THE USE OF QUATERNARY STRATIGRAPHY IN THE MEMPHIS-SHELBY COUNTY EARTHQUAKE HAZARD MAPPING PROJECT: A WANNABE GLACIATED TERRANE

Although never glaciated, the Memphis, Tennessee, area was profoundly impacted by Wisconsinan and earlier glaciations to the north. Memphis and Seattle, Washington, which was glaciated, have embarked on parallel projects to map the near-surface geology for the purpose of large-scale urban seismic hazard mapping. Memphis was selected as typical of the central and eastern U.S., having a paucity of seismically engineered buildings and infrastructure and relatively low regional attenuation. Memphis has a dense urban population near major faults and a high probability of a moderate earthquake in the near future. Furthermore, a sound scientific foundation has already been established the region through years of earthquake hazards studies. The strength of ground shaking expected from earthquakes depends, in part, on the geological materials beneath the site, particularly in the upper 60 m. Thus, providing state-of-the-art hazards maps has necessitated a comprehensive program to map and understand the Quaternary geology beneath the city of Memphis, a task hampered poor exposures and a built-up urban environment. A few exposures do exist, but we are highly dependent on geotechnical boring logs, electric logs from water and hydrocarbon exploratory wells, and inferences from surface geophysics and geotechnical testing contracted for this project. To handle the vast dataset involved, we have created the Shelby County Subsurface Database [http://gwidc.gwi.memphis.edu/website/introduction/], which can be queried via a web-browser or ArcView interfaces. We are using the database to construct a three-dimensional model of the Memphis-Shelby County geology. In addition to the subsurface database itself, the products will be 1:24,000-scale surficial geology maps (several of which have been completed), liquefaction- and landslide-susceptibility maps, ground motion amplification maps, and probabilistic ground motion maps. All products will be accessible digitally.