larger image [JPEG, 279.7 kb, 1000 X 803]

A relative hazard map of Greater Victoria (for moderate shaking). NOTE: This map should not be used for potential hazard evaluation of specific properties.

When the Ground Moves The effects of an earthquake of given magnitude vary due to the distance from the epicentre as well as local geological and topographic conditions. Ground shaking can be increased (amplified) or reduced (attenuated) by soils overlying bedrock. The map above shows the estimated variation in ground motion due to differences in soils for an earthquake that produces moderate shaking. For this situation, the most common to be expected in the Greater Victoria area, amplification potential is greatest (red) in areas underlain by thick deposits of soft clay, particularly where they are capped by peat and organic soils and lowest (green) where bedrock is exposed. For very strong shaking caused by a nearby earthquake the pattern would be very different. For a full discussion of this subject the reader is referred to the reference listed in "Additional Reading". Water saturated loose sands are susceptible to liquefaction during prolonged strong shaking. When soils liquefy they lose their strength and cannot support structures built upon them. In the Victoria area the liquefaction potential is greatest in geologically-young beach-sands and artificial fills. Many sandy shoreline deposits along the east coast of Vancouver Island liquefied during the 1946 Vancouver Island earthquake.

Earthquake-induced shaking can cause slope instability (landslides). In Greater Victoria, the slope instability hazard is greatest along sea cliffs such as those bordering Cordova Bay and some cliffs facing the Strait of Juan de Fuca as well as in valleys cut into soft glacial sediments. Most rock slopes appear to be relatively stable, although some areas of less stable bedrock occur in the Mount Finlayson/Malahat/Goldstream River region.

Additional information can be obtained on the Government of British Columbia, Ministry of Energy and Mines - Programs and Servies web site.