"The Cascadia subduction zone stretches from Cape Mendocino in northern California to Brooks
Peninsula on Vancouver Island in British Columbia, a distance of about 700 miles (1,130 km). All along
this zone, which begins beneath the seafloor to the west and extends inland towards the Cascade and
Coastal mountains, the subducting plates are forced beneath the North American Plate. At a relatively
shallow depth (less than about 20 miles/30 km down), the plates have become stuck. Below this locked
zone, warmer temperatures make the plates more pliable, allowing them to move more readily past
each other. This freer movement deeper down causes strain to accumulate along the locked zone.
Once that strain is great enough to overcome the friction that keeps the plates locked, the fault will
rupture: the edge of the North American Plate will lurch suddenly upwards and southwestwards as the
subducting plates slip under and northeastwards. With this movement, the deformed western edge of
the North American Plate will flex, causing the land along large sections of Cascadia" [0]
Also check the diagram(s) for a better understanding.
[0]http://crew.org/sites/default/files/cascadia_subduction_scen...