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.
Earthquake Distribution Map
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, Figures a and 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).
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.
Waiting for the 'Big One'
Giant earthquakes have occurred along the Cascadia Subduction Zone throughout the past several million years. The last one, on January 26, 1700, caused a tsunami that produced large waves on Vancouver Island and waves which crossed the Pacific Ocean bringing damage to coastal Japan. Geological evidence of this earthquake and earlier large earthquakes is well preserved along the coasts of British Columbia and northwestern United States where buried salt-marsh peat deposits attest to sudden sinking of coastal areas during the earthquakes.
This mathematical simulation shows the tsunami created by the Cascadia Subduction Zone earthquake on January 26, 1700, as it reaches Hawaii on its way across the Pacific Ocean.
Changes in the elevations and distances between positions on the Earth’s surface can be determined from precise measurements obtained from satellites of the global positioning system (GPS).
With the use of GPS satellite technology and other geodetic methods it is possible to make extremely accurate measurements of distances between points on the Earth’s surface and to detect changes in their elevations. Employing these and other techniques, scientists have concluded that the Juan de Fuca Plate is currently locked to the North America Plate along the Cascadia Subduction Zone fault beneath the edge of the continent. Where the fault is locked, the temperature is estimated to be less than 350°C. Whereas at temperatures less than 350°C, rocks behave in a brittle fashion and are thus susceptible to earthquakes, those at higher temperatures tend to flow while under stress, with no earthquakes occurring. Due to locking, the western part of Vancouver Island is being flexed upward (shown here as greatly exaggerated) and compressed horizontally as the North America and Juan de Fuca plates converge upon one another. Eventually the locked zone must release, causing a giant earthquake (magnitude 8 to 9) to occur at any time within the next few hundred years. At that time, areas along Vancouver Island’s west coast will suddenly drop by as much as 1 m or more.
NOTE: This map should not be used for potential hazard evaluation of specific properties.
A relative hazard map of Greater Victoria (for moderate shaking).
When the Ground Moves The effects of an earthquake of given magnitude vary due to the distance from the epicentre as well as local geological and topographic conditions. Ground shaking can be increased (amplified) or reduced (attenuated) by soils overlying bedrock. The map above shows the estimated variation in ground motion due to differences in soils for an earthquake that produces moderate shaking. For this situation, the most common to be expected in the Greater Victoria area, amplification potential is greatest (red) in areas underlain by thick deposits of soft clay, particularly where they are capped by peat and organic soils and lowest (green) where bedrock is exposed. For very strong shaking caused by a nearby earthquake the pattern would be very different. For a full discussion of this subject the reader is referred to the reference listed in "Additional Reading". Water saturated loose sands are susceptible to liquefaction during prolonged strong shaking. When soils liquefy they lose their strength and cannot support structures built upon them. In the Victoria area the liquefaction potential is greatest in geologically-young beach-sands and artificial fills. Many sandy shoreline deposits along the east coast of Vancouver Island liquefied during the 1946 Vancouver Island earthquake.
Earthquake-induced shaking can cause slope instability (landslides). In Greater Victoria, the slope instability hazard is greatest along sea cliffs such as those bordering Cordova Bay and some cliffs facing the Strait of Juan de Fuca as well as in valleys cut into soft glacial sediments. Most rock slopes appear to be relatively stable, although some areas of less stable bedrock occur in the Mount Finlayson/Malahat/Goldstream River region.