The biggest cities in the US are sinking, a study found. The phenomenon could tilt buildings and damage infrastructure.
If cities don't do something about this land-sinking phenomenon, called subsidence, it could damage buildings and infrastructure. It's already driving flooding in many places.
"We did not expect to see such widespread land subsidence," Manoochehr Shirzaei, a geophysicist at Virginia Tech, told Business Insider.
Shirzaei co-authored a study, published Thursday in the journal Nature Cities, using satellites to measure the millimeter-by-millimeter vertical movement of US cities.
He said there was a clear pattern in the data: Urban centers are sinking faster than other areas, and have a greater risk of building damage.
He found that subsidence is distorting 28 cities — all the places they checked — including Seattle, San Diego, Denver, Dallas, Chicago, and New York City.
About 34 million people live in the affected areas.
Uneven sinking can damage cities
Sinking can directly boost flooding, since coastal cities get lower while sea levels rise around them. Even inland, it can create new troughs in the land that pool stormwater.
Take Houston, which was the fastest-sinking city in the new study, with 40% of its land area sinking five millimeters each year and some areas sinking two inches per year. Shirzaei's previous research found that subsidence had left standing water in some unexpected parts of the city after Hurricane Harvey.
The risks are especially high, though, in places where sinking is happening unevenly within a small area.
When part of the ground sinks slower, or even rises while the spot just next to it sinks, that puts a lot of strain on whatever is built there.
At New Orleans' Louis Armstrong International Airport, the differential sinking has already cracked pavement and left huge puddles on the plane taxiway.
In the worst-case scenario, uneven subsidence could tilt buildings, crack foundations, and hobble bridges.
Sinking alone "would not probably cause a significant risk, but when it compounds with other hazards — flood and storms and winds or poor maintenance or poor construction code — then that can be deadly and result in building failure," Shirzaei said.
Hotspots of risk to 29,000 buildings
About 29,000 buildings are at risk, the study found, even though uneven subsidence only affects about 1% of the land area in the studied cities.
That's sort of good news, because it means the problem areas are concentrated in small, identifiable parts of cities.
"These are areas that could potentially be exposed to a hazard if some immediate action is not taken in the next couple of years," Leonard Ohenhen, a postdoctoral researcher at Columbia's Lamont-Doherty Earth Observatory and the lead author of the study, told BI.
He hopes cities will focus on those hotspots "to enforce some building codes, maybe strengthen the integrity of infrastructure."
Why cities are sinking
Groundwater extraction seems to be the main culprit behind America's sinking cities, according to the study.
That means cities can make a big difference by practicing better groundwater management — balancing withdrawals with rainfall that replenishes underground aquifers.
A few other factors are also at play.
Oil and gas extraction contribute to the problem in Texas, deflating the ground in a similar way to groundwater extraction.
The sheer weight of a city's buildings can also push it down into the Earth, as a 2023 study found in New York City.
In parts of the East Coast and across the center of the US, some subsidence is a leftover reaction from the disappearance of the Laurentide ice sheet. When this ice covered much of North America during the last Ice Age, it squeezed the land around its edges upward. Those regions are still settling back down today.
The study authors said that this was a contributing factor in New York, Indianapolis, Nashville, Philadelphia, Denver, Chicago, and Portland.
Recent studies in Chicago and Miami have suggested that heat from the city and vibrations from construction, respectively, could also contribute to buildings sinking.
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