Oregon puts the odds of a magnitude 7.1-plus Cascadia quake in the next 50 years at 37%

Oregon’s Office of Emergency Management says scientists see about a 37% chance that a magnitude 7.1-plus megathrust earthquake will strike the Cascadia fault zone within the next 50 years. The same agency describes a worst case far larger than that threshold: a magnitude 9.0-plus rupture and a tsunami of up to 100 feet in height along the Oregon coast.

Save more, waste less: The Cool Down’s free newsletter sends smart, simple changes that help you and the planet in your everyday life.

Subscribe to The Cool Down free →

We may earn a commission if you sign up.

The figure describes a single fault that has been quiet for more than three centuries, and the state treats it as the organizing hazard for its disaster planning.

The megathrust estimate behind 37%

The number comes from the state’s own hazard page, which states that scientists estimate about a 37% chance of a megathrust earthquake of magnitude 7.1 or greater in this fault zone over the next 50 years. The scope matters. The estimate applies to the megathrust, the shallow boundary where the Juan de Fuca Plate slides beneath the North American plate, and it counts any event at or above magnitude 7.1, not only the largest possible rupture.

A magnitude 7.1 quake is a damaging event by any standard, yet the same page keeps its attention on the far bigger scenario. Oregon could face a magnitude 9.0-plus earthquake, the page says, with shaking estimated at five to seven minutes in the most affected areas and weakening with distance inland.

The January 1700 rupture and the Cascadia fault

The fault runs roughly 700 miles from northern California to British Columbia, about 70 to 100 miles off the coast, according to the Oregon agency. Its most recent major earthquake occurred on January 26, 1700, with an estimated magnitude of 9.0, dropping the coastline several feet and sending a tsunami across the ocean. The agency also reports that the fault has produced 43 earthquakes over the last 10,000 years, which tells planners that the 1700 event was one in a long series and not an outlier.

No written account from the Pacific Northwest records that night, so the date rests on evidence from the far side of the Pacific. Japanese records describe a destructive tsunami that arrived with no felt earthquake to explain it. A U.S. Geological Survey publication, The Orphan Tsunami of 1700 by B.F. Atwater and colleagues, traces how those Japanese accounts were tied to a parent earthquake in North America through a series of discoveries in the mid-1990s. The authors note that the work informs current earthquake and tsunami precautions in the Cascadia region.

Geologists do not read the long record as a clock. The Oregon Encyclopedia, which summarizes the science for general readers, says intervals between Cascadia earthquakes vary widely, from a few hundred years to more than a thousand. That spread is the reason the state expresses risk as a probability across a half century rather than as a predicted year, and it explains why a 37% figure and a statement that the fault is building pressure can both be true at once.

Isolated coastal communities and the Oregon Resilience Plan

The Oregon agency’s page warns that coastal populations could become isolated islands, cut off by landslides, liquefaction and damaged infrastructure such as bridges. It describes a Cascadia Island Mapping Pilot Project that compares those populations with likely damage to roads and bridges, and it says Oregon may be without services and assistance for at least two weeks, if not longer.

The scale of that disruption was spelled out in the 2013 Oregon Resilience Plan, prepared by the Oregon Seismic Safety Policy Advisory Commission for the legislature. For a magnitude 9.0 earthquake and tsunami under conditions at that time, the plan projected fatalities ranging from 1,250 to more than 10,000. Restoring electricity was estimated at one to three months in the valley and three to six months on the coast, while drinking water and sewer service on the coast could take one to three years to return. Those recovery times are the practical reason emergency managers focus on household supplies. The plan was written in 2013, so its numbers describe the state as it stood then, but the underlying logic has not changed: a long outage on the coast is built into the scenario.

Being two weeks ready

The state’s advice is to plan now to be two weeks ready, a standard that the agency applies to individuals, businesses, schools, government and communities alike. Reporting from KUOW explained that the recommended stockpile was raised from three days to a two-week supply, partly because a multistate exercise suggested help could take up to two weeks to reach some areas.

Andrew Phelps, the director of Oregon’s Office of Emergency Management, described the household preparedness goal in that reporting as almost a preparedness moonshot and acknowledged that the state did not yet know exactly how it would reach it. The candor is notable, because the target was large: the Oregon Resilience Plan set a goal of 250,000 households at that level by 2021.

For residents, the 37% estimate is best read as a planning input rather than a forecast. It says the odds of a significant megathrust quake in a lifetime are substantial, while the 1700 record and the 43-earthquake history say the largest events are part of the fault’s normal behavior. The state’s preparedness pages cover earthquake safety, tsunami safety and the Great Oregon ShakeOut, and the agency’s central message is that waiting for a prediction is not an option because none exists.

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