Offshore Pacific Northwest — northern California to Vancouver Island · Subduction megathrust (thrust)
The Cascadia Subduction Zone is the only fault in the continental United States capable of magnitude-9 earthquakes. It is the plate boundary where the Juan de Fuca plate dives beneath the North American plate, running about 1,000 km offshore from Cape Mendocino in northern California to northern Vancouver Island. It last ruptured fully on January 26, 1700, in an earthquake estimated at M8.7–9.2 — and the tsunami it sent across the Pacific struck Japan with no earthquake felt there, the famous "orphan tsunami."
On the evening of January 26, 1700, the entire ~1,000 km megathrust ruptured. The earthquake's date and magnitude were pinned down by an elegant convergence of evidence: "ghost forests" of trees killed by 1–2 meters of sudden coastal subsidence in winter 1699–1700 (dated by tree rings), Indigenous oral histories, and Japanese written records of a tsunami that struck Honshu the night of January 27–28 with no local earthquake felt. The case was cracked between 1986 and 2005 (Satake, Atwater; USGS Professional Paper 1707). At least 19 great (M8+) megathrust ruptures have occurred along most of the coast in the past ~10,000 years — the most paleoseismically well-constrained subduction zone on Earth.
The shallow megathrust is fully locked — strong friction holds it to about 30 km depth — so the overriding North American plate is being slowly squeezed, storing strain that released in minutes in 1700. GPS shows the whole region deforming as that strain rebuilds. Deeper, the fault slips silently in episodic tremor and slip (ETS) every ~8–22 months, imperceptible at the surface; USGS notes no known link between ETS and megathrust earthquake hazard. Two rupture modes are recognized: full-margin (~M9) and partial-margin (~M8, more common in southern Cascadia).
Full-margin M~9 ruptures average about every 500 years (the figure the USGS National Seismic Hazard Model uses), though intervals have ranged from ~200 to over 800 years — it is not periodic. The USGS model considers Cascadia earthquakes up to M9.3 and publishes an M9.3 full-margin scenario. Roughly 326 years have elapsed since 1700 versus a ~500-year mean; "overdue" is loose language for an aperiodic process. The time-dependent probability is about 15% in 50 years for a full-margin rupture, and ~30% for an M8+ in southern Cascadia.
A full-margin rupture would produce minutes of shaking and a trans-oceanic tsunami. FEMA and Oregon emergency planners cite waves up to ~100 feet (~30 m) on the outer coast, arriving in roughly 15–30 minutes, with some coastal communities cut off from road access for about two weeks. FEMA's Cascadia Rising scenario puts more than 8 million people in the projected damage zone, with the broader affected region home to tens of millions across Oregon, Washington, and British Columbia.
Cascadia is the Pacific Northwest's dominant natural hazard and the reason for the region's tsunami preparedness. See the Oregon and Washington pages and our seismic risk lookup.