Seismic Attenuation Structure of the Seattle Basin, Washington State, from Explosive-Source Refraction Data

tigation of Puget Sound) seismic refraction experiment to constrain the attenuation structure of the Seattle basin, Washington State. We inverted the spectral amplitudes of compressional- and shear-wave arrivals for source spectra, site responses, and one- and two-dimensional Q1 models at frequencies between 1 and 40 Hz for P waves and 1 and 10 Hz for S waves. We also obtained Q1 models from t * values calculated from the spectral slopes of P waves between 10 and 40 Hz. One-dimen-sional inversions show that Qp at the surface is 22 at 1 Hz, 130 at 5 Hz, and 390 at 20 Hz. The corresponding values at 18 km depth are 100, 440, and 1900. Qs at the surface is 16 and 160 at 1 Hz and 8 Hz, respectively, increasing to 80 and 500 at 18 km depth. The t * inversion yields a Qp model that is consistent with the amplitude inversions at 20 and 30 Hz. The basin geometry is clearly resolved in the t * inversion, but the amplitude inversions only imaged the basin structure after removing anom-alously high-amplitude shots near Seattle. When these shots are removed, we infer that Q1 values may be 30 % higher in the center of the basin than the one-dimensional models predict. We infer that seismic attenuation in the Seattle basin will significantly reduce ground motions at frequencies at and above 1 Hz, partially countering amplification effects within the basin