HR: 0800h
AN: S51A-0228    [Abstracts]
TI: Vs30 from site response and local magnitude corrections
AU: * Pratt, T
EM: tpratt@usgs.gov
AF: USGS, School of Ocean., Univ. of WA, Seattle, WA 98195, United States
AU: Hartog, R
EM: renate@ess.washington.edu
AF: Earth and Space Sci., Univ. of WA, Seattle, WA 98195, United States
AU: Gomberg, J
EM: gomberg@usgs.gov
AF: USGS, Earth and Space Sci., Univ. of WA, Seattle, WA 98195, United States
AU: Frankel, A
EM: afrankel@usgs.gov
AF: USGS, Box 25046, Denver Fed. Center, MS 966, Denver, CO 80225, United States
AU: Williams, R
EM: rawilliams@usgs.gov
AF: USGS, Box 25046, Denver Fed. Center, MS 966, Denver, CO 80225, United States
AU: Wong, I
EM: Ivan_Wong@URSCorp.com
AF: URS Corp., 1333 Broadway, Suite 800, Oakland, CA 94612, United States
AU: Haugerud, R
EM: rhaugerud@usgs.gov
AF: USGS, Earth and Space Sci., Univ. of WA, Seattle, WA 98195, United States
AB: We used site response amplifications and local magnitude (ML) station corrections to derive Vs30 estimates (S- wave velocity in the upper 30 m) in the U. S. Pacific Northwest. To obtain Vs30 from site response amplifications, we used the average of the amplifications at 1, 2, 3 and 5 Hz to estimate an impedance ratio, which we assumed represents the ratio between bedrock (Vs=760 m/s) and the shallow, 30-m thick layer (Vs30). We used the velocity and density relations in Brocher (2005, BSSA) to convert this impedance ratio into a velocity and density for each layer. Comparing these Vs30 estimates with nearby (<300 m distance) velocity measurements generally shows agreement within about 30%. To estimate Vs30 from ML station corrections, we show that the station correction is proportional to the logarithm of the site amplification. The magnitude corrections are relative to the average of the network, so they first needed to be adjusted so that a bedrock reference site (Vs=760 m/s) has a correction of 0. We used the adjusted magnitude corrections to compute the site amplification that would be required to cause the ML correction, and from this the impedance ratio and Vs30 as before. The Vs30 estimates made from the magnitude corrections again show agreement to within 30% of nearby measured Vs30 values. The resulting Vs30 values correlate with the age of surface geologic units, showing a progressive increase in Vs30 from ~280 m/s for Quaternary alluvial deposits and artificial fill, to ~1300 m/s for Oligocene volcanic rocks. The Pleistocene glacial tills that cover much of the Puget Lowland region show an average velocity of 512 m/s.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7219 Seismic monitoring and test-ban treaty verification
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting