Portland is slowly sinking, new study shows
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Illustration: Aïda Amer/Axios
Portland is sinking millimeter by millimeter, and new research shows it's more widespread than once suspected.
Why it matters: Land subsidence is an invisible but growing threat to urban infrastructure — cracking roads, destabilizing buildings and making low-lying areas even more flood-prone.
- In Portland, it combines with seismic instability and aging buildings to heighten long-term risk.
Driving the news: In a peer-reviewed study published this month in Nature, researchers analyzed satellite radar data from 2015-2021 and found that more than 80% of Portland's land area is sinking at measurable rates, at an average of around 2 millimeters per year.
The intrigue: In Portland's case, tectonic activity and sediment compaction appear to be the dominant drivers — not groundwater pumping, which is a primary culprit elsewhere, per the study.
The big picture: The study found that 25 of the 28 most populous U.S. cities are subsiding, affecting more than 33 million people — over 10% of the U.S. population — who live on sinking land.
Threat level: A few millimeters a year means more than you might suspect.
- "The latent nature of this risk means that infrastructure can be silently compromised over time, with damage only becoming evident when it is severe or potentially catastrophic," researchers wrote.
Zoom in: Bill Burns, an engineering geologist with the Oregon Department of Geology and Mineral Industries (DOGAMI), told Axios that subsidence in Portland can vary greatly depending on the soil type.
- Volcanic soil that's rich in clay is present in much of Portland, and it can shrink when it dries and expand when it's wet.
- That can be problematic when it happens under buildings.
- "Concrete can't stand that," Burns told Axios. "It doesn't like to be moved, especially differentially, like if one part of the building goes and the other part stays there."
Reality check: Burns said the biggest threat concerning ground movement in Portland is still earthquakes.
- "We're talking about feet or meters or tens of meters in some cases, versus millimeters or centimeters in the subsidence study," he said.
- He recommended residents check out the DOGAMI's Statewide Hazard Viewer, a tool that shows what hazards — from earthquakes to landslides to radon exposure — are present in every part of the state.
What's next: Researchers urge cities to factor subsidence into adaptation strategies, including zoning changes and green infrastructure, as well as raising roads or installing tide gates.