SURFACE- AND BODY-WAVE MEASUREMENTS OF SHALLOW P- AND S-WAVE VELOCITIES IN ST. LOUIS, MISSOURI

WILLIAMS, R.A., ODUM, J.K., STEPHENSON, W.J., and WORLEY, D.M., U.S. Geological Survey, Denver Federal Center MS966, Box 25046, Denver, CO 80225, rawilliams@usgs.gov.

As part of the seismic hazard mapping efforts in the St. Louis metropolitan area we determined the compressional- and shear-wave velocities (Vp and Vs) to about 40-m depth at 17 locations in this area. The Vs measurements were made using both active source (3.6-kg sledgehammer) high-resolution seismic-refraction and reflection methods and by inversion of the dispersion curve generated from passive recordings of ambient surface waves. Compared to our reflection/refraction models, the inverted passive data models are unable to accurately capture the high-impedance alluvium-Paleozoic (Pz) bedrock boundary encountered at several sites in our study. We find a clear difference in the Vs profiles between sites located on the river floodplain and those located in the upland urban areas of St. Louis that will likely result in differences in ground shaking and building damage in an earthquake. At our sites along the Missouri and Mississippi River floodplains the depth to Pz limestone bedrock is in the range of 30 to 45 m. Vs30 (average Vs to 30 m depth) values in these areas range from 200 to 250 m/s (NEHRP category D) and contrast with sites on the upland areas of St. Louis which have Vs30 values ranging from 450 to 650 m/s (NEHRP category C). Paleozoic bedrock depths in the upland areas range from 8 to 15 m. The lower Vs30 values in the floodplains suggest a greater potential for stronger and more prolonged ground shaking in an earthquake. In addition to these broadband site amplification effects there is a potential for narrow-band resonances developing within the alluvium. Based on S-wave Pz reflection times potentially damaging S-wave resonant frequencies generated by earthquake shaking within this alluvium-bedrock layer could be about 2 Hz for the floodplains and 4 Hz for the upland areas, either of which could impact certain low- to mid-rise buildings.