Mashhad city is located in Mashhad Plain in the northeast of Iran. Mashhad plain is one of the most important plain in Khorasan Razavi province. In the last decades, rapid population growth coupled with agricultural expansion has tremendously increased pressure on the groundwater resources in the plain. Large increases in water demand with little recharge have strained the plain groundwater resources resulting with 65 m drawdown of water levels in the past 40 years in some part of the plain. Due to declining of the groundwater levels, digging new wells has been forbidden since 1965. As a result of dropping in groundwater levels, land subsidence appears in some parts of the region. For example, intensive dropping of groundwater level has created cracks and fractures on the land and buildings in Tous area in the northwest of the city. This paper deals with declining of groundwater level and mechanism of land subsidence in this part of the plain. Based on land subsidence data available from Tous permanent GPS stations, the annual rate of land subsidence was measured about 20 cm/yr. This rate of land subsidence is confirmed with the results was obtained by the InSAR(Interferometric Synthetic Aperture Radar) method. The results show that land subsidence in the study area is due to compaction of clay beds (Aquitard layers) within the aquifer systems as a result of a decrease in pore-fluid pressure (ground water decline) and resultant increase in effective stress.

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