First USGS Urban Seismic Hazard Maps Predict the Effects of Soils

Urban seismic hazard maps are designed to reflect the expected ground shaking from earthquakes that affect a particular location. Thus they must incorporate the effects of local geology, including soils, which are not normally included in national or regional seismic hazard maps. The U.S. Geological Survey (USGS) has released its first set of urban seismic hazard maps for Memphis, Shelby County, Tennessee. State-of-the-art methods are used to incorporate soil effects in these maps. Both probabilistic and scenario (deterministic) maps have been produced. This article briefly summarizes the seismic hazard methodology and model used to produce the maps and the knowledge uncertainties incorporated in them. Memphis, Tennessee, is a location where damaging earthquakes are only moderately likely, but where the consequences of earthquakes, mainly in the New Madrid seismic zone, can be very high (Figure 1). This densely populated urban area is built on a 1-kilometer-thick sequence of sediments deposited in a structural trough known as the Mississippi embayment. Th is thick sequence of sediments significantly affects earthquake ground motions (Bodin et al. , 2001; Hashash and Park, 2001). These urban seismic hazard maps and their derivative products represent the collaborative efforts of the USGS and its partners. A USGS website (http://www.ceri.memphis.edu/usgs/) and Open-File Report 04-1294 (Cramer et al. , 2004) document how these seismic hazard maps were generated. The Memphis maps complement the USGS national seismic hazard maps (Frankel et al. , 2002), which currently do not include the effects of local geologic structure and variations in soil conditions. Seismic hazard maps can represent the ground-motion hazard from earthquake shaking that affects a particular location. The USGS national seismic hazard maps provide an overview of seismic ground-motion hazard across the United States and are currently for a uniform soil condition (Frankel et al. , 2002). The Memphis maps …