Oregon geologist Lauren Gabel said sinking often stems from soil characteristics
PORTLAND, Ore. (KOIN) -- Portland is one of several U.S. cities that gradually sinks each year, according to a recently-published study.
Scientific journal Nature uncovered the report, titled "Land subsidence risk to infrastructure in US metropolises," earlier this month. After tracking "space geodetic measurements" in the country's 28 biggest cities from 2015 to 2021, researchers found that 25 of them are sinking by at least a millimeter annually.
Scientists determined that Portland - and other places including New York, Philadelphia and Chicago - lose between one and three millimeters of elevation per year, on average. The study also shows that more than 80% of the Rose City experiences subsidence.
According to researchers, this phenomenon is commonly seen as a hazard for coastal areas -- but it can also increase flood risk, damage properties and hinder transportation in urban destinations.
"Regional variability in the [vertical land motion] observed in cities along the western coast of the United States, such as Seattle, Portland and San Francisco, may be influenced by tectonic activities associated with the active plate margins and/or sediment compaction," the study reads.
While researchers noted that subsidence has some natural causes, they added that a majority of it is caused by human-driven activities like groundwater withdrawal. Within the Rose City, the Oregon Department of Geology and Mineral Industries' Lauren Gabel said sinking often stems from soil characteristics.
"Much of Portland is underlain by volcanic soils rich in clay, which shrink when wet and expand when dry," Coastal Field Geologist Gabel told KOIN 6. "These fluctuations can be problematic when they occur under the foundations of buildings and other large concrete-based infrastructure such as bridges and fuel or water tanks. However, by far the biggest threat concerning ground movement in Portland is earthquakes."
She noted that the city is at risk of an earthquake due to its proximity to the Cascadia Subduction Zone, in addition to the "numerous crustal earthquake sources" that move the ground. But Gabel said earthquake-induced changes aren't as easy to predict as factors like soil compaction.
Nationwide, researchers estimated that around 34 million people are impacted by subsidence occurring in at least 20% of major urban areas. They noted that the gradual sinking also threatens the more than 29,000 buildings found within the risk areas.