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.