Two decades of land subsidence in Kolkata, India revealed by InSAR and GPS measurements: implications for groundwater management and seismic hazard assessment

Land subsidence due to the over drafting of groundwater has been a major environmental crisis for decades in Kolkata, one of the most populous and largest cities in India. In this study, we map spatio-temporal evolution of the land subsidence of Kolkata region using Interferometric Synthetic Aperture Radar (InSAR) analysis of ENVISAT (2003–2010), ALOS (2007–2011) and Sentinel-1 (2017–2021) data sets along with GPS position time series (2012–2015) and Piezometric level measurements. During 2003–2011, the eastern part of the Kolkata City (east of Hooghly River) experienced ~ − 4 to − 12 mm/yr subsidence. Between 2017 and 2021, several new subsidence zones were evolved towards west of the Hooghly River as a response to the rapid decrease in groundwater level in this region (~ − 0.3 m/yr). However, during 2017–2021, the widespread subsidence pattern in the eastern part of the city became segmented into 5–20 km2 patches of low (~ − 2 to − 3 mm/yr) and significant deformation (> − 10 mm/yr). This local deceleration in the subsidence is reflected as a reduction in groundwater decline rate, probably associated with the improved artificial recharge scenario. The correlation of the decadal-scale land subsidence with groundwater level changes suggests an inelastic mechanism for the confined aquifer system of Kolkata. The present results and published seismic micro-zonation results together suggest that the areas having significant subsidence in Kolkata are associated with large Peak Ground Acceleration (PGA) values and thus are more vulnerable to seismic hazard.