The Threat of a Great Earthquake in
Southwestern British Columbia

Adapted from an article of the same title, published in
The BC Professional Engineer in 1995 (v. 46, no. 9, p. 4-8).

J.J. Clague
Geological Survey of Canada, Vancouver, British Columbia

P.T. Bobrowsky
British Columbia Geological Survey Branch, Victoria, British Columbia

R.D. Hyndman
Geological Survey of Canada, Sidney, British Columbia

Public awareness of the significant earthquake hazard in southwestern British Columbia has increased considerably in the last decade, due largely to media coverage of recent geological and geophysical findings in the area as well as the effects of destructive earthquakes in Mexico (1985), California (1989, 1994), and Japan (1995). During this period, earthquake preparedness initiatives have intensified, earthquake education has become more common in Vancouver-Victoria area schools, and some critical facilities, including dams, bridges, schools, and hospitals, have been seismically upgraded or replaced.

An accurate assessment of seismic hazards requires knowledge of the causes, sources, frequencies, and effects of earthquakes of different magnitudes, which is obtained through a wide range of geological and geophysical studies. In addition, ground motion attenuation with distance from the earthquake source zone must be determined and local site conditions assessed.

Written and instrumental records from the past 200 years show that there have been many moderate to large (Richter magnitude 6 to 7) earthquakes in southwestern British Columbia and in adjacent Washington state (Rogers, 1992, 1994). All well located earthquakes have originated either in the continental crust of the North America plate or deeper within the underthrusting Juan de Fuca oceanic plate, which moves beneath the North America plate at a rate of about 4 cm/year (Fig. 1). Until recently, it was thought that these were the only types of earthquakes to affect the region, but there is now strong evidence that much larger (magnitude 8 to 9) earthquakes have occurred beneath the continental slope and shelf off the west coast in the more distant past. These “great” quakes originate on the 1000 km long thrust fault that separates the Juan de Fuca and North America plates, in what is known as the Cascadia subduction zone.

For engineers involved in structural design, the first source of information on earthquake hazard requirements is the National Building Code of Canada (NBCC), whose seismic zoning and earthquake provisions currently reflect much smaller (magnitude 6 to 7), more frequent, and more widely distributed earthquakes, although the Code is being revised to more adequately incorporate the hazard associated with great subduction earthquakes. For some structures, additional detailed seismological and engineering studies are required to adequately deal with the earthquake hazard.

The following reviews the geophysical and geological evidence for great subduction earthquakes in southwestern British Columbia and argues that they must be considered in assessments of seismic risk for the region.