Horizontal-to-vertical spectral ratio of S-waves and SH-wave transfer functions at the vertical seismic and strong-motion arrays in the Central United States

Spectral analyses were performed on S-waves of earthquake recordings from two vertical seismic and strong-motion arrays in the central United States to obtain horizontal-to-vertical spectral ratios (HVSR) and transfer functions. Spectral analyses were also performed on ambient noises to obtain HVSRs at both sites. Theoretical SH-wave transfer functions were obtained using Thomson-Haskell propagation matrices and soil profiles at both sites. The results demonstrate that the S-wave HVSR is similar to the observed and theoretical SH-wave transfer functions in terms of peak frequencies up to the fifth mode. In other words, the S-wave HVSR could be used as an empirical SH-wave transfer function and provide in situ constraints on the shear-wave velocity structure of soils at these sites. The results also demonstrate that the ambient-noise HVSR is different from the S-wave HVSR and SH-wave transfer functions of the soils. Thus, the ambient-noise HVSR is not applicable for characterizing earthquake ground-motion site-effect, as well as determining the shear-wave velocity structure.