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SEISMIC IMPACT ZONES Preliminary map of horizontal acceleration (expressed as percent of gravity) in rock with 90 percent probability of not being exceeded in 250 years. From  Algermissen, S.T., Perkins, P.C., Thenhous, P.C., Hanson, S.L., and Bender, B.L., 1982, Probabilistic Estimates of Maximum Acceleration and Velocity in Rock in the Contiguous United States: U.S. Geological Survey Open-File Report 82-1033, Plate 3.          return to Iowa Earthquakes Homepage