HR: 08:55h
AN: S41C-04    [Abstracts]
TI: Attenuation Within Sedimentary Basins, and the Shapes of Site Response Curves 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 91895 United States
AU: Brocher, T M
EM: brocher@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, MS 977 , Menlo Park, CA 94025 United States
AB: Comparison of simple spectral ratio (SSR) and horizontal-to-vertical (H/V) site response estimates at 47 sites in the Puget Lowland of Washington State document significant attenuation of shear wave arrivals within the sedimentary basins there. Amplitudes of the horizontal components of shear-wave arrivals from three local earthquakes were used to compute SSRs with respect to the average of two bedrock sites, and H/V spectral ratios with respect to the vertical component of the shear-wave arrivals at each site. SSRs at thick basin sites show peak amplifications of 2 to 6 at frequencies of 3 to 6 Hz, and decreasing spectral amplification with increasing frequency above 6 Hz. In contrast, SSR site response curves at non-basin sites show a variety of shapes. We interpret the spectral decay at basin sites to attenuation within thick basin strata. Computing the frequency-independent, depth-dependent attenuation factor (Qs,int) from the SSR spectral decay gives values of about 30 for shallow sedimentary deposits and 220 for the deepest sedimentary strata. H/V site responses show less spectral decay than the SSR responses but contain many of the same resonance peaks. We hypothesize that the H/V method yields a flatter response because P-to-S mode conversion occurs at shallow horizons within the basin, such that attenuation in the deep basin strata influences both the horizontal and vertical components of the shear-wave arrivals to nearly the same degree. Correcting the SSR site responses for attenuation within the basins results in better agreement between SSR and H/V estimates of site response.
DE: 7223 Seismic hazard assessment and prediction
DE: 7294 Instruments and techniques
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting