Surface Seismic-Reflection/Refraction Measurements of Near-Surface P- and S-Wave Velocities at Earthquake Recording Stations, Seattle, Washington Poster presented at 1997 Fall Meeting of the AGU R. Williams (US Geological Survey), W. Stephenson, J. Odum, D. Worley, and A. Frankel; AGU EOS Transactions, v. 78, no 46, p. F433. We acquired high-resolution P- and S-wave seismic-refraction/reflection data on the ground surface at 13 sites in the Seattle urban area, where portable digital seismographs recently recorded earthquakes. We used these data to determine the compressional- and shear-wave velocities (Vp and Vs) at the earthquake recording sites to about 40-m depth. Reversed seismic profiles along a 30-station (3-m geophone interval), 87-m-long geophone array were acquired at each site. These data allow assessment of the importance of shallow Vs in determining weak-motion site amplification and help explain the observed variation in earthquake site response. see METHODS Given measurements of site amplification relative to a reference rock site, 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 south of the Kingdome (SODO), are located on artificial fill and Quaternary alluvium and have an average Vs in the upper 30 m (Vs30) of 150 to 170 m/s. These values ofVs30 place these sites in soil profile type E (Vs30 < 180 m/s) using the 1994 NEHRP guidelines. The reference "rock" site, located at Seward Park on Tertiary sedimentary deposits, has a Vs30 of 433 m/s, which is NEHRP soil type C (Vs30: 360 to 760 m/s). The Seward Park "rock" site Vs30 is about equal to, or up to 200 m/s slower than sites which were located on till or glacial outwash. Thus, sites located on Quaternary till or related deposits appear to have the highest Vs30 in the Seattle urban area. see P- and S-wave velocity results. click on image for 88 Kbyte version Generalized geologic map of southern Seattle and vicinity showing five principle geologic units. Short black bars show the approximate location of the seismic refraction/reflection profiles. Geology generalized from Waldron et al. (1962) and a digital compilation by J. Yount (unpublished data). We also recorded high-amplitude P- and S-wave seismic reflections at several locations that appear to correspond to strong resonances observed in earthquake spectra. Two of these sites are located 250 and 300 m east of the seismograph station located near the Kingdome. The reflection has a two-way traveltime of about 0.23 to 0.27 s (about 17 to 22 m depth) and may mark the boundary between overlying artificial fill (Vs <180 m/s) and a higher velocity material (Vs about 400 m/s). This reflector may cause strong 2-Hz resonance that is observed in the earthquake data near the Kingdome. An S-wave reflection at a till site, generated by a about a 7-m-deep reflector, appears to cause a strong 7-Hz resonance in the earthquake spectrum. However, not all S-wave reflections appear to directly correspond to resonances in earthquake spectra. For example, a high-amplitude reflection recorded in the SODO area that should correspond to about a 1-Hz resonance, is not clearly observed in earthquake spectra for this site. see Seismic Reflection-Resonance Correspondence. The Seattle area faces seismic hazards from interplate and intraplate earthquakes. Since 1949, two earthquakes >M6.5 have occurred within 80 km of the study area causing significant damage, a few deaths, and pockets of Mercalli intensity VIII in West Seattle (Algermissen and Harding, 1965). The U.S. Geological Survey, in collaboration with local governments, Universities, and private industry, is studying the earthquake hazard of Puget Sound. This study of near-surface P- and S-wave velociities is part of that larger effort. Previous studies have consistently shown that decreasing mean S-wave velocity in the near surface generally correlates with an increase in the average amplification of earthquake ground motion (Borcherdt and Gibbs, 1976; Hartzell et al., 1996). Determination of near-surface seismic velocities is also motivated by the National Earthquake Hazard Reduction Program's (NEHRP) efforts to place a special significance for shallow S-wave velocities into recent building code provisions (Building Seismic Safety Council, 1994). Conclusions Measurements of P- and S-wave velocities in the Seattle area via surface-seismic-refraction/reflection methods, has helped explain some of the earthquake site response results discussed by Frankel et al. Also, these data helped map the seismic velocity characteristics of the main geologic units of the southern and West Seattle areas in the upper 30 m of the ground surface. S-wave velocities in much of the Duwamish industrial area south of the Kingdome and on Harbor Island probably average less than 180 m/s in the upper 30 m. Thus, most of the soils in this area would classify as type E under the NEHRP classification scheme and would probably be subject to stronger ground shaking. Earthquake data recorded in these areas supports this speculation (Frankel et al., 1997). S-wave velocities in the upper 30 m for sites on glacial till are equivalent to or higher than the velocity measured on Tertiary 'rock' at Seward Park. S-wave velocities measured at two locations in the West Seattle area are lower than those measured on most till sites, but seemingly not low enough to cause larger ground motions like the pockets of Mercalli Intensity VIII observed in the 1965 M6.5 earthquake. We show three cases of a correspondence between a resonance observed on earthquake spectra and strong seismic reflections in the near surface. One of these resonances appears to be generated by high-impedance contrast at 7-m depth. At the Kingdome, a resonance at about 2 Hz appears to be areally consistent over much of the Kingdome facility, and caused by a high-impedance contrast at the boundary between soft alluvium and older till and alluvium. This boundary occurs in the 17- to 20-m-depth range at two locations on the Kingdome facility. Preexisting geotechnical data supports our interpretation of the seismic-refraction/reflection data at the Kingdome. Based on a deeper S-wave seismic reflection about 2 to 4 km south of the Kingdome, we infer that earthquake resonances should be about 1 Hz. But, earthquake spectra recorded in this vicinity are uncertain at these frequencies at this time and are being further analyzed. References Algermissen, S.T., and S.T. Harding, 1965, The Puget Sound, Washington earthquake of April 29, 1965: Preliminary seismological report: U.S. Coast and Geodetic Survey. Borcherdt, R.D., and J.F. Gibbs, 1976, Effects of local geological conditions in the San Francisco Bay region on ground motions and the intensities of the 1906 earthquake: Seism. Soc. Am. Bull., v. 66, no. 2, p. 467-500. Building Seismic Safety Council, 1994, NEHRP recommeded provisions for seismic regulations for new buildings, Part 1-Provisions: Federal Emergency Management Agency, 290 p. Frankel, A., Carver, D., Cranswick, E., Meremonte, M., Bice, T., and Overturf, D., 1997, Site response for Seattle and source parameters of earthquakes in the Puget Sound region: Am. Geoph. Union, EOS Trans., 1997 Fall Meeting. Harris, J., Street, R., Kiefer, D., Allen, D., and Wang, Z., 1994, Modeling site response in the Paducah, Kentucky, area: Earthquake Spectra, v. 10, no. 3, p. 519-538. Liesch, B.A., Price C.E., and Walters, K.L., 1963, Geology and Ground-Water Resources of Northwestern King County, Washington, State of Washington Department of Conservation, Division of Water Resources. Water Supply Bulletin No. 20: Mooney, H.M., 1984, Handbook of geophysical exploration, volume 1, seismic: Bison Instruments, Inc. Shannon & Wilson, 1994, Kingdome seismic evaluation Seattle, Washington: Report W-6836-01. Street, R., Woolery, E., Wang, Z., and Harris, J., 1995, A short note on shear-wave velocities and other site conditions at selected strong-motion stations in the New Madrid seismic zone: Seismological Research Letters, v. 66, no. 1, p. 56-63. Waldron, H.W., Liesch, B.A., Mullineaux, D.R., and D.R. Crandell, 1962, Preliminary geologic map of Seattle and vicinity, Washington: U.S. Geological Survey, Miscellaneous Geologic Investigations Map I-354, 1:31680. Williams, R.A., Stephenson, W.J., Odum, J.K., and Worley, D.M., 1996, Shallow P- and S-wave velocities at eleven aftershock recording stations of the Northridge earthquake, San Fernando Valley, California: U.S. Geological Survey Open-File Report 96-261, 10 p. Williams, R.A., Stephenson, W.J., Odum, J.K., and Worley, D.M., 1997, High-resolution surface-seismic imaging techniques for NEHRP soil profile classifications and earthquake hazard assessments in urban areas: U.S. Geological Survey Open-File Report 97-501, 42 p. Yount, J.C., Vick, G.S., and McCoy, G, 1990, Geotechnical drill-hole logs from the southern Seattle area, Washington: U.S. Geological Survey Open-File Report 90-90, 143 p. Acknowledgements The authors thank Susan Rhea (USGS) for providing the digital version of the Seattle area geologic map. We also thank Ivan Wong, Bob Metcalf, Drew Sparks (Woodward-Clyde), and John Peterson (City of Seattle) for providing copies of geotechnical data near our study sites. This study was supported by funding from the National Earthquake Hazard Reduction Program.

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