Interseismic Strain Accumulation Across the Main Recent Fault, SW Iran, From Sentinel‐1 InSAR Observations

• Published in: Journal of geophysical research. Solid earth Vol. 127; no. 2
• Main Authors: Watson, Andrew R., Elliott, John R., Walters, Richard J.
• Format: Journal Article
• Language: English
• Published: 01.02.2022
• Subjects: InSAR; interseismic slip; locking depth; Main Recent Fault; Zagros
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB022674

The Main Recent Fault is a major right‐lateral strike‐slip fault in the western Zagros mountains of Iran. Recent geodetic and geological studies estimate a low slip rate of 1–6 mm/yr at an unknown depth which, when combined with a non‐ideal fault geometry, makes the Main Recent Fault a difficult but interesting target for InSAR analysis. This analysis would further cement the estimated slip rate and provide an opportunity of estimate the depth to the base of the locked seismogenic zone, both important constraints on the seismic hazard posed by the fault, as well as for understanding how oblique convergence is accommodated and partitioned across the Zagros. We use 200 Sentinel‐1 SAR images from the past 5 years, spanning two ascending and two descending tracks, to estimate the first InSAR‐derived slip rate and locking depth for a 300 km long section of the fault. We utilize two established processing systems, LiCSAR and LiCSBAS, to produce interferograms and perform time series analysis, respectively. We constrain north‐south motion using GNSS observations, decompose our InSAR line‐of‐sight velocities into fault‐parallel and vertical motion, and fit 1‐D screw dislocation models to three fault‐perpendicular profiles of fault‐parallel velocity, following a Bayesian approach to estimate the posterior probability distribution on the fault parameters. We estimate an interseismic slip velocity of 2.4 ± 1.2 mm/yr below a loosely constrained 14 km locking depth, the first such estimate for the fault, and discuss the challenges in constraining the locking depth for low magnitude interseismic signals. Plain Language Summary Convergence between the Arabian and Eurasian plates is causing deformation of the Earth's crust in Iran. Some of this motion is taken up by movement at depth on the Main Recent Fault, which is stuck by friction near the Earth's surface and is therefore accumulating strain which may then be released in an earthquake. We use 5 years of satellite radar images to measure the average velocity of the ground surface either side of the fault. By looking at the velocity difference across the fault, along with the gradient, we can estimate the rate at which the fault is accumulating strain and the depth below which this is occurring. Our estimated rate of 2.4 ± 1.2 mm/yr is in agreement with previous estimates from GPS studies, while our locking depth estimate of 14 km with a large uncertainty of 6–40 km, is the first such estimate for the fault. The broad range of possible values for the locking depth highlights the difficulties of studying tectonic signals when they are close in magnitude to the sensing limit of our satellite imagery method (1 mm/yr). Key Points We derive East and Vertical surface velocities from 5.5 years of Sentinel‐1 synthetic aperture radar images over the western Zagros We estimate an interseismic slip rate of 2.4 ± 1.2 mm/yr (2σ) for the Main Recent Fault, in agreement with previous GNSS studies We estimate a geodetically‐determined interseismic locking depth of 14 km with a large uncertainty of 6–40 km, a first for this fault