InSAR constraints on interseismic slip-rate of the Esfarayen fault, northeastern Iran

In the last decades, GNSS constraints and geological estimates of the fault slip rates improve the understating of the kinematics of faulting across Iran, particularly in the northeastern part of the country. Here, we complete the sparse GNSS vectors from previous reported studies around the Baghan Quchan fault zone (BQFZ) in northeastern Iran, by processing the Sentinel-1 archives covering this zone. According to tectonic and geodetic studies, the right-lateral BQFZ and the left-lateral Esfarayen fault constitute the northeastern and southern limits, respectively, of the easternmost part of the South Caspian Basin. While the BQFZ is limiting the SCB towards Eurasia, the Esfarayen fault is its border towards the Iranian microplate. We constructed 452 interferograms with 102 images from 2014.10.29 to 2019.10.27 (5 years) in descending geometry using the NSBAS package. We combined all three swaths (iw1, iw2, iw3) to cover the area of interest. The revisit time is 24 days between 2014.10.29 and 2017.02. 15, and 12 days from 2017.02.15 to 2019.10.27. To remove the hydrogeological land displacement effect (charge and discharge of aquifers), we chose the first (2014.10.29) and the last image (2019.10.27) at the same time of the year. Following the SBAS time series analysis approach, we created interferograms with short temporal (one or two months) and spatial baselines. Also, to avoid introducing any artificial signal in the mean velocity map, we created some interferograms with longer temporal baselines (maximum one year). We removed the neutral atmospheric delay using global reanalysis data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). Then, we filtered and unwrapped the generated interferograms. We applied the SBAS time series analysis on the generated interferograms to obtain displacement variations in time and a mean velocity map in the line of sight (LOS) direction of the satellites. The first noticeable point is the LOS mean velocity change across the BQFZ fault reaching up to ~1.5 mm/yr in the LOS direction, compatible with right-lateral displacement. Moreover, the mean velocity map varies significantly across the Esfarayen fault, in a sense coherent with left-lateral displacement. This velocity map points out that the NW motion of the South Caspian basin is effectively accommodated by the Esfarayen fault, while previous work based on the sparse GNSS network (Mousavi et al., 2013) suggested that the Bojnord fault further north is accommodating this NW motion. In particular, the new InSAR map indicates that the velocity vector of the permanent GNSS station ESFN used by Mousavi et al. (2013) is contaminated by subsidence motion and cannot be representative of a tectonic motion. This study brings new information for assessing seismic hazard in NE Iran with large population centers. Moreover, the retrieved mean velocity map indicates significant subsidence in Nishabour and Jajarm cities and Joveyn, Chahar Borj, Chenaran, Faruj and Ribat Jaz villages in Iran, as well as in the Yashklik city in Turkmenistan. This is the first report of subsidence occurring in Turkmenistan.