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During an earthquake, energy is released when rocks under stress slide past one another along a fault, or fracture in the Earth's crust. These situations most commonly occur at, or close to the boundaries of moving tectonic plates that make up the outer shell of our planet. Southern Vancouver Island is situated above the boundary separating the oceanic Juan de Fuca Plate and the continental North America Plate. Beneath the boundary, or Cascadia subduction zone, the eastward-moving oceanic plate is descending, or being subducted, beneath the westward-drifting continent (b).

The magnitude scale (sometimes called the Richter Scale) is used to measure the size, or energy release of an earthquake. The scale is logarithmic, meaning that a magnitude 7 earthquake produces a ground displacement 10 times greater than a magnitude 6 event and 100 times greater than one of magnitude 5, and so on. The amount of energy released in an earthquake increases by about thirty-two times with each unit increase in magnitude.

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Distribution of epicentres (magnitudes 1 to 6) for earthquakes within the Juan de Fuca Plate (red) and the North America Plate (blue) for the period 1985 to 2000.

More than 200 small earthquakes are recorded each year in this region, several of which are felt. Those large enough to cause damage occur roughly once each decade, whereas the very large subduction zone earthquakes happen centuries apart.

On southern Vancouver Island, earthquakes occur within the subducting Juan de Fuca Plate (red dots, Fig. a, c) and in the overlying North America Plate (blue dots). Figure a. shows the locations and depths of those earthquakes above magnitude 1.5 recorded between 1985 and 2000 and within 10 km. on either side of the vertical face of the diagram. The vertical, east-west face passes just south of the city of Victoria as shown on Figure a. The largest earthquake recorded to date on Vancouver Island was a magnitude 7.3 event in 1946, located within the North America Plate beneath the central part of the Island (blue star on Figure c).

Additional information can be obtained on the Earthquakes Canada Web site.