Comment on “Microtremor observations of deep sediment resonance in metropolitan Memphis, Tennessee” by Paul Bodin, Kevin Smith, Steve Horton and Howard Hwang

Bodin et al. [Eng. Geol. 62 (2001) 159] reported three sets of maxima in observed H/V particle-motion spectra, where the longest-period set of peaks provided clear mapping information on the thickness of post-Cretaceous sediments in the Mississippi embayment, but the remaining peaks remained largely unexplained. A review of microtremor array studies for similar sites and similar frequencies suggests that the microtremor field is best interpreted in terms of Rayleigh-wave energy. Use of this approach, with modeling of particle-motion H/V ratios for realistic models of the Mississippi embayment, shows that the [Eng. Geol. 62 (2001) 159] second set of spectral maxima can be associated with higher-mode Rayleigh-wave energy, and may contain information on a possible velocity inversion in the shear-velocity profile of the site. A third spectral maximum appears to be associated with soft soils (loess) and may be able to be used for mapping the near-surface thickness (order 15 m) of this unit.