The Subduction Zone

The Cascadia Subduction Zone stretches from Northern Vancouver Island to Cape Mendocino California. It separates the Juan de Fuca Plate, a 700-mile chunk in the Pacific, and the North America plate, where we live.

For more than 300 years, the Juan de Fuca Plate has been pushing underneath the North America plate. But the subduction is stuck. Relatively soon, the Juan de Fuca plate will be released like a spring.

That massive shift is expected to cause a major earthquake and Tsunami, collapsing many structures and causing the ground to sink six feet or more in places.

It’s been 316 years since the last one in 1700. Here’s a PowerPoint by Scott Burns of Portland State on our chances of seeing a Big Cascadia quake and a video on Earthquakes in Oregon – Past, Present and Future.

The anticipated subduction zone earthquake and resulting tsunami would be similar to the 1964 Alaska earthquake and the 2011 Japanese Earthquake. If the whole shelf from Canada to California goes, it might be worse.

“Over the past 10,000 years, there have been 19 earthquakes that extended along most of the margin, stretching from southern Vancouver Island to the Oregon-California border,” according to Oregon State’s Chris Goldfinger. “These would typically be of a magnitude from about 8.7 to 9.2 – really huge earthquakes.”

If a full-margin rupture happens (as it has before), the Northwest will suffer the worst natural disaster in the history of North America, according to a widely quoted New Yorker article.

If only the southern part of the Cascadia subduction zone gives way the resulting quake will be somewhere between 8.0 and 8.6.

If the entire Cascadia subduction zone gives way – what seismologists call a full-margin rupture – the magnitude will be somewhere between 8.7 and 9.2.

“Major earthquakes tend to strike more frequently along the southern end – every 240 years or so – and it has been longer than that since it last happened,” according to Chris Goldfinger.

The last full margin earthquake to strike the Pacific Northwest occurred on Jan. 26, 1700. Researchers know this, Goldfinger said, because written records in Japan document how an ensuing tsunami destroyed that year’s rice crop stored in warehouses.

“Roughly 3,000 people died in San Francisco’s 1906 earthquake. Almost 2,000 died in Hurricane Katrina. Almost 300 died in Hurricane Sandy.

“FEMA projects that nearly 13,000 people will die in the Cascadia earthquake and tsunami. Another 27,000 injured. FEMA expects that it will need to provide shelter for 1 million displaced people, and food and water for another 2.5 million.

We’ve got a situation. A big earthquake is coming and we’re not prepared.