A 750-mile long subduction fault along the west coast of North America, stretching from Vancouver, BC to Cape Mendocino, CA. Called a subduction "zone" because, unlike the famous San Andreas Fault or others where the fault is linear as plates slide past each other, the subduction fault is along a plane as one tectonic plate sinks beneath another. In the case of the Cascadia subduction zone, it's the remnants of the ancient Farallon Plate subducting underneath the North American Plate: the Juan de Fuca Plate in the north and the Gorda Plate in the south. See the excellent ASCII map at the Juan de Fuca Plate node.

Diagram of the subduction zone:

              __/\__/  \_________
____________/     N. AMER. PLATE
 JUAN DE FUCA    _    \_
    PLATE       /|\     \_
                 |        \
                 |         \
    Subduction Zone------>  \
The subduction zone causes a continuous upward thrust along the coast, forming the Coast Range mountains. Also, the heat generated at the lower, farther inland parts of the zone causes rising magma, creating the volcanoes of the great Cascade Moutains.

While it was originally thought that the locked zone of the Cascadia Subduction Zone—the plane along which the plates interface—was relatively narrow, thus meaning the anticipated major "megathrust" earthquakes would tend to be smaller than in other subduction zones around the world. However, recent studies1 indicate that the zone extends farther inland than originally thought. Additionally, the zone appears to be completely locked, rather than the plates slowly sliding past each other, resulting in immense build-up of strain. This indicates that the next "big one" quake to be produced by the fault is more likely to be in the magnitude range of ~9.0 rather than ~8.0-8.5 originally thought. The last quake to strike the fault was on January 26, 1700 (as determined by a combination of carbon-dating and Japanese tsunami records), meaning that the next catastrophic earthquake will likely strike the region sometime between right about now and the year 2200.

(1) http://www.oregonlive.com/news/99/05/st050408.html

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