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Bulletin of the Seismological Society of America; October 1997; v. 87; no. 5; p. 1354-1361
© 1997 Seismological Society of America
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Soil response on the Fraser delta to the MW = 5.1 Duvall, Washington, earthquake

John F. Cassidy, Garry C. Rogers and Dieter H. Weichert

Geological Survey of Canada Pacific Geoscience Centre, P.O. Box 6000, Sidney, B.C., V8L 4B2 Canada

Abstract

Strong-motion recordings of the 3 May 1996 MW = 5.1 Duvall, Washington, earthquake made in southwestern British Columbia provide the best constraints, to date, on soil response to seismic shaking on the Fraser delta, just south of Vancouver. A total of nine strong-motion instruments in southern B.C. triggered during this earthquake, seven of which are on or near the Fraser delta. These recordings were made at epicentral distances of 140 to 180 km. A preliminary analysis of these data clearly shows that for low frequencies (<3 Hz) seismic spectral amplification (by a factor of 2 to 7) occurs on the Fraser delta relative to nearby bedrock or thin firm-soil sites. However, at higher frequencies, moderate attenuation is observed at the Fraser delta sites. The largest peak ground accelerations recorded (generally around 2 to 3 Hz) were not on the thickest delta sediments but nearer the edge of the delta. Peak amplification is more pronounced in the horizontal component of ground motion than in the vertical. The average SH/Z ratio for peak values at the Fraser delta sites is 4.5 ± 0.9 compared to 1.6 ± 0.6 for the bedrock and firm-soil sites. In addition to the strong-motion records, we also utilize five three-component broadband recordings of this earthquake made in southern B.C. (epicentral distances of 140 to 520 km), and 12 short-period vertical-component recordings (epicentral distances of 130 to 380 km) to document attenuation of peak acceleration with distance for this earthquake. The observed values are slightly lower than those predicted by the ground-motion relations proposed for the year 2000 National Building Code of Canada.




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