Geology
The zone separates the Juan de Fuca Plate, Explorer Plate, Gorda Plate, and North American Plate. Here, the oceanic crust of the Pacific Ocean has been sinking beneath the continent for about 200 million years, and currently does so at a rate of approximately 40 mm/yr.
The width of the Cascadia subduction zone 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. On the Hyndman and Wang diagram (not shown, click on reference link below) the "locked" zone is storing up energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture.
The Cascadia subduction zone runs from triple junctions at its north and south ends. To the north, just below Queen Charlotte Island, it intersects the Queen Charlotte Fault and the Explorer Ridge. To the south, just off of Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino fault zone at the Mendocino Triple Junction.
Read more about this topic: Cascadia Subduction Zone