Most recorded earthquakes are caused by stress accumulation within the plates. In BC and offshore earthquakes have occurred either within the Juan de Fuca or the North American plates. None have occurred along the junction where the two plates are actually in contact. The apparent inactivity within the zone of contact between the plates suggests that either the plates are locked together and are accumulating strain or that the contact zone is well lubricated and the plates are moving smoothly past each other. This stress accumulation can result in a potentially very destructive type of earthquake, called a megathrust earthquake.
Although megathrust earthquakes have not been recorded in British Columbia, past earthquakes have left a geological record of this occurrence. After the earthquake, the strain accumulated is released and vertical deformation results in the over-riding plate that is the reverse of that seen during stress build up. Thus areas like Vancouver Island, that have risen slightly before the quake, suddenly subside.
In this animation the Juan de Fuca plate (dark brown is sliding under the North American Plate (light brown). The red bar indicates that the plates become locked and the edge of the NA plate begins to slowly rise and buckle. over a period for 200-300 years the energy built up causes the locked portion to give way and the upper plate snaps back into its original position. The vertical displacement scale has been exaggerated for the purpose of the animation.