The Cascadia subduction zone (also referred to as the Cascadia fault) is a subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to northern California. It is a very long sloping fault that separates the Juan de Fuca and North America plates.
New ocean floor is being created offshore of Washington and Oregon. As more material wells up along the ocean ridge, the ocean floor moves toward and beneath the continent.
The North American Plate also moves, in a general southwest direction, overriding the oceanic plate. The Cascadia Subduction Zone is where the two plates meet.
Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism that has included such notable eruptions as Mount Mazama (Crater Lake) about 7,500 years ago, Mount Meager about 2,350 years ago and Mount St. Helens in 1980.
Major cities affected by a disturbance in this subduction zone would include Vancouver and Victoria, British Columbia; Seattle, Washington; Portland, Oregon; and Sacramento, California.
Source: www.wikipedia.com