Detailed satellite images have revealed that Tehran — a city of 8.694 million people — is sinking.
Some parts of the Iranian capital are plummeting by as much as 25 centimeters a year, and this includes the city’s airport.
Geoscientists Mahdi Motagh and Mahmud Haghshenas of the GFZ German Research Centre for Geosciences in Potsdam used satellite data to monitor this phenomenon across the region from 2003 to 2017. The sinking has been linked to the depletion of groundwater aquifers to irrigate nearby farmland and provide water to the 13 million or so residents of greater Tehran, the urban agglomeration around Tehran that covers central part of Tehran Province and eastern part of Alborz Province.
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It has been found that the Western Tehran Plain — a collection of Tehran’s urban sprawl, satellite cities and farmland — is subsiding by 25 centimeters a year. Meanwhile the Varamin Plain, an agricultural area to Tehran’s southeast is sinking at a rate of 5 centimeters a year.
This is not a problem exclusive to Iran. Subsidence, caused by growing populations and increased extraction of underground water, oil and gas, is a problem in urban centers the world over. The San Joaquin Valley in California for example, sinks 60 centimeters every year.
It has been estimated that 10 percent of urban Tehran has been affected by the phenomenon, along with a number of satellite towns and settlements to the city’s southwest. Fissures several kilometers in length have opened up here, and some of them have become threats to the local power transmission infrastructure.
“We see uneven street surfaces, shifted curbs, cracks in the walls and even tilted buildings, some of which have had to be demolished,” says Motagh.
In some cases the ground also suddenly collapses into sinkholes. “One farmer I met was locked up for hours when the ground gave way beneath him and he fell into a six-metre-deep crack,” says Ali Beitollahi, head of engineering seismology at the Building and Housing Research Center in Tehran.
Ironically, this phenomenon is making farmland unviable, as these cracks and sinkholes drain irrigation water, leaving crops parched.
Motagh and Haghshenas’ research also shows that this subsidence is steadily travelling eastward. According to them, population growth is exacerbating the problem. The city’s population has doubled in the past 40 years.