Issue 63
March 22, 2006

Weather Trivia

 You Asked Us

Environment Canada's knowledgeable specialists answer questions about wildlife, air pollution, water, weather, climate change and other aspects of the environment.

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What is an Earthquake?

When the earth formed 6 billion years ago, it was very hot. As it cooled a crust (the lithosphere) formed on the surface of the planet; similar to mud cracks forming as mud dries, this crust formed cracks dividing the crust into several large lithospheric plates. The viscous material under the lithosphere is the earth's mantle, which convects as it transports heat from the earth's core to the surface. As the mantle convects it moves the lithospheric plates on top of it, causing the plates to bump into one another, sliding over, under, or alongside an adjacent plate. Earthquakes are the result of the lithospheric plates bumping against one another and the stresses which build up within the plates.

The rates of plate movements range from about 2 to 12 centimetres per year. Sometimes, tremendous energy can build up within a single plate, or between neighbouring plates. If the accumulated stress exceeds the strength of the rocks making up these brittle zones, the rocks can break suddenly, releasing the stored energy as an earthquake. Ninety percent of the world's earthquakes occur along the boundaries of plates.

With the present state of scientific knowledge, it is not possible to predict earthquakes and certainly not possible to specify in advance their exact date, time and location. However, scientists know much more than they used to; it is now possible to forecast the geographical region within which earthquakes occur, a magnitude range associated with damaging earthquakes within that region, and the approximate time interval within which these large damaging earthquakes would occur. A great deal of research is being conducted to develop further understanding of earthquakes and to refine the ability to forecast them.

What About Canada?

Canada is very seismically active. Two-thirds of Canada's earthquakes occur in western Canada, because of the Juan de Fuca plate that goes beneath the North America plate in southwestern British-Columbia, and because there is a fault, between the Pacific and North America plates, which stretches along the coast from the northern tip of Vancouver Island to the Yukon. The remaining one-third of Canada's earthquakes occur in eastern Canada, primarily within the craton, an old and stable part of the continental crust, which comprises much of the interior of the North America plate. In much of Canada, although many faults are visible on the surface of the ground, most are millions of years old and are not currently active. About 3000 earthquakes occur every year in the country, with major earthquakes occurring several times each century.

The greatest earthquake in Canadian history occurred on January 26, 1700, along the Cascadia Subduction Zone (This is the region along which the Juan de Fuca plate system goes underneath the North America plate), extending 1000 kilometres from Vancouver Island to the top of California. This magnitude 9 earthquake produced a 10 metre high tsunami and was recorded in native oral histories of the Pacific northwest, within the geological deposits along the coast, and by tidal height records in Japan. However, the largest earthquake recorded by seismometers in Canada was a magnitude 8.1 in the Queen Charlotte Islands archipelago on August 22, 1949.

Earthquakes have occasionally been damageable to citizens and infrastructures in Canada. Substantial damage to some building structures occurred during the 1946 earthquake in Courtenay, British-Columbia, and the 1989 Saguenay, Quebec earthquake. Brick facings fell off buildings, chimneys rotated and occasionally broke off, building slid several inches off their foundations, windows broke, and roads slumped. One death was related to the 1946 Courtenay earthquake and loss of life occurred caused by the tsunami created in the 1929 "Grand Banks" earthquake, in Newfoundland.

The Magnitude Scale

The Richter magnitude scale is a logarithmic scale which describes the amount of surface displacement associated with an earthquake hypocentre. Depending on the size of the earthquake and the region within which it occurred, seismologists may use more than one method of calculating magnitude, however all these different methods are based on the Richter magnitude scale – they are all logarithmic and describe surface displacement. The only exception is moment magnitude, MW, which describes the amount of energy released at the earthquake hypocentre.

Magnitude is measured on a scale from 1 to 10. A magnitude 3 earthquake is usually felt and objects start shaking. Earthquakes above magnitude 5 to 5.5 have the potential to cause damage.

If you Experience an Earthquake

If you have felt an earthquake, you can go to Earthquake Canada's website for current updates. Please also take a moment to fill in the "Did you feel it" questionnaire on the Earthquake Canada website, this provides valuable information concerning the strength of shaking in the regions affected by the earthquake.