Surface Velocity Studies - 1999 Journal of Earthquake Spectra

Surface seismic measurements of near-surface P- and S-wave seismic velocities at earthquake recording stations, Seattle, Washington

1999, Williams, R.A., Stephenson, W.J., Frankel, A.D., EERI, M., and Odum, J.: Earthquake Spectra, v. 15, no. 3, 20 p.

ABSTRACT:

We measured P- and S-wave seismic velocities to about 40-m depth using seismic refraction/reflection data on the ground surface at 13 sites in the Seattle urban area, where portable digital seismographs recently recorded earthquakes. Sites with the lowest measured Vs correlate with highest ground motion amplification. These sites, such as at Harbor Island and in the Duwamish River industrial area (DRIA) south of the Kingdome, have an average Vs in the upper 30 m (Vs30) of 150 to 170 m/s. These values of Vs30 place these sites in soil profile type E (Vs30 < 180 m/s). A "rock" site, located at Seward Park on Tertiary sedimentary deposits, has Vs30 of 433 m/s, which is soil type C (Vs30: 360 to 760 m/s). The Seward Park site Vs30 is about equal to, or up to 200 m/s slower than sites which were located on till or glacial outwash. High-amplitude P- and S-wave seismic reflections at several locations appear to correspond to strong resonances observed in earthquake spectra. An S-wave reflector at the Kingdome at about 17 to 22 m depth probably causes strong 2-Hz resonance that is observed in the earthquake data near the Kingdome.