Stress that causes earthquakes is created by movement of almost rigid plates that fit together and make up the outer shell of the Earth. These plates float on a dense, more fluid layer just beneath them. The plates move at a very slow rate, about the same as a fingernail grows. For example, the Juan de Fuca plate off the coast of British Columbia moves only about 4-5 cm/year relative to the North American plate. Over time this small movement can build up enough stress to produce significant earthquakes.
Earthquakes occur most frequently on, or near, the edges of plates where the stress is most concentrated. Just off the west coast, four plates meet and interact making offshore British Columbia the most active earthquake zone in Canada. The plates converge, diverge and slide past each other at transform boundaries. About 200 kilometres off the west coast of Vancouver Island, the Juan de Fuca plate and Pacific plate are diverging or spreading apart along the Juan de Fuca ridge. The fault boundary between the Pacific and the North American plates called the Queen Charlottefault, was the site, in 1949, of Canada’s largest earthquake.
Along the Canadian west coast, earthquakes large enough to cause damage occur about every ten years. In the last 100 years, several major earthquakes have occurred in B.C. or along its coast. In addition, large earthquakes occurring near the borders with Alaska and Washington were strongly felt in British Columbia. Property damage from these large quakes was minimal because the affected regions had only small populations at that time. However, if one of these large earthquakes occurred near an urban centre, the results could be disastrous.