Factsheet: Cascadia Subduction Zone

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• The Cascadia Subduction Zone goes from Cape Mendocino, California to northern Vancouver Island.
• Megathrust earthquakes occur at "subduction zones" which are the collision points between tectonic plates. Generally a huge slab of the ocean floor is forced underneath a continental plate. At magnitude 9 and higher, these are among the world's largest earthquakes. If this crack in the ocean floor rips apart in a vertical direction, it can lift a mountain of seawater (sometimes several kilometers deep), generating a series of enormous tsunami waves.
• The crack in the sea floor off Sumatra, where the devastating tsunami of December 26, 2004 occurred, is 1400 km long. This is almost exactly the same length (and it also has approximately the same width) as the Cascadia Subduction Zone.

City streets from Vancouver to Sacramento will be littered with glass and debris from a magnitude 9 earthquake in the Cascadia Subduction Zone.
Credit: Artifex Studios
• A megathrust earthquake at a magnitude of 9 or higher is highly likely for the cities of Sacramento, Portland, Seattle, Vancouver and Victoria plus hundreds of smaller towns from California to British Columbia. (Estimates of the Sumatra-Andaman Earthquake of 2004 are between 9.1 and 9.3.)
• Megathrust earthquakes can create 15-metre tsunami waves, which could hit the beaches on the West Coast in 20 minutes or less (just as they did in Sumatra). In a few local zones the Sumatra waves were as high as 30 metres (approximately 100 feet), and the same kinds of waves are expected from the Cascadia Subduction Zone.
• Marine Geologist Chris Goldfinger from Oregon State University shows viewers mud core samples that document 39 Cascadia fault ruptures over the past 10,000 years. 19 of those were full margin ruptures (magnitude 9 or higher) along the entire length of the subduction zone. Some of the quakes were 800 years apart, others only 200 years apart. The last full-margin megathrust earthquake happened more than 300 years ago - on the night of January 26th, 1700.