Geological parameters affected land subsidence in Mashhad plain, north-east of Iran

Mashhad plain is an area with a dry-to-semi-dry climate in Razavi Khorasan Province, Iran. About 60% of 6 million population of the province live in this plain, and mainly in Mashhad city. Overexploitation of ground water during the past decades has led to a severe drop in groundwater table and land subsidence. In this respect, some parts of the plain show an annual groundwater-level decline of 1.5 m. The surface subsidence features such as rising of the well casing, well collapse, and tension cracks observed in different parts of the area. Studying of ENVISAT ASAR images in a period of 2003/06/30 to 2010/05/24 shows the maximum subsidence rate of 32 cm/year in the north-west of the Mashhad city, but there was a mismatch between groundwater drop and ground subsidence in some parts of the plain. The evaluation of geological characteristics of subsidence area reveals, in addition to the groundwater decline, tectonic condition and active faults which change the bedrock depth have a key role in the pattern and rate of subsidence. Also, consolidation of the Pleistocene marl bedrock located in the central part of the plain is effective in the increased subsidence rate of the study area.