P- and S-Wave Velocity Results
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Seattle area average S-wave velocities to 30-m depth projected onto the surficial geologic map of Seattle shown in Figure 1. Height of the colored column indicates average velocity. Average velocities are also annotated on the columns. The lowest S-wave velocities were found in the Duwamish industrial area, which is located on mostly artificial fill at the surface. Highest average velocities were measured at sites located on glacial till. We also classified the sites based on a NEHRP scheme that considers the average S-wave velocity in the upper 30 m of the ground surface. NEHRP recommends that these S-wave velocities be considered at new building sites (Building Seismic Safety Council, 1994). These classifications are color coded according to the description shown to the right of this figure.
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The average S-wave velocity to 30-m depth for each site is listed in this graph. The bars are also color coded to indicate the NEHRP soil profile type. The data suggest that sites which are located in the warehouse district south of the Kingdome will probably be classified as type E. The two sites near the Kingdome are located on a thinner section of the low-velocity material and thus are classified as type D. S-wave refraction data from the Kingdome sites were generally poor, possibly because of a S-wave velocity inversion near the surface. S-wave reflection data were used at these sites to complete the velocity profile. The graph also shows that the sites located on till (BG0, PSC, and BHD) can have significantly higher velocities than site SQ0, which was located on Tertiary sedimentary deposits. The two West Seattle sites (WES and WEN), which were located on glacial outwash sand, generally have lower velocities than the sites located on till.
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P- and S-wave velocity interpretations to about 30-m depth for the 13 refraction profiles with site names annotated at the top of each rectangle. Seismic velocities are annotated in the shaded boxes. Boxes are shaded according to velocity value. The Vs annotation at the bottom of the S-wave velocity column gives the mean S-wave velocity to 30-m depth. Because the S-wave refraction phases were poorly developed or nonexistent in the upper 20 m at KD1 and KD2, velocities were determined primarily from the reflection data. The weak refractions at this site are likely due to the known S-wave velocity inversions that exist in the near-surface in this vicinity (Shannon and Wilson, 1994; Steve Palmer, unpublished downhole data).
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