The Cascadia Subduction Zone

At about nine o'clock on the night of January 26, 1700, the coast of the Pacific Northwest dropped and the sea came in. No written record survives from North America, but the tsunami crossed the Pacific and flooded villages in Japan ten hours later, where officials noted a wave with no earthquake felt. Three centuries on, scientists matched that orphan tsunami to drowned forests along the Washington and Oregon coast. The Cascadia Subduction Zone, where an ocean plate slides beneath North America, had produced a magnitude 9 quake. It has been almost silent since, and the cities that grew up above it, from Vancouver to Portland, are still adapting to what that means.

The zone runs about 1,000 kilometers from Cape Mendocino in northern California to the northern tip of Vancouver Island, roughly 100 to 150 kilometers offshore. There the small Juan de Fuca, Gorda, and Explorer plates sink eastward beneath the North American Plate at about 30 to 40 millimeters a year. The interface is locked, so the coast is being squeezed and slowly lifted, and the strain will be released when the fault slips. Melting of the descending slab feeds the Cascade volcanoes, including Mount St. Helens, Mount Rainier, and Mount Hood. Coastal towns from Eureka to Tofino face tsunami waves arriving within 15 to 30 minutes of a great quake, while Seattle, Portland, and Vancouver face long, strong shaking.

Geologist Brian Atwater's work on ghost forests and buried marshes in the 1980s, and the Japanese tsunami records analyzed in 1996, established that Cascadia has produced at least 19 great earthquakes over about 10,000 years. A 2025 USGS fact sheet gives full-margin magnitude 9 ruptures an average recurrence of about 500 years, with intervals ranging from a few centuries to about a thousand, and a 10 to 15 percent chance of one in the next 50 years. Southern Cascadia also breaks in smaller magnitude 8 events, raising its 50-year chance to about 30 percent. Oregon and Washington have since moved schools out of tsunami zones, built vertical-evacuation towers, and joined the ShakeAlert early-warning system.

• Japan
  An archipelago empire of volcanoes, technology, and deep tradition 1700 tsunami recorded in
• Pacific Ocean
  Earth's largest and deepest ocean beneath
• The 1960 Valdivia Earthquake
  The most powerful earthquake ever recorded, a magnitude 9.5 megathrust rupture in southern Chile with a Pacific-wide tsunami comparable megathrust earthquake
• The 2011 Tōhoku Earthquake and Tsunami
  Japan's strongest recorded earthquake, whose tsunami killed nearly 16,000 people and caused the Fukushima Daiichi nuclear accident comparable megathrust earthquake
• The Tectonic Plates
  The seven great slabs and dozens of smaller ones that make up Earth's moving shell convergent plate boundary
• Mount St. Helens
  The Cascade volcano whose 1980 blast tore off its own summit feeds
• Canada
  The world's second-largest country, from Atlantic to Pacific to Arctic located off
• United States
  A continental superpower of fifty states and astonishing variety located off
• The Ring of Fire
  The Pacific rim of subduction, where the ground does not hold still part of
• Vancouver
  Canada's Pacific gateway, between the sea and the Coast Mountains threatens