Portland and the northern Willamette Valley face a 6% chance of a magnitude 6.5 quake in 50 years

Portland sits above a different kind of earthquake threat than the one that dominates Pacific Northwest headlines. The U.S. Geological Survey puts the chance of a magnitude 6.5 or larger crustal-fault earthquake in the Portland or northern Willamette Valley region at 6 percent over the next 50 years, a figure that rests on an expected recurrence of roughly once every 800 years.

The Oregon event the number describes would be a different animal from the magnitude 5.6 Scotts Mills earthquake that damaged chimneys and school gables in 1993.

The 800-year recurrence interval behind the 6 percent

The figure comes from USGS Fact Sheet 2025-3050, “Earthquake probabilities and hazards in the U.S. Pacific Northwest,” written by USGS scientists Erin Wirth, Arthur Frankel, Brian Sherrod, Alex Grant, Audrey Dunham and Ian Stone with Julia Grossman of the University of Washington. Focusing on northern Willamette Valley crustal faults, the authors write that an “~800-year recurrence interval for M ≥6.5 crustal fault earthquakes” corresponds to a 6-percent chance of an M 6.5 or larger earthquake in the Portland or northern Willamette Valley region in the next 50 years.

The rates behind that statement come from the 2023 National Seismic Hazard Model. The National Seismic Hazard Model is the USGS tool that feeds building codes and insurance rate structures, and its 2023 update covered all 50 states and included targeted studies of the sedimentary basins under Seattle and Portland. The fact sheet’s own publication record dates it to Sept. 19, 2025, with a version 1.1 update on Sept. 30, 2025.

The fact sheet is also candid about what it cannot see. It states that observational and paleoseismic constraints are extremely limited for inland crustal faults outside the Puget Sound region, which includes the Portland area.

Puget Sound and Cascadia figures set beside Portland’s

Set beside its neighbors, Portland’s 6 percent is small. For the Puget Sound region the same fact sheet gives an 85 percent chance of a deep intraslab earthquake of M 6.5 or larger, a 17 percent chance of a crustal-fault earthquake of M 6.5 or larger, and a 5 percent chance of an M 6.5 or larger earthquake on the Seattle Fault alone, all over 50 years.

The Cascadia Subduction Zone carries the biggest numbers. The USGS gives a 10 percent chance of another M~9 earthquake in the next 50 years in the time-independent model and 15 percent in the time-dependent model, plus about a 30 percent chance of an M 8 or larger earthquake in southern Cascadia. The agency’s Pacific Northwest hazards page describes that fault as a 1,100-kilometer plate boundary beneath Washington, Oregon and northern California, and lists shallow crustal faults, capable of roughly magnitude 7.5, as the third source of danger.

The fact sheet offers no explicit explanation for why Portland’s crustal figure differs from Puget Sound’s 17 percent.

Portland Hills, Gales Creek and Mount Angel fault zones

The fact sheet does not name the individual faults behind the Portland figure, so the local geology comes from Oregon sources. Washington County’s emergency office lists three shallow fault zones: the Portland Hills zone east of the county, which it says has three faults trending northwest to southeast about two miles apart and could generate a magnitude 6.8 to 7.2 earthquake about once per millennium; the Gales Creek zone on the eastern flank of the Coast Range; and the Mount Angel zone in the northern Willamette Valley. Its scenario for a magnitude 6.8 Portland Hills event has shaking reaching violent (IX) in the Portland Hills and very strong (VII) in central Washington County.

Geologists at Oregon’s Department of Geology and Mineral Industries are more cautious on timing. A paper in DOGAMI’s Oregon Geology by Wong, Hemphill-Haley, Liberty and Madin concludes that the Portland Hills fault appears capable of large-magnitude earthquakes, estimates magnitude 6.8 to 7.2 over a 28 to 62 kilometer rupture, and puts the recurrence interval at 2,000 to 16,000 years based on slip rates of 0.05 to 0.4 millimeters per year. At North Clackamas Park, they report folded Missoula flood deposits, 12,800 to 15,000 years old, that suggest at least one surface-rupturing earthquake in that span.

The 1993 Scotts Mills earthquake as the regional benchmark

The largest recent crustal event nearby was far smaller. The Pacific Northwest Seismic Network records that the 1993 Scotts Mills earthquake struck at 5:34 a.m. on March 25, 1993, with a magnitude of 5.6 on the Mount Angel fault zone, about 54 miles south of Portland. It caused more than $28 million in damage in 1993 dollars, mostly to unreinforced masonry; Molalla High School lost its chimney and two gables. Shaking reached intensity VII near the epicenter and was felt widely across the Portland area.

The DOGAMI authors write that a major Portland Hills earthquake could produce ground accelerations exceeding 1.0 g in downtown Portland, enough to threaten older buildings lacking modern seismic design.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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