Pilot program to assess seismic hazards of the Granite City, Monks Mound, and Columbia Bottom quadrangles, St. Louis Metropolitan area, Missouri and Illinois

Three 1:24000 scale quadrangles were selected for a pilot program intended to evaluate seismic site response across the spectrum of geologic conditions underlying the St. Louis Metropolitan area, using the Granite City, Monks Mound and Columbia Bottom quadrangles. These evaluations included assessments of: (i) site amplification distributions; (ii) probabilistic hazard analysis of PGA, 0.2 second and 1.0 second spectral accelerations for 2%, 5% and 10% probability of exceedance in 50 years; (iii) two scenario earthquakes and their associated PGA, 0.2 sec, and 1 sec spectral accelerations; and (v) sensitivity and uncertainty analyses. These hazard maps were prepared using a fully-probabilistic approach, which considered uncertainties in the input data and in the predicted site amplification.;The results indicate that the variations of the soil conditions in the St. Louis area exert; (i) a significant influence on the amplitude, and (ii) contrasting shaking characteristics for each of the ground motion parameters. Accurate estimates of the soil cap thickness are most important for assessing amplification. The other important parameter affecting site amplification was the input earthquake time histories. The thickness and shear wave velocity of the weathered bedrock horizon (below the soil cap) appears to have little impact on site amplification, or upon the associated uncertainties. This study included the effects of the underlying geologic conditions, using virtual borings on a 500 meter grid. It has been predicted that on loess covered uplands, earthquake forces may be most severe for short period structures, while in the flood plains underlain by alluvium, earthquake forces can be expected to be more severe for long period structures.