New York City’s skyline may undergo significant changes as major cities continue to sink.
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Over 20 of the largest metropolitan areas in the United States are experiencing subsidence, impacting thousands of structures and millions of residents.
This phenomenon has been noted especially in coastal areas. However, utilizing satellite technology that gauges the duration it takes radar signals to reach the Earth’s surface and return, researchers discovered that 25 out of the 28 largest cities in the country are affected.
“By analyzing multiple images taken over time from the same region, we can identify subtle vertical shifts in the ground that can reach several millimeters annually,” explains team member Manoochehr Shirzaei from Virginia Tech. “It’s akin to capturing a high-resolution time lapse of the Earth’s surface, revealing whether it is rising or sinking over time.”
Fort Worth, Houston, and Dallas demonstrate the highest rates of subsidence among major cities, averaging over 4 millimeters per year. Cities like New York, Chicago, Houston, Columbus, Seattle, and Denver show average subsidence rates surpassing 2 millimeters annually.
“Houston, the fastest sinking city among the 28 most populous in the US, has 42% of its land area sinking at rates greater than 5 mm per year, with 12% sinking faster than 10 mm annually,” researchers reported.
Most subsidence is attributed to groundwater extraction, although cities like New York, Philadelphia, and Washington, DC, primarily experience sinking due to “glacial isoplasm regulation.”
“During the last glacial period, these regions were covered by massive ice sheets. The considerable weight of the ice compressed the Earth’s crust, akin to resting on a memory foam mattress,” says Shirzaei. As the ice melted thousands of years ago, the pressure released and the terrain began to slowly recover,” he explains.
“However, this rebound isn’t uniform,” Shirzaei notes. “In some areas, particularly along the East Coast and in the Midwest, the land is subsiding rather than rising due to the residual weight of nearby ice and their proximity to regions currently collapsing.”
In Seattle, Portland, and San Francisco, plate tectonics may account for some of the subsidence.
“We must address sinking as a gradually unfolding disaster,” Shirzaei argues. Researchers also noted that cities can sink at varying rates in different locations or sink continuously in one area while other regions remain stable. “This uneven movement can create angular distortions and stress, resulting in cracks in walls and foundations, misalignments in windows and doors, or even significant structural failures,” Shirzaei warns.
Jesse Cars from Kyoto University in Japan demonstrated similar findings using satellite data, showing that many cities in New Zealand are also experiencing subsidence. “A crucial challenge for the geophysical community remains understanding how the observed trends stem from particular causes, whether they are artificial or naturally occurring geological processes,” he states.
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Source: www.newscientist.com
