Land subsidence is typically considered a coastal problem: The dual threats of sinking land and rising seas intensify flooding, particularly in places like New York City and Louisiana. But even inland, major cities face infrastructure problems and flooding damage from sinking land beneath.
“Land subsidence does not stop at coastal boundaries.”
A study published in Nature Cities has found that all 28 of the most populous cities in the United States are sinking. Though some of this subsidence is due to long-term geologic processes, much of it is spurred by human activity, including groundwater pumping and the building of new infrastructure. Better groundwater management and stricter building codes could mitigate risks.
“Land subsidence does not stop at coastal boundaries,” said Leonard Ohenhen, a postdoctoral researcher at Columbia University and the first author of the new study.
From Coast to Coast, and in Between
Rates of sinking or uplifting land, also known as vertical land motion, can be measured from satellites via synthetic aperture radar (SAR), a technology that sends radar pulses to Earth and records how those pulses are reflected back. Ohenhen and the research team used SAR measurements from 2015 to 2021 from the Sentinel-1 mission to create maps of ground deformation in the 28 most populous U.S. cities.
The team found that in every city, at least 20% of the land area was sinking, and in 25 of the 28 cities, at least 65% of the land area was sinking. Estimates from the study show that about 33.8 million people live on sinking land in these 28 cities.
The study shows a “really good assessment of what the whole local and regional picture of vertical land motion looks like,” said Patrick Barnard, a geologist at the University of California, Santa Cruz Center for Coastal Climate Resilience, who was not involved in the new study. “It gives us more and more confidence and a greater understanding of how [subsidence] is influencing urban areas and increasing the risk to the population.”
Some of the highest rates of subsidence (>4 millimeters per year) were observed in several cities in Texas: Houston, Fort Worth, and Dallas. The fastest-sinking city in the country was Houston, with more than 40% of its land subsiding at a rate greater than 5 millimeters per year.
Chicago, Detroit, New York, and Denver were among the cities with the most land area affected by subsidence.
Some of the rates described in the study were “alarming,” Barnard said, because typical background subsidence is below a couple of millimeters per year. Rates above 2 millimeters per year can damage infrastructure and buildings, he said.
Vertical land motion is especially problematic where land is sinking unevenly, or where a subsiding region is next to an area that’s rising.
Analyzing building densities and land deformation, the researchers found that San Antonio faces the greatest risk, with one in every 45 buildings at a high risk of damage.
What may seem like slow sinking can build up over time to cause problems, Ohenhen said. “Four millimeters per year becomes 40 millimeters over 10 years, and so on…that cumulative effect can add up.”
Getting Ahead of Ground Deformation
A now-absent ice sheet may be responsible for some of the land deformation. Tens of thousands of years ago, the Laurentide Ice Sheet covered much of North America, compressing the land beneath. Now that the ice sheet has melted, North America is readjusting. Land once underneath the ice sheet is generally rising slowly, while land not covered by the ice sheet is sinking. Ohenhen compared this process to relieving pressure on a mattress: Once pressure is released, some parts of the mattress rise while others sink back to their original height.
Most of the subsidence described in the study, though, likely comes from groundwater pumping, which decreases pressure in the pore space of rock and sediment. The pore space slowly collapses and the ground sinks.
“We can’t just be pumping the ground without any regard to the potential long-term impacts.”
That can exacerbate flooding and infrastructure damage. Groundwater pumping and oil and gas extraction near Houston caused land subsidence that correlated with flood severity after Hurricane Harvey in 2017, for example.
As climate change continues to intensify drought conditions in some parts of the United States, land subsidence from groundwater pumping could become even more of a risk to infrastructure. An “increasing number of cities may face significant challenges in subsidence management,” the study authors wrote.
“It’s really a major issue we have to consider, especially in these urban areas,” Barnard said. “We can’t just be pumping the ground without any regard to the potential long-term impacts.”
The risks posed by land subsidence are high enough to warrant policy changes to better manage groundwater pumping across the country, Barnard said. Better enforcement of building codes could also prevent damage, the paper’s authors wrote.
“People are often not attuned to some of these subtle hazards they may be exposed to,” Ohenhen said. “[We should] make people aware of the situation so that we do not wait until the very last moment to respond.”
—Grace van Deelen (@gvd.bsky.social), Staff Writer
