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Down and out in the Middle Kingdom: Beijing is sinking

Journey to the centre of the Earth

By Richard Chirgwin, 27 Jun 2016

Beijing is one of the most water-stressed cities in the world, and research carried out using satellite interferometry shows one of the side-effects of that: the city is sinking.

Not by a trivial amount, either: according to this open-access study in the journal Remote Sensing, the eastern part of the city is subsiding by 100 mm per year.

Before you fetch a ruler and drop a comment that 100 mm per year doesn't sound like much, consider the impact it might have on something like water pipelines, especially since the subsidence isn't going to be perfectly uniform.

According to Christian Science Monitor, the city is particularly worried about what the subsidence could do to its trains [the Monitor also offers a delicious example of the challenge of science writing in non-metric America: “In the last decade alone, Beijing has sunk 14 inches. The city continues to sink at a rate of nearly 11 centimeters per year” -- Ed]

The causes are a combination of the rapid building on the surface; and the city's excessive groundwater extraction. Taking too much water from the aquifers makes them shrink.

The measurements were made using interferometric synthetic aperture radar (InSAR) measurements from the now-defunct Envisat and then the TerraSAR-X satellites.

Those measurements were correlated with GPS to confirm their accuracy. ®

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