ABSTRACT: The earthquake threat in southwestern British Columbia: A geological perspective.

CGRG Bibliography of Canadian Geomorphology
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Nine moderate to large earthquakes (moment magnitude, Mw, 6-7.5) have struck southwestern British Columbia and adjacent Washington state in the last 130 years, most recently in 1965 at Seattle. Some of the earthquakes occurred on faults within the crust of North America. Others had much deeper sources, in the Juan de Fuca plate which is subducting beneath North America at the Cascadia subduction zone. Geological evidence suggests that a third, much larger (Mw 8-9) type of earthquake has occurred in this region, i.e. at the boundary between the Juan de Fuca and North America plates. These plate-boundary, or ‘subduction’, earthquakes are much rarer than Mw 6-7 crustal and subcrustal events. However, by virtue of their size, they affect a much larger area. The last great plate-boundary earthquake, in January 1700 probably exceeded Mw 9 and produced a tsunami that devastated the Pacific coast from Vancouver Island to northern California. Geological evidence and historical earthquakes indicate that future quake damage in southwestern B.C. will result from fire, strong ground motions, tsunamis, landslides, liquefaction, and possibly coseismic land-level change. The intensity of ground shaking is critically depending on local geology and topography and can vary by a factor of three or four over relatively short distances. Seismic waves generated by Cascadia plate-boundary earthquakes attenuate over the 100-150 km that they travel from their source to major cities on the south coast, and ground accelerations at Vancouver and Victoria probably would not exceed those of a strong local quake. The strong shaking of a great earthquake, however, would last much longer (up to 2-3 minutes) than that of a large local quake. Western Vancouver Island is at greatest risk from tsunamis. A Cascadia plate-boundary earthquake would generate a large tsunami that would strike Vancouver Island soon after the shaking stops. The tsunami would attenuate as it moves eastward along Juan de Fuca Strait and into the Strait of Georgia, and probably would not significantly damage Vancouver. Highways and rail lines that connect Vancouver to the rest of Canada follow steep-walled valleys. They would probably be blocked in many places by landslides during a strong earthquake. Seismic microzonation mapping and geological studies have identified areas in and around Victoria and Vancouver that are susceptible to liquefaction. Buildings and other engineered structures in areas susceptible to seismic liquefaction must be designed to prevent damage due to irregular settling and fissuring of the ground above a liquefied layer. A great plate-boundary earthquake might cause western Vancouver Island to subside 1 m or more, resulting in some flooding of low-lying areas. Coseismic uplift or subsidence might also occur within the epicentral area of a large crustal earthquake.

Bibliography of Canadian Geomorphology