Why Is Portland Sinking Into the Earth?

Cities around the world are sinking. Causes include sheer mass of buildings (NYC), fracking (Houston), and tapping of underground aquifers (L.A.). Portland is subsiding as well. However, we have few skyscrapers, no fracking, and piped-in water. How do geologists explain our slow descent? —Ray in Southeast

Given the condition of the rest of the country, Ray, it’s tempting to suggest that Portland is trying to disappear into the earth out of sheer embarrassment, like Homer Simpson receding into a bush. Cheap shots aside, you’re right: Portland’s (relatively modest) sag isn’t due to any of the causes you mention.

Urban subsidence seems to have entered the zeitgeist with a 2024 article in the journal Nature. The study included many colorful maps, so folks could see how their cities were faring without learning to read. (Houston is dropping like a turd in a well.)

The leading cause of subsidence is, as you note, groundwater depletion. Sediment layers are supported in part by water; pump the water out and the layer compacts. Big in Southern California and the Central Valley, this one is also a factor in Las Vegas, Phoenix and Houston.

Second is oil and gas extraction. Both fracking and conventional drilling reduce pressure in underground reservoirs, causing the surface to slump. This one is more common in wide-open places—West Texas, Oklahoma—but it is happening near some cities, like Houston, and even in Los Angeles.

New York’s subsidence comes from the earth’s crust shifting back into place after glacier-induced buckling during the last ice age. (It takes a long time.) The city’s estimated 1.7 trillion pounds of buildings certainly don’t make it any more buoyant, however. And Portland’s drop is just workaday sediment compaction—our alluvial soil was deposited relatively recently and it’s still settling.

You could argue all these places are moving (slowly) in the direction of hell thanks to each city’s respective original sin. L.A.? Everyone knew there was no water. Houston? It grew around the same oil industry that undermines it today. New York’s crime? Maybe just being too big.

Our city’s original sin? Being built on crappy, loose ground. Our soil is not only compressible, it also liquefies when you shake it, which is why we’re all going to die when the earthquake hits. Technically, this is only original sin in the Calvinist sense, where those destined for damnation are born already doomed. But hey, if the shoe fits...

Questions? Send them to dr.know@wweek.com.