Variation in Nisqually Earthquake Ground Shaking across Greater Victoria, British Columbia

On February 28th 2001, the largest earthquake in more than fifty years struck the Pacific Northwest. The M_W=6.8 Nisqually earthquake, one of the largest earthquakes ever generated within the subducting Juan de Fuca plate, triggered strong motion instruments and was widely felt as far away as southern British Columbia. The Nisqually earthquake provides the first opportunity to examine the variation in shaking across Victoria to compare with recently published ground amplification maps based on the local geology. The Nisqually earthquake generated substantial ground motion in greater Victoria reaching maximum Modified Mercalli (MM) intensities of V-VI with reports of minor damage such as broken windows, broken pipes and chimney damage. Over seven hundred responses across Greater Victoria were collected by the Geological Survey of Canada, Pacific Division from an online web form and by detailed survey in areas of potential ground amplification comprising fifteen city blocks. Community Decimal Intensities (CDI) were computed and mapped in order to compare with published ground amplification maps for the greater Victoria area. Regions previously labelled moderate to high ground amplification suffered the highest CDI's. Correspondingly regions thought to be of very low to low ground amplification had the lowest CDI. Seven strong motion instruments recorded peak horizontal ground accelerations ranging from 1.5 to 3.5 % g. The strong motion data also provides the first opportunity to examine local site response and to compare with the predicted response based on detailed geologic information.