Large-scale crustal deformation and strain rate distribution along the central-eastern Altyn Tagh fault (NW Tibet) from Sentinel-1 InSAR and GNSS data

The left-lateral Altyn Tagh Fault (ATF) is one of the longest active strike-slip faults in the world. Investigating the present-day state of the ATF is critical for our broader understanding of the India-Asia collision zone and the current motion of the Tibetan Plateau. Previous geodetic studies of the ATF using InSAR focused on relatively small areas, which is insufficient for a whole-fault understanding, but with the launch of the Sentinel-1 SAR constellation and the development of InSAR techniques, we can measure the crustal deformation and stress fields associated with interseismic motion along the fault more systematically as Sentinel-1 has provided high spatial coverage, better spatial resolution compared to GNSS, and shorter repeat times compared to previous SAR satellites. The large spatial coverage from such research could not only allow a better understanding of along-strike variations of fault slip rate and locking depth, but provide an opportunity to see how fault bends influence the deformation and strain fields, both of which are important for synthetic evaluation of future seismic risk along the fault. In this research, we use interferograms, which are produced by LiCSAR processing system, on 7 ascending tracks and 6 descending tracks to map surface velocities for a total area of ~ 600,000 km² (~ 1,300 km × 450 km) around the central and eastern segment of ATF. Each track uses nearly 180 epochs between October 2014 and July 2022. To reduce the impact of phase biases and nontectonic seasonal signals, we combine both short temporal (< 4 months) and 1-year to 7-year long summer-to-summer baseline interferograms in the network, which generates an average of nearly 2000 interferograms in each LiCSAR frame (a track includes 1 or 2 frames). We use the Generic Atmospheric Correction Online Service (GACOS) to reduce the tropospheric delay in the unwrapped phase. Time-series analysis was applied using LiCSBAS. We estimate 83 3D GPS velocities using the data measured during 1998-2021 from the Crustal Movement Observation Network of China-I/II and then solve for the best-fit model of surface velocities and strain rates for the central-eastern Altyn Tagh fault zone based on both InSAR and GNSS velocities. Our results suggests that deformation and strain in the study area is concentrated along the ATF and show an along-strike variation from west to east. Using a screw dislocation model, we constrain best fit values for the slip-rate, locking depth, creep rate, and fault dip, for 12 fault-perpendicular velocity profiles along the length of the ATF using a Bayesian inversion and the Markov chain Monte Carlo (MCMC) sampler. Our results provide an important constraint on the present-day motion and structure of the Eastern and Central ATF. Additionally, by comparing with previous geodetic and geological investigation results, our study could bring some new thoughts and directions for future research about the ATF and other active faults.