Puget Sound has an 85% chance of a deep magnitude 6.5 quake within 50 years, USGS says

Deep earthquake odds for the Seattle-Tacoma region are far higher than the odds of the Cascadia megaquake that dominates the headlines. The U.S. Geological Survey puts the chance of a magnitude 6.5 or larger earthquake originating deep inside the subducting Juan de Fuca plate beneath Puget Sound at 85 percent over the next 50 years. The figure dates to September 2025.

The fact sheet, numbered 2025-3050, was 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. According to the authors, each percentage applies only to the source named beside it.

The 85 percent figure and its 26-year clock

In USGS Fact Sheet 2025-3050, the 85 percent applies to an earthquake of magnitude 6.5 or greater in the “deep,” or intraslab, category within the Puget Sound region across a 50-year window. The agency pairs that probability with an average recurrence interval of about 26 years, the shortest of any source in its table. The authors write that earthquake probabilities are often computed assuming earthquakes are time independent, with no memory of when the last one struck.

The record the USGS cites shows the pattern: a magnitude 6.8 quake near Olympia in 1949, a magnitude 6.5 quake at Seattle in 1965, and the magnitude 6.8 Nisqually earthquake in 2001. Older public material, including a Pacific Northwest Seismic Network page, still quotes 84 percent for the same category, so the 2025 fact sheet moved the figure by a single point. The gaps between the three most recent events were 16 years, from 1949 to 1965, and 36 years, from 1965 to 2001, which brackets the roughly 26-year average the USGS reports. Counted from 2001, the span since the last deep shock now exceeds 25 years.

Intraslab ruptures inside the Juan de Fuca plate

The Pacific Northwest Seismic Network, which operates the region’s seismometers, describes these events as originating within the sinking Juan de Fuca plate at depths between 30 and 70 kilometers. The slab bends as it dives beneath North America, and the stress produces normal faulting inside the plate itself. The same network credits the 1949 shock with more than $100 million in damage, including harm to the Capitol Building in Olympia, and the 1965 shock with more than $50 million.

The Washington Department of Natural Resources, in its Nisqually earthquake fact sheet, records that the February 28, 2001 magnitude 6.8 event struck at a depth of 32 miles and shook the ground for about one minute. It damaged the SR 99 viaduct in Seattle and the Capitol dome in Olympia, with the strongest shaking in the southern Puget lowland, and was felt as far away as central Oregon and western Montana.

Crustal faults and Cascadia in the same table

For comparison, the USGS gives a 17 percent chance of a magnitude 6.5 or larger earthquake on a crustal fault in the Puget Sound region over the same 50 years, with a recurrence interval near 265 years. The Seattle Fault Zone alone carries a 5 percent chance, tied to a recurrence interval of roughly 1,000 years; its last big rupture, estimated at magnitude 7.5 to 7.8, dates to 923 or 924 C.E. For the Portland and northern Willamette Valley region the fact sheet lists 6 percent. The fact sheet states a 10 percent chance of a magnitude 9 earthquake on the Cascadia Subduction Zone when earthquakes are treated as time independent, and 15 percent when the time since the 1700 rupture, which the fact sheet estimates at magnitude 8.7 to 9.2, is factored in; both use a recurrence interval of about 500 years. A magnitude 8 or larger event confined to southern Cascadia is put at about 30 percent, with a recurrence interval of roughly 300 to 350 years.

Percentages over a 50-year window

Probabilities of this kind describe the whole 50-year span, not a date. Because each figure covers a different source, the 85 percent for a deep quake should not be added to the 17 percent for a crustal fault or to the Cascadia numbers; they answer separate questions about separate faults. An 85 percent chance in 50 years does not mean a quake is due this decade, and a 15 percent chance of a magnitude 9 does not make it remote for anyone living in a tsunami zone. A GovTech account of Washington’s seismic history, citing the same agency, calls the deep Nisqually-type quake the most frequently observed damaging source in the state, with events in 1949, 1965 and 2001.

The USGS publication page lists version 1.1, dated September 30, 2025, as current, with the 85 percent line unchanged.

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

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