Intraplate Seismicity Along Inherited Faults: Insights from the 2020 Earthquake Sequence in SW Romania

Abstract Intraplate shear can localize along inherited faults at major crustal boundaries, even long after orogenic convergence has ceased. A typical case is the bending SW segment of the South Carpathians located in the Danubian area. This study investigates the June 2020 earthquake sequence near Orşova city, which provides a well-recorded example of deformation at the contact between the rigid Moesian Platform and the Miocene-aged South Carpathians. Using an automated machine-learning workflow for phase picking and association, we identified three previously unreported events and relocated the sequence, revealing a tightly clustered hypocenter distribution between 17 and 20 km depth along the Cerna-Jiu Fault (CJF). Moment tensor inversion for the Mw 4.1 mainshock confirms strike-slip faulting on a NE–SW oriented structure, consistent with regional shear strain rates. We applied empirical Green’s function and spectral ratio techniques to estimate source parameters for the sequence. The scaling results for the study area indicate relatively high stress drop values for the larger shocks (Mw 3 – 4) and a deviation from self-similar scaling, suggesting the engagement of high-strength fault patches during the rupture process. Furthermore, azimuthal variations in source time functions (STF) and the northeastward migration of aftershocks are consistent with a predominantly unilateral rupture propagating toward the NE, although other phenomena that control aftershocks migration (e.g., fluid migration) could be considered as well. Together, these observations demonstrate that even low-strain intraplate regions with low background seismicity can generate abrupt, high-stress-drop earthquakes when strain localizes along inherited faults at the boundaries of mechanically strong lithospheric blocks, posing a distinct challenge for local hazard assessment.