This polygon shapefile represents seismic hazards in the United States. The data represent a model showing the probability that ground motion will reach a certain level. This map layer shows peak horizontal ground acceleration (the fastest measured change in speed, for a particle at ground level that is moving horizontally due to an earthquake) with a 10% probability of exceedance in 50 years. Values are given in %g, where g is acceleration due to gravity, or 9.8 meters/second^2. The lines of equal hazard, which are the lines between the polygons, were determined by interpolating from a grid of equally spaced points in latitude and longitude. Each point was weighted based on the seismic hazard at that location. The grid spacing is 0.1 degrees for Alaska, 0.05 degrees for the conterminous United States, and 0.02 degrees for Hawaii. This layer is part of the 1997-2014 edition National Atlas of the United States. This map summarizes the quantitative information, available from geologic and geophysical sources, about seismic ground motion hazard in the United States. The data are intended for geographic display and analysis at the national level, and for large regional areas. The data should be displayed and analyzed at scales appropriate for 1:2,000,000-scale data. No responsibility is assumed by the U.S. Geological Survey or the National Atlas of the United States in the use of these data. National Atlas of the United States. (2012). Seismic Hazard Map for the United States. National Atlas of the United States. Available at: http://purl.stanford.edu/rm034qp5477. This map layer was prepared by combining spatially-smoothed historic seismicity information with information from fault-specific sources. The acceleration values contoured are the random horizontal component. The reference site condition is firm rock, defined as having an average shear- wave velocity of 760 meters/second in the top 30 meters corresponding to the boundary between National Earthquake Hazards Reduction Program (NEHRP) site classes B and C. For more information about the USGS National Seismic Hazard Maps, please visit http://earthquake.usgs.gov/hazards/index.php. A description of how the underlying geologic and geophysical data were prepared as well as the methodology used in calculating seismic hazard for a geographic location can be found in the following reports: Petersen, M.D., Frankel, A.D., Harmsen, S.C., Mueller, C.S., Haller, K.M., Wheeler, R.L., Wesson, R.L., Zeng, Yuehua, Boyd, O.S., Perkins, D.M., Luco, Nicolas, Field, E.H., Wills, C.J., and Rukstales, K.S., 2008, Documentation for the 2008 Update of the United States National Seismic Hazard Maps: U.S. Geological Survey Open-File Report 2008-1128, 61 p. Klein, F.W., A.D. Frankel, C.S. Mueller, R.L. Wesson and P.G. Okubo, 2001, Seismic Hazard in Hawaii: high rate of large earthquakes and probabilistic ground motion maps, Bulletin of the Seismological Society of America, v. 91, pp. 479-498. Wesson, Robert L., Boyd, Oliver S., Mueller, Charles S., Bufe, Charles G., Frankel, Arthur D., Petersen, Mark D., 2007, Revision of time-Independent probabilistic seismic hazard maps for Alaska: U.S. Geological Survey Open-File Report 2007-1043, 33p. This layer is presented in the WGS84 coordinate system for web display purposes. Downloadable data are provided in native coordinate system or projection.