Surface seismic refraction/reflection measurement determinations of potential site resonances and the areal uniformity of NEHRP site class D in Memphis, Tennessee
We determined S-wave velocities (Vs) to about 40-m depth at 65 locations in the Memphis-Shelby County, Tennessee, area. The Vs measurements were made using high-resolution seismic refraction and reflection methods on the ground surface. We find a clear difference in the Vs profiles between sites located on the Mississippi River flood plain and those located to the east, mostly covered by loess, in the urban areas of Memphis. The average Vs to 30-m depth at 19 sites on the modern Mississippi River floodplain averages 197 m/s (?? 15 m/s) and places 17 of these sites at the low end of NEHRP soil profile category type D (average Vs 180-360 m/s). The two remaining sites are type E. Vs to 30-m depth at 46 sites in the urban areas east of the modern floodplain are more variable and generally higher than the floodplain sites, averaging about 262 m/s (??45 m/s), still within category D. We often observed the base of the loess as a prominent S-wave reflection and as an increase in Vs to about 500 m/s. Based on the two-way travel time of this reflection, during an earthquake the impedance boundary at the loess base may generate resonances in the 3- to 6-Hz range over many areas of Memphis. Amplitude spectra from four local earthquakes recorded at one site located on loess indicate consistent resonance peaks in the 4.5- to 6.5-Hz range.