Ground motion amplification of soils in the upper Mississippi Embayment

The 1811–1812 New Madrid earthquakes affected a large area in the Central United States. The seismic hazard is low since large-magnitude earthquakes such as these are infrequent. However, the seismic risk is considerable due to the geology and building codes that do not adequately reflect the seismic hazard. The strong ground motions observed during the 1811–1812 earthquakes are attributed to the soft deposits and the deep soil column encountered in the Mississippi Embayment. This study evaluated the effect of embayment deposits on dynamic site response. Remote sensing imagery was evaluated to identify soils susceptible to ground motion amplification. Regional and local shear wave velocity (V_s) profiles were developed to model conditions at soil sites based on the age of near-surface geologic deposits.; A parametric study was conducted to assess the effect of several geotechnical and seismological factors on site response. A stochastic approach using band-limited white noise (BLWN) was used to model rock motions at the base of the soil column. Peak parameters were calculated using random vibration theory (RVT). Shear wave velocity profiles and dynamic soil properties were randomized to account for uncertainty. Based on one-dimensional site response analyses, the key factors affecting Fourier amplitude and response spectra are the age of geologic deposits, the depth of the soil column, and the effect of nonlinear soil behavior.; Amplification spectra compare the response of a soil site relative to a rock site. Ground motions for periods longer than 0.5 second may be more than twice that observed at rock sites due to amplification from low-velocity deposits and resonances from the large impedance contrast at the base of the embayment. Published amplification factors do not account for the amplification at long periods. At shorter periods, damping controls site response and deamplifies ground motions.; The results of this study were also compared with published attenuation relationships developed for the Central United States. The attenuation relationships overestimate ground motions at short hypocentral distances due to nonlinear soil behavior. However, at large distances, attenuation relationships may underestimate ground motions since amplification from low-velocity deposits and resonances within the soil column control site response.