U.S. cities are sinking. Here's what that means for homeowners

The land below many U.S. cities is sinking, including New Orleans, New York City, Miami and south San Francisco.

This phenomenon, known as land subsidence, can severely affect the integrity of buildings and infrastructure. When coupled with a sea-level rise, it can greatly increase the incidence of flooding.

Problems associated with land subsidence can cost U.S. homeowners 6% of their home value. In areas with high subsidence, that number can jump to 8.1%, according to forthcoming research done by assistant professor of public policy Mehdi Nemati at the University of California, Riverside, and his colleagues. Their research focused on the Central Valley of California, but Nemati said the findings could be extrapolated nationwide.

Standard homeowners insurance usually does not cover land subsidence issues, according to Policygenius, although in some areas, you may be able to purchase specific coverage for subsidence caused by nearby mines or mining activity.

Consumers are likely to see the effect of land subsidence directly, in the form of problems with their home, and indirectly, in the form of issues for their local economy.

What causes land subsidence

Both natural and man-made processes cause land subsidence.

As glaciers are receding from the land in the U.S. and Canada, the process creates a “see-saw” effect where the land in the U.S. falls but rises in Canada, researchers say.

Manoochehr Shirzaei, a professor of geophysics and remote sensing at Virginia Tech, also attributed some land subsidence to tectonic processes.

“For example, earthquakes can make the land rise, but also in some places [make it] fall. So these two are considered to be natural processes,” he said.

Human-induced land subsidence relates to how we have developed our cities, namely groundwater extraction and building practices.

“We use groundwater to drink and for other purposes. And as we take water out from the land, the space beneath it becomes compact because we’ve built on top of it,” said Rob Freudenberg, vice president of energy and environment at the Regional Plan Association. Heavy building materials also compact the land, putting infrastructure at risk.

When designed, most infrastructure does not account for shifting land. This can be dangerous, experts say.

“If you think of something like a rail line and the rail lines running across ground that’s sinking, some of that will sink, some of it will not,” Freudenberg said. “So now you might have kind of erosion beneath the track where you didn’t have it before. You might have to realign the tracks.”

Watch the video above to learn more about how land subsidence is putting U.S. infrastructure in a precarious situation and how much it could cost to fix it.