In this study, site amplification factors for the deep soil deposits of the Mississippi embayment are computed using a nonlinear site response analysis program first to develop a model for nonlinear soil response for possible use by ground motion developers and second to address site amplification estimation. The effects of geology, sediment depth, and average shear wave velocity at the upper 30 m of soil ranging 180–800 m/s, as well as the effect of peak ground acceleration at the bedrock on nonlinear ground motion amplification for the upper embayment, are investigated. The site response computations cover various site conditions, sediment depth of 70–750 m, and peak acceleration of input rock motions of 0.01–0.90 g. The amplification (or de-amplification) at various frequencies implied by the sediment depth is greater than that implied just by site classification of the top 30 m of soil.

A new ground-motion prediction model is developed for the response spectral ratio of vertical-to-horizontal (V/H) components of earthquakes for the Gulf Coast region. The proposed V/H response spectral ratio model has the advantage of considering the earthquake magnitude, source-to-site distance, and the shear-wave velocity of soil deposits in the upper 30 m of the site (V S30) for the peak ground acceleration, and a wide range of spectral periods (0.01-10.0 s). The model is based on a comprehensive set of regression analyses of the newly compiled Next Generation Attenuation-East database of available central and eastern North America recordings with the moment magnitudes M ≥ 3:4 and the rupture distances R Rup < 1000 km. The 50th percentile (or median) pseudospectral acceleration values computed from the orthogonal horizontal components of ground motions rotated through all possible nonredundant rotation angles, known as the RotD50 (Boore, 2010), is used along with the vertical component to perform regression using a nonlinear mixed-effects regression algorithm. The predicted V/H ratios from the proposed model are compared with the recently published V/H spectral ratio models for different regions. The derived V/H ratios can be used to develop the vertical response spectra for the sites located within the Gulf Coast region, which include the Mississippi embayment.