| Where do earthquakes occur? 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 such a slow rate, about the same as a fingernail grows, that we do not notice it in our everyday lives. For instance, the Juan de Fuca plate off the coast of B.C. moves only about 4-5 cm/year relative to the North American plate. Over time, however, this small movement can build up enough stress to produce significant earthquakes. Earthquakes occur most frequently on, or near, the edges of plates where stress is most concentrated.
British Columbia Plate TectonicsJust off the west coast, four plates meet and interact making offshore B.C. the most active earthquake zone in Canada.
Plates move towards each other at converging, apart at diverging and past each other at transform boundaries. All three of these boundary types occur in offshore B.C. 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. Further east, the Juan de Fuca plate is converging with and sliding beneath the North American plate. Another small plate, the Explorer, is also sliding underneath the North American plate, and at the same time the Juan de Fuca plate is sliding past it along the Nootka fault. In the north, there is a major transform fault boundary between the Pacific and the North American plates called the Queen Charlotte fault. This fault was the site, in 1949, of Canada's largest earthquake.
Earthquake patternsThe locations of several hundred earthquakes caused by the complex plate motions within B.C. are shown on the map below. Most of these were minor earthquakes in unpopulated regions and were not noticed by many people.
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 B.C. The largest dots on the map indicate the locations of large, historic quakes. 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.
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This page was last updated Wednesday, March 08, 2006 |
