SOIL RESPONSE ON THE FRASER DELTA TO THE M-W=5.1 DUVALL, WASHINGTON, EARTHQUAKE

Strong-motion recordings of the 3 May 1996 M-W = 5.1 Duvall, Washingto n, earthquake made in southwestern British Columbia provide the best c onstraints, to date, on soil response to seismic shaking on the Fraser delta, just south of Vancouver. A total of nine strong-motion instrum ents in southern B.C. triggered during this earthquake, seven of which are on or near the Fraser delta. These recordings were made at epicen tral distances of 140 to 180 km. A preliminary analysis of these data clearly shows that for low frequencies (<3 Hz) seismic spectral amplif ication (by a factor of 2 to 7) occurs on the Fraser delta relative to nearby bedrock or thin firm-soil sites. However, at higher frequencie s, moderate attenuation is observed at the Fraser delta sites. The lar gest peak ground accelerations recorded (generally around 2 to 3 Hz) w ere not on the thickest delta sediments but nearer the edge of the del ta. 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-moti on 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 (epicent ral distances of 130 to 380 km) to document attenuation of peak accele ration with distance for this earthquake. The observed values are slig htly lower than those predicted by the ground-motion relations propose d for the year 2000 National Building Code of Canada.