Non-linear one-dimensional seismic ground motion propagation in the Mississippi embayment

Deep unconsolidated deposits of the Mississippi Embayment overlie the New Madrid Seismic Zone, considered the most seismically active zone in the Eastern US. The deposits range in thickness from less than 100 m in the St. Louis area to about 1 km in the Memphis area and consist of silts, clays and sands. The influence of these deposits on propagation of seismic waves remains a major source of uncertainty for site response analysis. This paper describes the development of a new non-linear one-dimensional site response analysis model for vertical propagation of horizontal shear waves in deep soil deposits. Soil response is modeled using a modified hyperbolic model with extended Masing criteria to represent hysteretic loading and unloading of soil. The new soil model accounts for the influence of large confining pressures on strain dependent modulus degradation and damping of soil. The model is calibrated using measured shear modulus degradation and damping data from resonant column tests on sand samples under confining pressures up to 3.5 MPa. The new model is used to estimate ground motion amplification and attenuation for three soil columns 100, 500 and 1000 m thick, representative of soil thickness variability within the Embayment. The new model shows that some high frequency components of ground motion are transmitted through these deep deposits. These components are usually filtered out using conventional wave propagation methods. Longer period waves develop in deposits of 500–1000 m thickness. Spectral amplification factors of deep deposits are greater than unity and can be as large as 5 in the longer period range of 2–10 s. Preliminary evaluation of model results show that computed surface response spectra in the period range of 0.5–2 s are larger than the 1997 NEHRP recommended design response spectrum. The proposed model highlights the need to account for depth dependence of modulus and damping properties of soils in seismic wave propagation through deep soils.