Site Response Impacts of the Memphis Sand Layer within the Mississippi Embayment

In this study, the site response impacts of accurately characterizing and resolving the Memphis sand layer at the Central United States Seismic Observatory (CUSSO) site are explored. The Memphis sand, also known as the “500 ft sand,” is a major shallow impedance contrast in the Mississippi Embayment, influencing the site response estimates across the embayment. However, the Mississippi Embayment’s available velocity information, including the Central United States Seismic Velocity Model, often does a poor job of properly identifying the depth and impendence contrast associated with the Memphis sand layer. To understand the impacts of the Memphis sand layer on site response estimates, a parametric study using linear viscoelastic transfer function and nonlinear site response analysis is performed on a suite of shear-wave velocity (V_S) profiles with varying depths to the Memphis sand and varying formation velocity of the Memphis sand. The linear and nonlinear site response results indicate that varying the depth and V_S of the Memphis Sand layer within reasonable ranges can significantly impact site response results, with a variation of 10%–30% observed for periods less than 1.0 s. However, the largest variations are observed at periods between 1.0 and 2.0 s with variations of up to 45%. This highlights the need to properly resolve shallow impedance contrast, such as the Memphis Sand in the Mississippi embayment, for site response analyses.