미국의 주요 도시들의 침강하는 이유 (WP)
Land under the country’s largest cities is sinking. Here’s where — and why.
- The movement is slow — sinking on the scale of millimeters per year in the United States — but the effects accumulate over years.
The land underneath the largest cities in the United States is sinking, a phenomenon threatening buildings, roads and rail lines, according to new research. But that sinking, known as subsidence, is not happening in the same way in each place, or even the same way across one city.
Researchers mapped out how land is moving vertically across the 28 most populous U.S. cities and found all the cities were compressing like a deflated air mattress to some extent. Twenty-five of them are dropping across two-thirds of their land. About 34 million people — about 10 percent of the U.S. population — live in the subsiding areas, according to the study published Thursday in Nature Cities.
The cities with the most widespread declining, where 98 percent of the city’s area was affected, include: Chicago; Dallas; Columbus, Ohio; Detroit; New York; Indianapolis; Charlotte; Denver; Houston; and Fort Worth.
Many of these sinking cities are located in the country’s interior. Subsidence has typically been a high concern in coastal cities, where rising sea levels can more easily come ashore and inundate areas. But the researchers say sinking land inland can destabilize infrastructure as well as worsen flooding during storms.
“Land subsidence often does not stop at the boundary of coastal regions. It’s a problem that [affects] everywhere, both inland areas and coastal areas,” said Leonard Ohenhen, lead author of the study and a researcher at Columbia University.
The movement is slow — falling on the scale of millimeters per year in the United States — but the effects can notably accumulate over years. For instance, a stadium key to the annual U.S. Open tennis tournament has been steadily sinking over time, requiring a lighter roof to reduce its weight. One runway at New York’s LaGuardia Airport, which is located on a former landfill and has undergone renovations, is also a key piece of sinking infrastructure.
“Land subsidence alone can amplify hazards,” said Manoochehr Shirzaei, a study co-author and researcher at Virginia Tech. “We can use this framework to identify where there might be a risk.”
Sinking occurs when the underlying support structure of land weakens.
Buildings, roads and even the soil itself all put pressure on everything below them. The ground beneath consists of several layers of sediment, including aquifers with a lot of porous space between sediment grains. Water fills the spaces and helps support some of the weight from above. But if water is removed and soil is compressed, the land is weighed down.
The reasons for subsidence can vary from city to city.
Along the East Coast and Great Lakes region, the land is mostly slowly sinking in response to the melting of the glaciers from the last Ice Age — known as glacial isostatic adjustment.
In large cities like New York, the weight of the buildings is literally pushing down the soil. The concrete jungles also absorb more heat, which can be transferred underground and can deform the soil and rocks — as seen in Chicago. The Pacific Northwest is subject to tectonic plate activity, where one plate goes underneath another plate and drive sinking.
Washington, D.C., for instance, has several factors driving subsidence. The city is sinking as Ice Age glaciers melt. The greatest rates occur around East Potomac Park (more than 5 millimeters per year), which was built on reclaimed land filled with soft, unstable sediments. But along the Anacostia River, some land lifts up as the river recharges the groundwater system.
The most dominant cause of sinking across most locations is the pumping of groundwater for drinking and agriculture, the researchers found. They determined that removing groundwater caused 80 percent of sinkage across the cities.
“The usage of groundwater is not going to decline,” Ohenhen said. “In most places, you are not going to tell people to stop extracting groundwater because that may be the only available resource for a particular region or city.”
Texas is home to the fastest subsiding places in the country, which pump groundwater but also a lot of oil and gas. Houston has long been known for sinking at the fastest rate, more than 5 millimeters per year. But the team found several other Texas cities further inland moving at similarly high rates, including Dallas and Fort Worth.
Sinking land exacerbates flooding risks because it can trap stormwaters for longer periods, Shirzaei said. For instance, during Hurricane Harvey in 2017, he found that 85 percent of the flooded area in the Houston-Galveston area subsided by more than 5 millimeters per year. During Hurricane Helene in 2024, subsided areas may have made it harder to clear the flooding in the following days.
“Land subsidence changed the drainage network so water could not drain out of the city,” Shirzaei said.
Damage to critical infrastructure can occur when a city experiences different levels of subsidence across a small area. For instance, a building sitting on partly elevated and partly sinking ground is more likely to be destabilized. It doesn’t take a large difference in elevation either.
Across the 28 cities, the densest urban cores had issues with precarious destabilized ground — threatening 29,000 buildings. The study found San Antonio had the highest risk for buildings even as it didn’t have the fastest subsidence rates. There, 1 in 45 buildings were at a high risk of damage; Austin showed a high risk for 1 in 71 buildings; Fort Worth was 1 in 143; and Memphis was 1 in 167.
“We don’t want to create panic,” Shirzaei said. He added, “the good thing about land subsidence is that rates are very slow” and gives time for communities to prepare or mitigate the effects.
Some places have been able to slow and pause subsidence by using a system that collects water from rain, floods, rivers or treated wastewater, and returns it to underground aquifers. Such systems exist in California’s Coachella Valley, Santa Clara and Santa Ana, in Spain’s El Carracillo district and in Beijing.
“Having detailed maps of ground movement as well as the information of what causes it can aid in designing policies,” said Pejman Tahmasebi, a subsidence researcher at the Colorado School of Mines and Hamburg University of Technology who was not involved in the study.
The findings, Tahmasebi said, can also help create better groundwater-management plans or improve urban planning in a way that curbs damage and any danger to people’s lives.
“We should start talking about those solutions right now,” said Ohenhen, who outlined some possible approaches in the study. “This problem is always only going to increase as we progress into the future.”
