New York City’s skyline may start to look very different if the metropolis keeps sinking
Gary Hershorn/Getty Images
More than two dozen of the biggest cities in the US are sinking, which could affect thousands of buildings and millions of people.
The problem has been reported before, particularly in coastal areas. But by using satellite technology, which sends radar signals towards Earth’s surface and measures the time it takes for them to bounce back, scientists have found that it affects 25 out of 28 of the country’s biggest cities.
“By comparing multiple images taken over time from the same area, we can detect tiny vertical movements of the ground, down to a few millimetres per year,” says team member Manoochehr Shirzaei at Virginia Tech. “It’s like taking a high-resolution time-lapse of Earth’s surface and watching how it rises or sinks over time.”
Fort Worth, Houston and Dallas exhibited the highest subsidence rates of all the large cities, exceeding 4 millimetres per year, on average. For New York, Chicago, Houston, Columbus, Seattle and Denver, the average subsidence was greater than 2 millimetres per year.
“Houston – the fastest sinking city out of the 28 most populated US cities – has 42 per cent of its land area subsiding faster than 5 mm per year, and 12 per cent subsiding faster than 10 mm per year,” according to the researchers.
They say that most of the subsidence is caused by groundwater extraction, however, in some cities, such as New York, Philadelphia and Washington DC, the sinking is primarily caused by “glacial isostatic adjustment”.
“During the last ice age, these areas were covered by massive ice sheets. The sheer weight of the ice pushed down on Earth’s crust, like sitting on a memory foam mattress,” says Shirzaei. When the ice melted thousands of years ago, the pressure lifted, and the ground began to slowly rebound, he says.
“But this rebound isn’t uniform,” says Shirzaei. “In some areas, like the US East Coast and Midwest, the land is still sinking rather than rising, because they’re near the forebulge, a zone that had been pushed up by the weight of the ice nearby and is now collapsing.”
In Seattle, Portland and San Francisco, plate tectonics is probably to blame for the subsidence.
“We need to start treating subsidence like the slow-moving disaster it is,” says Shirzaei. The scientists also found that some cities are sinking at different rates in different spots, or sinking in some places and rising in others. “This uneven movement creates angular distortion and stress, potentially leading to cracks in walls and foundations, misaligned windows and doors, or worse, structural failure,” says Shirzaei.
Jesse Kearse at Kyoto University, Japan, has used similar satellite data to show that many New Zealand cities are also subsiding. “A key challenge that remains for the geophysics community is how to attribute the observed trends to specific causes, whether they be anthropogenic or natural geological processes,” he says.
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