Near-surface Vs Profiling in the New Madrid Seismic Zone Using Surface-wave Methods

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Abstract
Surface-wave tests were performed at eleven sites in Shelby County and Memphis, Tennessee and northeast Arkansas representing soil deposits of differing types and ages to determine near-surface shear-wave velocity (Vs) profiles for site-response analyses. Active surface-wave tests were conducted using a harmonic source and an irregular linear array of receivers. At two of the eleven sites, a two-dimensional array of receivers was used to record surface waves generated by passive sources including microtremors and cultural noise. Dispersion curves for both types of tests were calculated using frequency-wavenumber methods and were inverted using an algorithm that encourages smoothness of the resulting Vs profile. The surface-wave test results compare well with profiles obtained by other researchers using a variety of methods. We believe that most observed differences are due to the nature of surface-wave propagation and the characteristics of the inversion algorithm. At the Mud Island B site, we obtained a profile to a depth of 170 m by combining active and passive dispersion data to form a composite dispersion curve. The combination of active and passive data significantly enhances the capabilities of surface-wave test procedures. Finally, the measured Vs profiles were used to calculate linear amplification spectra, which clearly indicate differences in site response between Holocene- and Pleistocene-age deposits.

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