Ground surface response to continuous compaction of aquifer system in Tehran, Iran: Results from a long-term multi-sensor InSAR analysis

Most of the developed groundwater basins in Iran are subject to land subsidence hazards resulting from the over- extraction of groundwater. Several areas in Tehran, the capital city and a provincial center in north-central Iran, have been reported to be subsiding at different rates. In this study, we present the results of an Interferometric Synthetic Aperture Radar (InSAR) time series analysis of Tehran using different SAR data between 2003 and 2017. By constructing more than 400 interferograms derived from 39 Envisat ASAR (C-band), 10 ALOS PALSAR (L-band), 48 TerraSAR-X (X-band), and 64 Sentinel-1 (C-band) SAR datasets, we compile displacement time series from interferometric observations using the Small Baseline (SB) technique. Our analysis identifies 3 distinct subsidence features in Tehran with rates exceeding 25 cm/yr in the western Tehran Plain, approximately 5 cm/yr in the immediate vicinity of Tehran international airport, and 22 cm/yr in the Varamin Plain to the southeast of Tehran city. The temporal pattern of land subsidence, which is dominated by a decreasing trend, generally follows the regional decline in groundwater level, which suggests that anthropogenic processes caused by excessive groundwater extraction are the primary cause of land subsidence. Integrating a decadal time series of subsidence constructed from multi-sensor InSAR with in-situ observations suggests that inelastic and permanent compaction dominates the main deformation regime of the Tehran aquifer, and the ratio between elastic and inelastic deformation is approximately 0.4. A geological analysis indicates that the shape of the subsidence bowl in the western Tehran Plain does not follow the trend of major mapped faults in the region. In contrast, the subsidence bowl in Varamin is controlled by the Pishva Fault, which suggests that either this fault acts as a hydrologic barrier to the groundwater flow in this region or that the differences in sediment thickness causes the discontinuity in land subsidence.