2017 M w 6 Sefid Sang earthquake and its implication for the geodynamics of NE Iran.

The 2017 April 5 M w 6 Sefid Sang earthquake occurred east-northeast of the town of Fariman between the Binalud Mountains, at the northeast boundary of the Arabia-Eurasia collision zone, and the Doruneh Fault which defines the northern limit of the Lut Block. The pattern of present-day deformation in this area is not well understood. We combine results from studies of earthquake location, focal mechanism solutions and Interferometric Synthetic Aperture Radar (InSAR) to investigate the coseismic displacements. The active deformation of the source region is investigated through a synthesis of the structural geology, complimented by geological fieldwork. Our results reveal that the mainshock nucleated at a depth of ∼15 km on a reverse fault plane dipping north-northeast, with a component of dextral faulting. Rupture in the mainshock was almost unidirectional towards the southeast. The temporal growth of the 24-km long aftershock cloud reflects migration of seismicity towards the southeast. Analysis of ascending and descending interferograms of TOPS S1 SAR images show a maximum of 9 and 11 cm of surface deformation, respectively. A rupture model based on the inversion of the InSAR interferograms features a maximum slip of 83 cm on the NNE-dipping fault plane. Inversion of fault-slip data from earthquake focal mechanisms and geological field measurements indicates a contemporary regional transpressional tectonic regime characterized by a NE-trending maximum compression (mean direction of N020°E for seismological slip data and N029°E for geological fault kinematics). This implies a transpressional active kinematics according to which the Central Iran–Eurasia convergence is taken up by reverse dextral faulting on several NW-striking faults such as the eastern termination of the Doruneh fault and the southeast termination of the Chakaneh-Neyshabur fault systems. Our seismological, geodetic and geological results consistently indicate accommodation of active deformation by oblique-slip reverse faulting that is distributed in the region connecting the distinct block-bounding fault systems of northeast and eastern Iran. We propose a transpressional relay zone structural model to explain the nature, kinematics and structural pattern of the boundary of the collision zone between the Doruneh Fault and the Binalud Mountains. Our observations and results could shed light on structural aspects of other intracontinental collision zone boundaries, where deformation is taken up by discontinuous continental block boundaries. [ABSTRACT FROM AUTHOR]