Comparison between the coseismic surface displacement during the 29 December 2020 Mw 6.4 Petrinja earthquake (Croatia) from optical image correlation and long-term geomorphological observations of cumulative displacements

<p>The Petrinja-Pokupsko fault-system is a NW-SE right-lateral fault system that ruptured during the 29 December 2020 Mw 6.4 earthquake (~40km south-east of Zagreb, Croatia). Field analysis revealed opening of cracks and offsets of several centimeters (3 to 40 cm) along a ~20 km long fault zone extending from the Kupa river (in the northwest) to the Petrinj&#269;ica river (in the southeast). Optical image correlation based on WorldView satellite images has been used to document the first-order near-field rupture signal. The pre-event image was acquired on 7th December 2017, and the post-event image on 15th January 2021. The first results indicate a right-lateral displacement of &#8776;75 cm with a small (<10 cm) extensional dip-slip component localized on the Petrinja fault. Using 1:5,000 topographic maps, a WorldView-derived DEM (1 m), and field observations, we identified and quantified cumulative dextral offsets along the central and southern section of the fault (south of &#381;upi&#263;). Right-lateral offsets range from 5 to 200 m near Kri&#382; and Cepeli&#353; (central sector). Diverted streams also extend southeast of the Petrinj&#269;ica river, where no surface ruptures have currently been reported to date. To the northwest, perched valleys, wind gaps, and karst features all testify to ongoing uplift across NW-SE-trending anticlines. It is unclear if the primary component of faulting changes from strike-slip (in the SE) to reverse (in the NW), or if these folds merely record a transpressive component across the fault. The activity of this fault system is poorly known. The region experienced a magnitude Mw 5.8 in 1909, ~30 km northwest of Petrinja, which may have been associated with the Petrinja-Pokupsko fault system. The recent 29 December 2020 earthquake confirms the seismic potential of this fault system to generate Mw>6 earthquakes. Since the fault extends farther NW and SE, from the Vukomeri&#269;ke Gorice hills to Mount Kozara (Bosnia), for a total length of about 100 km, it could generate potentially larger events. It is also noteworthy that the 2020 Petrinja event occurred only 9 months after the Zagreb March 2020 (Mw 5.3) earthquake. This event occurred on an ENE-WSW-trending thrust fault, broadly orthogonal to the right-lateral Petrinja-Pokupsko fault system, ~45 km north of Petrinja, and raises the prospect of potential interplay between strike-slip and thrust faults in moderate strain-rate intra-plate settings. To address this problem, future works will aim at constraining the geometry of this fault network and its seismogenic potential.</p>