Site Response, Basin Effects, and Attenuation in the Puget Lowland, Washington State, U. S.

Site response, basin amplification, and attenuation within the Puget Lowland, in the Cascade forearc of Washington State, have been measured using large temporary deployments of seismometers during 3 Seattle Seismic Hazard Investigation of Puget Sound (SHIPS) experiments, and the permanent strong-motion network. Horizontal-component recordings of shear waves from teleseisms and local earthquakes were used to compute spectral ratios with respect to the average of several bedrock sites. Results show amplifications of 0.3 to 0.8 Hz seismic waves by factors of 6 to 12 at sites over the deep Seattle sedimentary basin (＞2.5km 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 that the deposits in the upper 2.5km extending beyond the margins of the deep basin are responsible for much of the amplification. The deep basin itself appears to amplify waves by factors of up to 2 in the 0.05 to 0.2 Hz frequency range, which is consistent with resonance in the deep strata. The horizontal to vertical ratios and crosscorrelations of arrivals at adjacent stations on the basin are consistent with the presence of scattered body-wave energy and Rayleigh waves in the later portions of the long-period signal. Estimates of the shear-wave attenuation factors (Qs) between 2 to 20 Hz increase from 5 to 40 for shallow sedimentary deposits to about 250 for the deep sedimentary strata (7km depth)