Surface Seismic Refraction/Reflection Measurement Determinations of Potential Site Resonances and the Areal Uniformity of NEHRP Site Class D in Memphis,Tennessee

Abstract

We determined S-wave velocities (V_s) to about 40-m depth at 65 locations in the Memphis-Shelby County, Tennessee, area. The V_s measurements were made using high-resolution seismic refraction and reflection methods on the ground surface. We find a clear difference in the V_s 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 V_s 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 V_s 180-360 m/s). The two remaining sites are type E. V_s 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 V_s 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.