The 2016 Mw 6.5 Nura earthquake in the Trans Alai range, northern Pamir: Possible rupture on a back-thrust fault constrained by Sentinel-1A radar interferometry

Abstract

On June 26, 2016 an Mw 6.5 earthquake occurred in the Trans-Alai Range, about 37 km west of a 2008 Mw 6.7 earthquake, both near the town of Nura, China. We use Interferometric Synthetic Aperture Radar (InSAR) images from Sentinel-1A to image coseismic surface displacements from the most recent Nura earthquake. We invert the interferograms for both fault geometry and slip distribution. We find that the InSAR data can be described by either a steeply north-dipping fault or shallowly south-dipping fault, although only when slip on the south-dipping fault is constrained to be blind. The InSAR misfit for the north-dipping fault plane is slightly lower, and the north-dipping fault better describes the gradient of deformation in the near-field. We prefer the north-dipping fault geometry (~67° dip and 266° strike) as the source for the 2016 Nura earthquake, not only due to the better fit to the InSAR, but the resulting coseismic slip model is more consistent with the focal mechanism solutions and regional seismogenic thickness; however, it is important to reiterate that the InSAR data cannot uniquely determine which model is the definitive representation of the Nura earthquake. The maximum inferred coseismic slip is about 0.75 m at ~14 km depth beneath the Trans Alai range, with the largest slip in the depth range of about 8–20 km. Slip does not reach the surface, and is dominated by thrust displacement with a right-lateral strike-slip component. The geodetic moment of our preferred slip model is 6.61 × 1018 N·m, equivalent to Mw 6.5. The surface projection of our preferred fault is to the south of the Pamir Frontal Thrust (PFT), and we argue it is a back-thrust of the listric PFT, suggesting that the Trans Alai range is a ‘pop-up’ structure.