URBAN SEISMIC HAZARD MAPPING IN MEMPHIS, TENNESSEE: PROCESS, METHODOLOGY, RESULTS, AND PRODUCTS

MEMPHIS HAZARD MAPPING WORKING GROUP,( CRAMER, C.,) U.S. Geological Survey, 3890 Central Ave., Memphis, TN 38152, cramer@usgs.gov,

http//www.ceri@memphis.edu/usgs/.

The U.S. Geological Survey and its partners have completed high-resolution seismic hazard maps and derivative products for Memphis, Tennessee. While focusing on generating products for public use, this effort stimulated new analytic developments including approaches to characterizing local geology, incorporating local sediment response, and calculating fully probabilistic ground motion estimates. It also stimulated research on outstanding scientific questions about nonlinear sediment response, seismic wave attenuation, and long-term deformation and recurrence in intraplate environments. Products include geology maps, databases of well and engineering logs and their interpretations, new shear-wave velocity profiles, probabilistic and scenario ground-motion maps that include the effect of local sediments, liquefaction susceptibility maps, a web site, and publications for technical and lay audiences. All products are available digitally via the web. Comparing the results from the Memphis maps with the national seismic hazard maps (which do not take local soils into account) illustrates the importance of local geology in making seismic hazard estimates. The thick soft sediments under Memphis dampen high-frequency seismic waves (up to 30% at 0.2 s) and reduce regional ground-motion gradients (making ground motion hazard more uniform) that can affect smaller structures. These sediments also amplify low frequency waves that predominantly affect large structures such as high-rise buildings and long bridges (double the amplitudes at 1.0 s). We also quantify liquefaction potential in a new way that accounts for sparse and uncertain observations, and includes probabilistic estimates of susceptibility. Ensuring that mapping products are of maximum use to a public not familiar with earthquake hazard issues is the challenge we now face.