HR: 1330h
AN: S22B-0457    [PDF]
TI: Basin Attenuation and the characteristics of simple spectral ratio and H/V site response estimates
AU: * Pratt, T L
EM: tpratt@ocean.washington.edu
AF: U. S. Geological Survey, School of Oceanography, University of Washington, Seattle, WA 98195 United States
AU: Brocher, T M
EM: brocher@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 90425 United States
AB: Site response estimates are commonly used to characterize potential ground shaking during future earthquakes. These estimates are generally computed by taking either the spectral ratio (SR) of ground shaking at the site relative to a nearby bedrock site, or the spectral ratio of the horizontal to vertical components of the shear wave arrivals at the site (H/V). In the H/V method, the vertical component of the shear wave arrivals is assumed to be a converted phase from the base of the shallow deposits, making it a proxy for the signal entering the base of the shallow deposits. The site response at frequencies of 1 to 20 Hz is generally interpreted in terms of the velocity structure of shallow sedimentary deposits. Seismic waves reaching sites above thick sedimentary basins, however, undergo attenuation in the deeper basin-filling sediments. This frequency-dependent attenuation results in decreasing spectral ratios with higher frequency in the SR method, but will have little effect on the H/V method because all three components used in computing the latter ratio have undergone nearly the same attenuation. To demonstrate the effects of attenuation in deep sedimentary basins on both SR and H/V site response estimates, we calculated the site response at 50 locations in the Puget Lowland of Washington State using both methods. Spectra were computed from recordings of three local earthquakes of magnitude 2.1 to 2.8, which provided a useful signal-to-noise ratio across the spectrum. The SR estimates at sites over deep basins consistently show a decrease in the spectral ratio with increasing frequency, whereas at non-basin sites the SR estimates do not exhibit a systematic decrease with frequency. Using the H/V method, responses determined at the deep basin sites show little or no systematic decrease in spectral ratio with increasing frequency, suggesting that the method is indeed measuring primarily the response of the shallow deposits. Computing values for the attenuation factor (Q) from the SR site response estimates using the method of Anderson and Hough (1984) results in reasonable values of 15 to 200 depending upon the total thickness of the basin sediments. Removing the trend of decreasing amplitude with frequency from the SR ratio using these attenuation estimates significantly improves the fit of the SR site response estimates with those computed using the H/V method.
DE: 7212 Earthquake ground motions and engineering
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting