Sentinel-1 Insights into interseismic coupling along the plate boundary of the Western Makran Subduction Zone

The Makran Subduction Zone (MSZ) of Iran and Pakistan, where the oceanic Arabian plate is sinking beneath the overriding continental Eurasian plate, is among the least explored subduction zones. Limited geodetic measurements, especially in the Western MSZ (WMSZ) with lower seismicity, have posed challenges in assessing the potential for future seismic events. The extensive spatial coverage offered by the Interferometric Wide-Swath (IW) mode of Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) allows for measuring tectonic deformation at a scale of millimeters per year across distances spanning hundreds of kilometers. Nevertheless, the presence of other signals and errors­­­­­—with similar spatio-temporal patterns to the signal of interest—poses challenges to accurately estimating the low-amplitude, large-scale subduction-induced deformation from InSAR observations.

In this contribution, we analyze more than eight years of continuous Sentinel-1 InSAR data from both ascending and descending orbits in the WMSZ area of Iran, to capture the Line-Of-Sight (LOS) interseismic crustal deformation rate. Our approach integrates a comprehensive and novel atmospheric mitigation strategy, accompanied by corrections for non-tectonic processes and rigid plate motion, aiming to isolate the tectonic-related signal from other non-tectonic signals and errors. In the following step, we investigate three trench-perpendicular profiles to infer the spatial and along-dip distribution of plate coupling from the Line-Of-Sight (LOS) deformation rates obtained through InSAR.

Due to the limited InSAR coverage near the trench (as located beneath the sea), it is not possible to constraint coupling in that area that extends 150 km far from the trench and reaches a depth of 10 km. Our findings reveal significant variations in interseismic coupling from west to east. We observe regions of weak and strong coupling, located near Jask (the westernmost part of the WMSZ) and Chabahar (the easternmost part of the WMSZ), respectively. The middle profile, located near the epicenter of a 5.9 magnitude earthquake that occurred in 1989 (Mw 5.9), exhibits a moderate coupling of 65 percent. Additionally, the coupling is notably high at depths between 10 and 20 km, gradually decreasing to zero at depths between 30 and 40 km.

In summary, the enhanced spatial resolution of InSAR, along with the high precision of deformation rates provided by the advanced error mitigation on the long time series of Sentinel-1 significantly improves our ability to characterize the locking depth at which the boundaries between two plates are accumulating stress in WMSZ.