A Study of Horizontal-to-Vertical Component Spectral Ratio in the New Madrid Seismic Zone

The horizontal-to-vertical component (H/V) spectral ratio of the small and moderate earthquake ground motions for the shear-wave window was used as an estimation of the site response in the New Madrid seismic zone (NMSZ). The database used in this study consisted of 500 broadband seismograms from 63 events of magnitude M(w) 2.5 to 5.2, recorded on 11 stations operated by the University of Memphis Center for Earthquake Research and Information (CERI) at the University of Memphis. All the broadband stations were located within the Mississippi embayment. Soil deposits overlying the rock basement of the embayment strongly affected the amplitudes of the ground motions. The horizontal-to-vertical component ratios were evaluated for the frequency range of 0.2 to 20 Hz. The observed average H/V ratios suggested site amplification between 2 and 4 in the low-frequency range (f <= 5 Hz) for stations located on the lower shear-wave velocity deposits (lowlands). The higher shear-wave velocity deposits (uplands) indicated low-frequency amplification between 1.5 and 3 Hz. The observed average H/V ratios were also compared with the soil amplifications in the upper Mississippi embayment developed by Romero and Rix (2005) from the 1D (equivalent linear) method for generic regional profiles. The H/V ratios were also compared with the theoretical quarter-wavelength approximation. These comparisons suggested that the H/V ratios could be a first estimate of the site amplifications. Finally, the variability of the H/V ratios with distance was examined and no discernible trends were found; therefore, the path effect model developed by Zandieh and Pezeshk (2010) for the vertical ground motions in NMSZ using the database of this study was also applicable for the horizontal ground motions.