New insights into the 2017 Sefidsang earthquake by Coulomb stress change pattern and aftershock distributions: implication for active tectonics of NE Iran

The April 5th 2017 Mw 6.15 Sefidsang earthquake occurred east-northeast of the town of Fariman near the Cimmerian arc-related Fariman complex. The stress distribution was estimated by seismic parameters and Coulomb stress distribution in the source region. Inferred Coulomb stress field and general pattern of aftershocks distribution revealed that the Sefidsang earthquake occurred on a northeast-dipping listric fault with dextral reverse movement. Proximity of the Sefidsang sequence to Fariman complex proposes the reactivation of the pre-existing Cimmerian arc-related faults in the present-day stress field. The crustal-penetrating low-angle inverted faults coupled with Mesozoic mafic–ultramafic magmatism can shed light on the structural aspects of the region. The kinematics was also investigated by GPS velocity fields and morphotectonic features. The counterclockwise block rotation under the left-lateral regional shear between the Doruneh fault system and the North Kopeh Dagh fault system and NE-oriented coeval shortening led to the formation of rhombic structures. Cimmerian-related basement faults in NE Iran confined the expansion of rhombic cells and sense of block rotation. The results of this study improved our understanding about kinematics of active deformation in NE Iran and had important implications for seismic hazard assessment of the region and potential future failure area.