The Cascadia subduction zone is a very long sloping fault that stretches from mid-Vancouver Island to northern California. It separates the Juan de Fuca Plate and the North American Plate. Here, the oceanic crust of the Pacific Ocean is pushed toward and beneath the continent at a rate of 40 mm/yr.

The width of the Cascadia subduction zone fault varies along its length, depending on the temperature of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten, it eventually loses the ability to store mechanical stress and generates earthquakes.

Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. The last known great earthquake in the northwest was in January of 1700, the Cascadia Earthquake, just over 300 years ago. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 400 to 600 years. There is also evidence of accompanying tsunamis.

Other similar subduction zones in the world usually have such earthquakes every 100–200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.

Famous volcanoes associated with this zone are Mt. St. Helens and Mt. Rainier.

See also

• North Cascades National Park
• Cascade Mountains
• Subduction zone
• Plate tectonics

External links

• http://www.pgc.nrcan.gc.ca/seismo/hist/mega1.htm