The Cascadia subduction zone can produce very large earthquakes ("megathrust earthquakes"), magnitude 9.0 or greater, if rupture occurs over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Great Subduction Zone earthquakes are the largest earthquakes in the world, and can exceed magnitude 9.0.
Earthquake size is proportional to fault area, and 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 and North American plates. Because of the very large fault area, the Cascadia Subduction Zone could produce a very large earthquake. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.
In 1999, a group of Continuous Global Positioning System sites registered a brief reversal of motion of approximately 2 centimeters (0.8 inches) over a 50 kilometer by 300 kilometer (about 30 mile by 200 mile) area. The movement was the equivalent of a 6.7 magnitude earthquake.
The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.
Source: www.wikipedia.com