Megathrust earthquakes are the most powerful earthquakes known to occur, and can exceed magnitude 9.0. They occur when enough energy (stress) has accumulated in the "locked" zone of the fault to cause a rupture known as a megathrust earthquake. The magnitude of a megathrust earthquake is proportional to length of the rupture along the fault. The Cascadia Subduction Zone, which forms the boundary between the Juan de Fuca and North American plates, is a very long sloping fault that stretches 600 miles from mid-Vancouver Island South to the Oregon California border.
The San Andreas Fault in California is not a powerful mega thrust fault. The mega thrust Cascadia Fault just off the Oregon and Washington Coast, is one of only five mega thrust faults worldwide that are known to exist.