Cascadia Subduction Zone: Earthquake Risk, History & Live Activity
The Cascadia Subduction Zone is a roughly 1,100-kilometre megathrust where the Juan de Fuca, Explorer, and Gorda plates dive beneath the North American plate, offshore from northern California to Vancouver Island. Unlike California's strike-slip faults, this is a subduction boundary capable of magnitude 8 to 9 earthquakes and ocean-crossing tsunamis. The plates converge at roughly 30 to 45 millimetres a year, and the offshore fault is largely locked between events.
How likely is a large earthquake?
Paleoseismic turbidite and coastal studies, led over decades by Chris Goldfinger and colleagues at Oregon State University, put the average interval between great Cascadia earthquakes at roughly 500 years, but the record is irregular: the southern margin has ruptured more often, on the order of every 200 to 300 years, than the northern margin. The USGS and regional scientists estimate roughly a 10 to 15 percent chance of a full-margin magnitude 9 earthquake in the next 50 years, and a higher chance, commonly cited near 30 percent, of a magnitude 8 or larger event on the southern portion in that window.
The last great earthquake struck on 26 January 1700, a date pinned down because its tsunami was recorded in Japan. With more than 300 years elapsed against an average near 500, the margin is well within the range where a future event is plausible, though not predictable to any specific date.
Live seismic activity near this fault
Counts include M2.5+ events — the smallest size USGS reports consistently worldwide.
| Magnitude | Time (UTC) | Depth | Location |
|---|---|---|---|
| M4.2 | Jul 23, 19:16 UTC | 10 km | 150 km WNW of Port Orford, Oregon |
| M3.0 | Jul 22, 05:09 UTC | 10 km | 189 km W of Bandon, Oregon |
History
Cascadia produces great earthquakes infrequently, so its record comes mainly from geology and from written tsunami accounts abroad rather than from instruments.
- 1700 Cascadia (estimated M8.7 to 9.2) — a full-margin rupture whose orphan tsunami was documented along the coast of Japan.
- Paleoseismic events — turbidite records identify roughly 40 great earthquakes over the past 10,000 years.
- 1992 Cape Mendocino (M7.2) — a thrust earthquake at the southern end of the subduction zone in northern California.
- 2001 Nisqually (M6.8) — a deep earthquake within the subducting plate beneath Puget Sound, not on the megathrust itself.
Frequently asked questions
Is Cascadia overdue for the Big One?
The average interval between great Cascadia earthquakes is about 500 years, and the last was in 1700, roughly 325 years ago. That places the margin within its typical window but not past its average, so overdue overstates it. The southern margin, which ruptures more often, is closer to its average interval. Scientists express the hazard as a probability over decades, not a countdown.
What would a magnitude 9 Cascadia earthquake do?
Regional planning scenarios project three to five minutes of strong shaking across western Oregon, Washington, and northern California, followed by a tsunami reaching low-lying coastlines within 15 to 30 minutes. Inland cities such as Portland and Seattle would see significant shaking damage, while the coast faces the added tsunami hazard. Agencies advise coastal residents to move to high ground as soon as shaking stops.
Nearby regions
Live counts from the USGS earthquake feeds within 400 km of the fault. Magnitudes and depths are automated readings and may be revised. Not for emergency use.