(Type: Oral)

PRELIMINARY SITE AMPLIFICATION INVESTIGATION FOR THE MEMPHIS-SHELBY COUNTY AREA OF TENNESSEE USING Vs30

CHRIS H. CRAMER, U.S.G.S., 3876 Central Ave Ste 2, Memphis, TN 38152-3050, cramer@ceri.memphis.edu

As a part of the U.S. Geological Survey’s Memphis – Shelby County Seismic Hazard Mapping project, shear-wave velocity (Vs) measurements have been gathered into a database of subsurface information. This information along with other subsurface geological, geotechnical, and geophysical measurements will be used to produce state-of-the-art seismic hazard maps, including geologic site conditions, for the Memphis area. Seventy shallow Vs profiles (depth less than 100m) in Shelby Co. have been analyzed to determine average Vs profiles and preliminary site amplification factors. Amplifications relative to a 760 m/s site condition for average Vs profiles were calculated using SHAKE (1972). The average Vs over the uppermost 30 m (Vs30) is used to group actual Vs profiles and determine one mean Vs profile for each group. Initially, the Vs30 groupings were 180-210, 210-250, 250-300, 300-360, and greater than 360 m/s. Generally, the 180-210 m/s Vs30 group corresponds to sites in the Mississippi flood plain, the 210-250 and 250-300 m/s groups correspond to sites in recent tributary fluvial drainages, the 300-360 m/s group corresponds to sites on older sedimentary deposits between tributary drainages, and the > 360 m/s group to an area of thinning sedimentary section in the SE corner of Shelby Co. The 180-210, 210-250, and 250-300 mean profiles were statistically the same except over the 10-35m depth range. Relative amplifications in the 0-10 Hz band for these three mean profiles are similar and these groups have been combined into one 180-300 m/s group. Preliminary results suggest average site amplifications of 1.5-2.0 in the 1-5 Hz band for sites with Vs30’s of 180-300 m/s. Sites with Vs30’s of 300-360 m/s should have average site amplifications of about 1.3 in the 2-8 Hz band. And sites with Vs30’s greater than 360 m/s should have little site amplification (~1.0), on average, in the 0-10 Hz band. These preliminary site amplification results are comparable to the revisions to the NEHRP site amplification factors proposed by Joyner and Boore (2000).