The Cascadia Subduction Zone extends about 1,000 kilometres off the Pacific coast from northern California through Oregon and Washington to Vancouver Island. Here the small Juan de Fuca Plate dives beneath the North American Plate, forming a megathrust capable of the planet's largest earthquakes.
As a subduction zone, one plate is thrust beneath the other, locking the interface and building enormous strain that releases in sudden, powerful ruptures. Such events can reach magnitude 9 and generate destructive tsunamis that strike the coast within minutes.
Geological and historical evidence, including a tsunami recorded in Japan, dates the last great Cascadia earthquake to January 1700 at an estimated magnitude 9.0. With recurrence intervals of a few hundred years, scientists regard a future 'Big One' as a leading natural hazard for the Pacific Northwest.