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The earthquake threat in British Columbia

Beneath our feet, Canada is constantly atremble. Earthquakes shake the country about 2,500 times per year, most too small to feel. But occasionally, and without warning, the earth's crust below Canada buckles and spasms to frightening effect. More dangerous are the tsunamis that such quakes can cause. CBC Archives looks back at notable Canadian quakes, fears about "the big one" predicted for the West Coast and scientists' efforts to better understand the threat from below.

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Due to their proximity to Canada's most volatile fault line British Columbians live every day under the threat of a possible earthquake. But researchers have repeatedly warned of a so-called 'megaquake' that looms deep beneath the coastline and could affect millions. This clip from a 1988 CBC Television documentary investigates the likelihood of a such a megaquake in the country's earthquake hotspot. 
• For the more than 2.7 million people who live in southwest British Columbia earthquakes are an accepted fact of life. Scientists estimate that shallow 'crustal' quakes, which result in barely detectable tremors, are a near weekly occurrence in the area. Major 'subcrustal' quakes, which are capable of significant damage, happen about every 10 years.
• West Coast seismologists are now most worried about the possibility of a rare 'megathrust' earthquake which would submerge a large swath of coastline and send strong tremors hundreds of kilometers inland.

• Also called subduction zone quakes, these kinds of earthquakes are believed to occur every 500 years. Unpredictable and extremely powerful, they are the result of the sudden slipping of tectonic plates that have been slowly rubbing against each other for centuries.
• As researchers in this clip explain, British Columbia's dominant tectonic feature is the Cascadia Subduction Zone which is located far below the southwestern part of the province.

• The Juan de Fuca plate is currently being forced beneath the North America plate which is responsible for most of the quake activity in the region.
• If the Juan de Fuca plate slips, the resulting megaquake would instantly hit Vancouver, Victoria, Seattle and Portland, Ore. The tremors would also likely hit several hundred kilometres inland.
•Researchers say there is a 70 per cent probability that a subduction quake will occur in British Columbia within the next 200 years.

• The last subduction zone quake in British Columbia happened in January 1700 and had a magnitude of 9. One of the history's greatest earthquakes, it was felt on both sides of the Pacific Ocean and caused extensive destruction and death in Japan.
• Native oral traditions from Vancouver Island suggest the tsunami destroyed villages in Pachena Bay and that the shaking damaged houses in the Cowichan Lake region on the southern island.

• Earthquakes are caused by the abrupt movement of rocks separated by a geological crack, or fault. The resulting vibrations can cause damage to buildings, roads, bridges and pipelines. If the fault is underwater, the movement can cause a sudden displacement of water and a series of large waves called a tsunami. Earthquakes usually begin with a loud noise, such as a bang or a roar, followed by vibrations or swaying lasting up to several minutes.

• Earthquake hotspots in Canada are: the west and southwest regions of British Columbia, including Vancouver Island and the Queen Charlotte Islands; the St. Lawrence Valley, particularly Quebec's Charlevoix region; the high Arctic; and Yukon.

• Ninety per cent of the world's earthquakes occur along the edges of tectonic plates that make up the earth's outer crust. There are 13 major plates. Heating and cooling of rock beneath them causes the plates to move between two and 12 centimetres per year. They sometimes bump and rub against each other, building up pressure. When the pressure is stronger than the brittle rock edges, the rock breaks and the plates lurch, releasing tremendous energy.

• When scientists describe the strength of an earthquake as its magnitude, the number given is a composite of several types of equations and instruments (of which Richter is only one). At magnitude 5, a moderate quake, people can begin to feel movement; dishes might rattle. Magnitude 6 is considered strong while magnitude 7 is a major quake that can do significant damage and be felt for hundreds of kilometres. The biggest quake ever was a magnitude 9.5 in Chile in 1960.

• The Geological Survey of Canada, part of the federal Natural Resources department, monitors earthquake activity around the clock using seismograph stations across the country. The agency does research on causes and hazards that are incorporated into the national building code. Universities also do earthquake research. Tsunami research is also conducted by agencies within Fisheries and Oceans Canada.
Medium: Television
Program: The Journal
Broadcast Date: Nov. 28, 1988
Guest(s): John Adams, Dick Austin, Fred Cooper, Gary Rogers, Dieter Weichert
Reporter: Jerry Thompson
Duration: 14:27

Last updated: March 27, 2014

Page consulted on September 10, 2014

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