Along a tectonic rift, an area where plates are still attached to each other. The Cascadia subduction zone stretches from California to British Columbia. All along this zone, the subducting plates are forced beneath the North American Plate, but around 30 km down the plates have become locked. Below this zone the plates are 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, causing earthquakes.