Fault Slip Distribution and Inhomogeneous Frictional Properties in the Southeastern Zagros Mountains of the 2022 Iran doublet Earthquakes Inferred from Bayesian Inference and InSAR observations

Abstract

On July 1, 2022, a doublet of earthquakes, with a magnitude of Mw6.0 and a Mw5.7 aftershock between them, occurred within a two-hour period in the southeastern part of the Zagros Mountains near the Persian Gulf in Iran. This doublet earthquake event provides a unique opportunity to study the geometric properties of geological faults and the frictional attributes of rocks in the southeastern of the Zagros Mountains, particularly in the vicinity of the Hormoz salt layer. Here, we acquired co-seismic and post-seismic InSAR ascending and descending observations to simultaneously determine fault geometry and slip distribution models for the doublet earthquakes based on Bayesian inference. The inversion results reveal that the doublet earthquakes occurred on two distinct faults with similar strike (101.93°, 93.7°) but notable differences in dip (56.2°, 31.3°), and the slip distribution of the mainshock 2 is shallower and more westward compared to the mainshock 1. Moreover, the reliability of the fault geometry and slip distribution was confirmed through detailed discussions on the distributions of post-seismic kinematic afterslip, the relocated aftershocks beyond five months after the mainshocks, and the changes in positive Coulomb stress triggered by co-seismic events. Additionally, our post-seismic deformation modeling elucidated that post-seismic deformation is predominantly driven by stress induced by co-seismic event, accompanied by the release of both afterslip and aftershocks. Afterslip is distributed both up- and down-dip of the coseismic region on the two faults, with the maximum afterslip concentrated in the shallow portions, reaching approximately 0.45 m. By comparing the temporal evolution characteristics of stress-driven afterslip distributions with those of kinematic afterslip, we observed significant inconsistencies in the frictional properties within the southeastern Zagros Mountains, particularly between the Hormoz salt layer and its upper region. Specifically, above the Hormoz salt layer, the friction is stronger, and the relaxation time of afterslip is shorter. Finally, we also discussed the triggering potential of the mainshock 1 and the Mw5.7 aftershocks on mainshock 2, and from the perspective of Coulomb stress transfer, we found that mainshock 1 and Mw5.7 aftershocks may have triggering effects on mainshock 2.