America’s biggest cities are slowly sinking—and not just the ones near the ocean, according to a study published today in the journal Nature Cities. The satellite-based study shows that all 28 U.S. cities with over 600,000 people are subsiding, putting infrastructure in fast-growing urban areas increasingly at risk.
Researchers used satellite data to investigate the vertical land movements in large U.S. cities, finding that all of them are sinking to some extent. Groundwater extraction seems to be the most common culprit, and its impact on land movement has direct implications for the infrastructure in the country’s most populated neighborhoods.
“As cities continue to grow, we will see more cities expand into subsiding regions,” Leonard Ohenhen, lead author of the study and a postdoctoral researcher at Columbia Climate School’s Lamont-Doherty Earth Observatory, said in a university statement. “Over time, this subsidence can produce stresses on infrastructure that will go past their safety limit.”
Ohenhen and his colleagues used recent satellite data to map vertical land movement in the U.S.’s most populated cities, analyzing it in systematic 90-square-foot (28-square-meter) grids. While many people have heard that large coastal cities such as New Orleans are sinking, the study reveals that even municipalities far inland—including Denver, Oklahoma City, and Fort Worth—are subsiding.
Overall, the results show that in 25 of the 28 cities analyzed, at least two-thirds of the land is sinking. Houston is the fastest sinking metropolis, with over 40% of its land dropping more than 0.20 inches (5 millimeters) every year—and 12% subsiding at twice that speed. Cities including New York, Las Vegas, Washington D.C., and San Francisco also have relatively small but fast-sinking zones.
This contributes to differential motion: regions sinking at different speeds, or some regions subsiding while others uplift. This uneven movement can endanger infrastructure by causing buildings to tilt.
While only around 1% of the documented areas are experiencing strong enough differential motion to impact infrastructure, these regions tend to be the most urbanized, accounting for around 29,000 buildings. According to the study, one in 45 buildings in San Antonio are at high risk, as are one in 71 in Austin, one in 143 in Fort Worth, and one in 167 in Memphis. An even more detailed study would be needed to assess the specific risk for individual buildings, Ohenhen said in the statement.
“Unlike flood-related subsidence hazards, where risks manifest only when high rates of subsidence lower the land elevation below a critical threshold, subsidence-induced infrastructure damage can occur even with minor changes in land motion,” the authors wrote in the study. Nevertheless, sinking land is generally more vulnerable to floods.
By analyzing groundwater extraction alongside vertical land movements, the researchers determined that this activity is responsible for 80% of the documented subsiding. That’s because when humans remove water from aquifers—layers of porous rock that hold groundwater—the pores can collapse, lowering the land surface above it. The researchers argue that this phenomenon will only worsen in some areas because of elements such as population growth (meaning increasing water needs) and climate-driven droughts.
There are also other forces at play, however. Some sinking areas are still recovering from the bulging caused by long-gone ice age glaciers in North America’s interior. The monumental weight of buildings might also impact some cities’ land movement (cough cough, New York).
Ultimately, the researchers hope their study will inspire a push for mitigation efforts, which could include land raising, building retrofitting, and updated construction codes.
“As opposed to just saying it’s a problem, we can respond, address, mitigate, adapt,” Ohenhen concluded. “We have to move to solutions.”
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