Gravity Change and Its Relation to Land Subsidence and Underground Water Table Variation at Kerman, Iran

Abstract

The gravity value at the surface of the Earth can be changed due to land subsidence and underground water depletion. Absolute gravity measurements show a gravity increase of ~ 169 µgal at Kerman station in southeastern Iran during 2004–2017. InSAR vertical map (2017–2019) reveals displacement rates of -3.5 cm/year at the Kerman site and a maximum of -25 cm/year at the center of the plain. Kerman GPS measurements (2011–2018) indicate -4.3 cm/year of vertical displacement rate. The geometrical contribution of the subsidence to the gravity variation at this site is + 140.2 and + 172.2 μgal using InSAR and GPS, respectively. In situ measurements of the groundwater table show a 17 cm/year depletion rate, leading to minimum and maximum values of − 27.8 and − 46.4 µgal in the induced gravity change assuming a 30–50% porosity range. The sum of induced hydrological and geometrical gravity changes is found to be smaller than the observed gravity variation at Kerman station, underlying a variable subsidence rate in time. The decrease in subsidence rate, observed at some urban leveling benchmarks, is probably due to the westward development of Kerman city, the lack of a proper sewage system, as well as the decrease in water extraction because of land use change. Assuming that the subsidence rate was larger at the beginning of the absolute gravity measurement period and decreases with time, most of the gravity increase at the Kerman station can be explained by subsidence with only a small water mass change contribution.