VARIATION IN GROUND SHAKING ON THE FRASER RIVER DELTA(GREATER VANCOUVER, CANADA) FROM ANALYSIS OFMODERATE EARTHQUAKES

摘要

The thick, soft soils of the Fraser River delta, just south of Vancouver, Canada, are home to critical infrastructuresuch as one of North America's busiest port facilities, Canada's second busiest airport, and key transportation andpower-transmission facilities for 2-3 million people. This area is one of the most seismically active regions inCanada.We have utilised recordings of recent moderate (1996 M=5.1 at 180 km distance, 1997 M=4.3 at 40 km distance)and large (2001 M=6.8 at 230 km distance) earthquakes to examine site response in the greater Vancouver, region,with an emphasis on the Fraser River delta. These suites of accelerograms have relatively low amplitudes ( 0.015gfor the 1996 records, 0.024g for the 1997 records, and 0.035g for the 2001 records). The 1997 data set is significantas it contains the first three-component recordings made on bedrock in greater Vancouver, and the 2001 data set issignificant as it contains longer-period energy. We compute spectral ratios to estimate the site response for each ofthe soil sites.We find frequency-dependent amplification of up to 12 times (relative to competent bedrock) near the edge of thedelta. Here, the amplification occurs over a relatively narrow frequency range of 1.5-4 Hz (0.25-0.67 s period). Nearthe centre of the delta (where the soft soils are thickest) peak amplification of 4-10 times (relative to bedrock) ismeasured. Relative to firm soil, the amplification ranges from 2-5 for the thick soil delta centre sites, and 2-6 for thedelta edge sites. At higher frequencies, little or no amplification or even slight attenuation is observed.The Geological Survey of Canada is currently deploying a dense urban seismograph network (～1km spacing) whichcrosses the northern edge of the Fraser delta to address varying site response in more detail.