Ground motion, the definitive characteristic of earthquakes, causes damage directly by vibration and indirectly through secondary effects such as landslides and liquefaction. Structural design in earthquake-prone areas such as southwestern British Columbia is based on anticipated peak horizontal ground acceleration and velocity values provided by the Geological Survey of Canada. These parameters are derived from statistical analysis of past earthquakes and require an understanding of the causes of earthquakes in various regions and estimates of ground motion attenuation relationships (how quickly shaking decreases with distance from an earthquake). This information is incorporated into the National Building Code of Canada in the form of seismic zoning maps.

The map above shows seismic zones in British Columbia. The zones are based on levels of peak horizontal acceleration with a 10% probability of being exceeded over a 50-year period. Zone 6 has the highest values, >0.32 g (32% gravity); zone 0 has the lowest values (<0.04 g). This acceleration zoning map is centred near 5 Hertz (5 oscillations per second) a frequency of ground motion that can damage small or rigid structures. A similar zoning map (not shown) in the National Building Code for peak seismic velocity is centred near 1 Hertz, a frequency that can damage larger structures (e.g. 10-storey buildings). Engineers use this information to design earthquake-resistant structures. These maps represent the intensity of shaking on rock. Local geology and topography may amplify ground shaking at some frequencies and may de-amplify shaking at other frequencies. There is no map yet for Vancouver and the Fraser Valley showing areas of expected ground amplification or de- amplification. The map shown here does not include possible effects of rare, very large, subduction-type earthquakes west of Vancouver Island; shaking produced by these earthquakes will be considered in the next generation of seismic zoning maps.

Map based on P.W. Basham, D.H. Weichert, F.M. Anglin, and M.J. Berry, 1982, New probabilistic strong seismic ground motion maps of Canada: a compilation of earthquake source zones, methods and results, Geological Survey of Canada, Earth Physics Branch Open File 82-33, 202 pp.