The October 09, 2023, Osku earthquake: seismological and morphotectonic evidence for transtensional deformation along the Azarshahr–Tabriz fault zone of northwestern Iran

The October 09, 2023, M_N 3.9, earthquake occurred near the NE-trending Azarshahr–Tabriz fault zone, in the Osku region of northwestern Iran, typically characterized as a stable continental region. Because of the lack of M_N ≥ 3.0 earthquake along this fault zone during the instrumental time period, there is serious ambiguity regarding its kinematics. The only earthquake contributed to this fault zone is a historical earthquake dated 1641 (Ms ~ 6.8) that its meizoseismal area corresponds to the Azarshahr–Tabriz fault zone. The 2023 earthquake has provided a good opportunity to reconsider the seismotectonic of this fault zone and try to understand its kinematics by analysis of the earthquake source mechanism. Strong ground motions recorded at nearby accelerometry stations show northeastward rupture directivity. The relocated epicenter of the 2023 Osku earthquake is located in a zone of transtensional structure formed within a left-stepping releasing stepover. The focal mechanism solution of the Osku earthquake indicates strike-slip faulting with a normal component, consistent with the existing releasing stepover. Structural elements of the Osku stepover may cause moderate-magnitude earthquakes, similar to the experience of earthquakes on the Hayward–Rodgers Creek fault stepover, in Francisco, California, and the Nima County fault stepover in central Qinghai–Tibet Plateau, China. This is a key issue in determining potential seismic sources for earthquake hazard analyses and risk assessments, especially for the Khosrowshahr–Osku region, with a population of ~ 180,000.