HR: 0800h
AN: S51C-1026    [Abstracts]
TI: Basin Effects and Ground Shaking in the Puget Lowland, Washington State
AU: * Pratt, T L
EM: tpratt@ocean.washington.edu
AF: U. S. Geological Survey, School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195 United States
AB: Ground shaking during earthquakes was measured over the Seattle sedimentary basin, western Washington State, using the 87 seismometers in the 2002 Seattle Seismic Hazard Investigation of Puget Sound (Seattle SHIPS, or SHIPS02) array. The array consisted of 2-Hz geophones recorded on Refraction Technology (Reftek) recording systems. The instruments were in place for 4 months, from the end of January through May, 2002. Horizontal-component recordings of shear waves from 5 teleseisms and 7 local earthquakes were used to compute spectral ratios with respect to the average of 5 bedrock sites located in the Olympic Mountains at the west side of the array and in the Cascade Range foothills on the east side of the array. Results show amplifications of 0.3 to 0.8 Hz seismic waves by factors of 6 to 8 at sites over the deep Seattle basin (>2.5 km of sedimentary strata), with the peak amplification being at about 0.3 Hz. All sites in the central Puget Lowland are amplified significantly, however, which indicates the deposits in the upper 2.5 km that extend beyond the deep basin are responsible for much of the amplification. The variance, the horizontal to vertical amplitude ratio, and the crosscorrelation of arrivals from nearby stations are consistent with the presence of scattered energy and surface waves from multiple directions in the later portions of the long-period signal. At frequencies above 1 Hz the amplification levels are greatest in areas of soft fill such as the Duwamish river valley.
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: Fall Meeting 2005