The U.S. Geological Survey (USGS) completed new probabilistic seismic hazard maps for the contiguous United States in 1996. These hazard maps form the basis of design maps used in the 1997 edition of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings prepared by the Building Seismic Safety Council and published by FEMA. The maps depict peak horizontal ground acceleration (PGA) and spectral response at 0.2, 0.3, and 1.0 sec periods, with 10%, 5%, and 2% probabilities of exceedance in 50 years, corresponding to return times of about 500, 1000, and 2500 years, respectively. The maps are the result of a set of regional workshops of geoscientists and engineers, where the methodology was revised based on feedback from the participants. The construction of the maps involved three basic components of the seismic hazard. First, we used spatially-smoothed historic seismicity as one portion of the hazard calculation. Second, we considered large background source zones based on broad geologic criteria to quantify hazard in areas with little or no historic seismicity, but with the potential for generating large events. Third, we included the hazard from specific fault sources. Recurrence estimates for large earthquakes in New Madrid and Charleston SC were taken from recent paleoliquefaction studies. While the probabilistic accelerations are lower in New Madrid than in California for a return time of 500 years, the probabilistic accelerations are similar between New Madrid and areas of California near the San Andreas fault system at the longer return time of 2500 years. The influence of large New Madrid earthquakes (moment magnitude about 8) varies depending on the location, return time, and ground-motion frequency. For St. Louis, the hazard for PGA is dominated at a 500 year return time by magnitude 4.5-6 events within about 75 km of the city. For spectral response at 1 sec period, the relative contribution to the hazard at St. Louis from large New Madrid events increases. At Memphis, the PGA hazard for a 500-year return time is influenced about equally by large New Madrid events and smaller, close-in earthquakes. However, for longer return times of 1000 and 2500 years, the PGA hazard at Memphis is dominated by large New Madrid earthquakes. The USGS hazard maps are for a firm-rock site condition. When using the maps for sites on the Mississippi Embayment, these probabilistic ground motions must be adjusted for propagation through the sedimentary deposits within the Embayment. The national seismic hazard maps, documentation, interactive mapping tool, hazard look-up by zipcode, hazard de-aggregations, and hazard look-up by zipcode are available from our website at http://geohazards.cr.usgs.gov/eq/.

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