Update of Urban Seismic-Hazard Maps for Memphis and Shelby County, Tennessee: Geology and VS Observations

Abstract

In 2004, Memphis urban seismic‐hazard maps were completed for a six‐quadrangle area centered on Memphis and southern Shelby County (Cramer et al. , 2004, 2006, 2008). Both probabilistic and scenario seismic and liquefaction hazard maps were generated for the six‐quadrangle study area. These urban seismic‐hazard maps, particularly the liquefaction hazard maps, have been used by consultants and public agencies to address seismic‐hazard mitigation in the Memphis area. Memphis has the second highest annualized expected loss (AEL, expected losses to building inventory times frequency of occurrence) and the highest seismic hazard and AEL ratio (normalized by the building inventory exposed to loss) in the central and eastern United States (Petersen et al. , 2008; Federal Emergency Management Agency [FEMA], 2008). Additionally, the U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) has been updated since the first Memphis urban seismic‐hazard maps were completed in 2004 with significant changes in the New Madrid source model (number and weighting on alternative fault‐rupture locations and distinctions in occurrence rates on the north, central, and south segments) and attenuation relations (updated and added relations showing on average lower ground motions; Petersen et al. , 2008) that influence Memphis hazard. Urban seismic‐hazard maps consider the effect of local geology on the earthquake ground‐motion response for the analysis to have practical benefit. To do this, information on the local distribution and thicknesses of soils is needed. Specifically, two pieces of information that affect ground‐motion amplification are needed: the thicknesses at a site of each soil type (lithology) and each soil type’s physical (geotechnical) properties. Site amplification is determined by taking a soil profile (soil type thicknesses and physical properties) and subjecting that soil profile to earthquake shaking (time history) originating in the solid rock at the bottom of the soil profile to calculate the expected shaking at …