CGRG Bibliography of Canadian Geomorphology
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Author : Cassidy, J.F.; and Rogers, G.C.
Date : 2003.
Title : Variation in ground shaking on the Fraser River Delta (Greater Vancouver, Canada).
Publication : On the Edge: Earth Science at North America’s Western Margin. Annual Meeting of the GAC, MAC and SEG. Sheraton Wall Centre, Vancouver, British Columbia. May 25 - 28, 2003.
Issue :
Page(s) :
Abstract
The thick, soft soils of the Fraser River delta, just south of Vancouver, Canada, are home to critical infrastructure such as one of North America's busiest port facilities, Canada's second busiest airport, and key transportation and power-transmission facilities for 2-3 million people. This area is also one of the most seismically active regions in Canada. We have utilised recent three-component, digital records of recent moderate (1996 M=5.1 at 200 km distance, 1997 M=4.3 at 40 km distance) and large (2001 M=6.8 at 300 km distance) earthquakes to examine the response to seismic shaking in the greater Vancouver region, with an emphasis on the site response of the Fraser River delta. These earthquake recordings have relatively low amplitudes (maximum accelerations of 0.015 gravity (g) for the 1996 records, 0.024g for the 1997 records, and 0.035g for the 2001 records). The 1997 data set is significant as it contains the first three-component recordings made on bedrock in greater Vancouver, and the 2001 data set is significant as it contains long-period signal (1-10 second energy). We use bedrock, and firm soil recordings as a reference to compute the site response for each of the strong motion instrument soil sites in greater Vancouver. Our results show frequency-dependent amplification, with factors of up to 12 times (relative to competent bedrock) near the edge of the delta. Here, the amplification is observed over a relatively narrow frequency range of 1.5-4 Hz (0.25-0.67 s period). Near the centre of the delta (where the soft soils are thickest) peak amplification of 4-10 times (relative to competent bedrock) is measured. Relative to firm soil, the peak amplification ranges from 2-5 for the thick soil delta centre sites, and 2-6 for the delta edge sites. At higher frequencies, little or no amplification, and in many cases slight attenuation is observed. The more distant earthquakes (200-300 km) present a simpler and more predictable picture of ground motion variation than that of the 1997 earthquake (40 km distant). The Geological Survey of Canada is currently deploying a demonstration dense urban seismograph network (~1km spacing) which crosses the northern edge of the Fraser delta in the greater Vancouver area to address varying site response in more detail.
Bibliography of Canadian Geomorphology