Earthquake hazard mapping at a scale of 1:250,000 in the Central United States Earthquake Consortium (CUSEC) region has been undertaken by the Association of CUSEC State Geologists (CUSEC-SGs). This association includes the states of Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri, and Tennessee. For this region, mapping directed at assessment of amplification of shaking and potential for liquefaction was given the highest priority. Ground motions generated by earthquakes can be amplified by the nonlithified geologic materials (NGM) resting on the bedrock (lithified geologic materials). The degree of amplification is directly related to variations in the geotechnical properties and thickness of the NGM. Mapping, therefore, requires the documentation of the NGM (soils) and their geotechnical characterization of the NGM (soils) in three dimensions.

The classification of map units follow the procedure introduced by Borcherdt, 1994. He developed a "soils" classification dependent on correlation among measured amplifications, shear-wave velocity characteristics, and the physical properties of the NGM as mapped at the surface in California. With these correlations, he established classification criteria for "Soil Profile Types" which are primarily dependent on the shear-wave velocity values of NGM. Shear-wave velocity values for similar "soil profile types" described in the Midwest and California were found to be essentially the same. However, for NGM's that are dissimilar to Borcherdt's descriptions, estimates or direct shear-wave velocity measurements were required for the CUSEC area.

Our maps show "soil profile types" coded A through F. Not only does each "soil profile type" amplify earthquake ground motions by various amounts, a single type will amplify earthquake ground motions a different amount depending upon the level of shaking of the underlying bedrock. This dependency is displayed in the legend as a range of amplification per "soil profile type" shown for acceleration on bedrock from 0.1g through 0.5g.

CUSEC-SGs are preparing both paper and electronic versions of our maps. Because we continue to collect borehole, shear-wave velocity, and standard penetration test data throughout the CUSEC region, the electronic version of the mapping program will be upgraded continuously. Any one of our maps can be used as a "stand-alone" product showing relative amounts of shaking among areas, or the map can be used in conjunction with other maps which show the expected acceleration on bedrock for various earthquake source locations and magnitudes. The combination of the acceleration map with the "soil profile type" map can be used to project the expected shaking on the ground surface for various earthquake scenarios.

Our mapping procedure has direct application to both the Federal Emergency Management Agency's (FEMA) Earthquake Loss Estimation Program (HAZUS) and the 1997 Unified Building Code use. Our maps were produced primarily for use by FEMA, State Emergency Management agencies, and other state and local governments for earthquake preparedness planning and exercises. However, the maps and associated databases also have application for the needs of the Federal and State Departments of Transportation and the Environmental Protection Agencies.

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