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

¹ Department of Earth and Space Science, Box 351310
University of Washington
Seattle, Washington 98195-1310
 (Q.L.)
² School of Oceanography, Box 357940
University of Washington
Seattle Washington 98195-7940
 (W.S.D.W.)
³ U.S. Geological Survey
School of Oceanography, Box 357940
University of Washington
Seattle, Washington 98195
 (T.L.P.)
⁴ Department of Geoscience
University of Nevada
Las Vegas Nevada 89154-4010
 (C.M.S.)
⁵ U.S. Geological Survey
345 Middlefield Road, MS 977
Menlo Park, California 94025
 (T.M.B.)

We used waveform data from the 1999 SHIPS (Seismic Hazard Investigation 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 Q^–1 models at frequencies between 1 and 40 Hz for P waves and 1 and 10 Hz for S waves. We also obtained Q^–1 models from t* values calculated from the spectral slopes of P waves between 10 and 40 Hz. One-dimensional inversions show that Q_p 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. Q_s 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 Q_p 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 anomalously high-amplitude shots near Seattle. When these shots are removed, we infer that Q^–1 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.

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