigated markers, fault slip rates range from 0.2 to 0.7 mm/yr and 0.7 to 1.6 mm/yr, respectively. In contrast, assigning Early Holocene ages would imply much higher - and likely unrealistic - slip rates of 1.6 to 3.9 mm/yr. Although the estimated loading rates vary greatly and depend on strong assumptions regarding the age of the markers abandonment, our results suggest a minimum fault slip-rate of 0.2 mm/yr for local seismic hazard assessments.