The 2020 Qotur earthquake doublet and its implication for the geodynamics of Northwestern Iran

The 23 February 2020 Qotur earthquake doublet (5.7 and 5.9 M_w) occurred near the Iran-Turkey border. The doublet ruptured along the northern segment of the Bashkale fault system, which forms the northernmost end of the Zagros' Main Recent Fault. To investigate the tectonics of the region, we combined geological fault mapping, precise location of the seismic cluster, moment tensors of the mainshocks and 28 larger fore- and aftershocks, InSAR source modeling, and inversion of source parameters for the regional stress field. We show that both mainshocks happened on NE-SW trending left-lateral strike-slip faults belonging to the Bashkale fault system. However, both NE-SW trending left-lateral and NW-SE trending right-lateral strike-slip faults were activated by the earthquake cluster. The InSAR images imply no surface rupture for the doublet event in agreement with the field observations. The first mainshock was deeper (centroid: 9 km) than the second mainshock (5 km). We model the InSAR data of the second mainshock as rupture along a 7-km-wide and 5-km-long fault with maximum displacement of 1.2 m at ∼3 km depth. The spatial distribution of aftershocks and the damaged region implies southwest directivity for the second mainshock. Stress inversion of the moment tensors indicates a transtensional regime with an NNW-SSE direction of maximum horizontal stress. This agrees with the stress regime deduced from the inversion of geologically measured fault planes and GPS vectors, and the focal mechanism of the 1930 Salmas earthquake. Our results show that strain accommodation at the northern end of the right-lateral strike-slip Main Recent Fault of the Zagros is distributed across a complex network of immature conjugate right and left-lateral strike-slip and normal faults.