SEISMIC SURFACE WAVE TESTING IN THE MEMPHIS, TN AREA
Gregory L. Hebeler , M. Catalina Orozco , Glenn J. Rix, School of Civil and Envir. Engrg., Georgia Inst. of Tech., Atlanta, GA 30332-0355
Recently, the interest in characterizing the dynamic soil properties in the New Madrid Seismic Zone (NMSZ), specifically the Memphis, TN area, has increased. It is well known that the effects of a large seismic event in the NMSZ may depend strongly on ground motion amplification in near-surface soils. To accurately quantify this response, the site-specific dynamic soil properties must be characterized. This study presents the shear wave velocity (Vs) and material damping ratio (D) profiles obtained from Spectral Analysis of Surface Waves (SASW) tests performed at eleven sites in the Memphis area. Active SASW tests were performed at all sites with a linear array of 15 sensors measuring the ground motions generated by a harmonic, electromagnetic source. At a few sites where conditions allowed, passive SASW tests were performed with a 16 sensor circular array measuring ground motions generated by microtremors. Passive surface waves mainly consist of lower excitation frequencies, allowing for deeper soil characterization. However, passive tests require a 2-D array of sensors, and more involved analyses. Dispersion calculations were conducted using frequency-wavenumber (f-k) spectral methods for both the active and passive tests. Surface wave dispersion and attenuation curves were then inverted using a constrained least-squares algorithm that encourages smooth variations of dynamic soil properties with depth. Dynamic soil properties were determined to approximately 40 meters using active techniques, and to a maximum depth of 170 meters when combined with passive measurements. Several of the current sites had been tested previously with a combination of piezocone, refraction, and downhole methods. Additionally, traditional SASW measurements (Stokoe et al., 1994) were performed at three sites allowing for the direct comparison of different SASW and in-situ seismic methodologies. The direct comparisons presented in this study should help to further validate the use and advantages of surface wave techniques, more specifically the advantages of the current dispersion and inversion calculation methods. Vs and D profiles were calculated for each site tested, and will be added to the existing database of dynamic soil properties for the NMSZ, increasing both the number and spatial extent of seismic measurements in the region
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