Estimating ground motions of the thick sediments of the Mississippi Embayment is difficult. Amplification of the thick layers (greater than 100 meters) of sediment with low shear-wave velocity, liquefaction, energy loss between the seismic source and the surface, and differences in ground motion between the floodplain in northeastern Arkansas and southeastern Missoui and loess bluffs in western Kentucky and Tennessee all contribute to this difficulty. To address these issues, a vertical seismic array is being installed in the most active part of the New Madrid Seismic Zone. This will allow us to evaluate the effect of deep soil conditions on earthquake ground motions in the seismic zone, characterize the dynamic soil properties, and validate geotechnical techniques currently being used to characterize deep soil sites. We will also be able to assess the provisions recommended by the National Earthquake Hazards Reduction Program (NEHRP) for the upper Mississippi Embayment. The results will be applicable to sites throughout the upper Mississippi Embayment, as well as deep soil sites elsewhere (such as Southern California). quantifying and characterizing the path and site effects in the transition zone between the floodplain and the loess bluffs is necessary for realistic ground-motion estimations in areas that are not within the immediate boundaries of the New Madrid Seismic Zone, but are subject to the effects of earthquakes that occur in the seismic zone. The current NEHRP recommended provisions classify the top 30 meters of soil at a site, without regard to the damping effects and natural period of the overall sediment thickness. These provisions must be quantified and validated so that local structures such as dams and bridges can be retrofitted according to adequate criteria.
