Re-evaluating the seismic hazard at U.S. Bureau of Reclamation dams

As part of the U.S. Bureau of Reclamation's program to re-evaluate their existing dams for seismic safety, we have been performing site-specific probabilistic seismic hazard analyses to estimate ground motions for a range of annual exceedance probabilities. These probabilities generally range from 10-4 to 2 x 10-5/yr. corresponding to return periods of 10,000 to 50,000 years, respectively. The ground motions are input into risk analyses performed by Reclamation to determine if further studies and mitigation measures are required. They are also used to perform pseudo-static analyses of their dams, and in some cases, to assess the potential for liquefaction and to develop time histories for dynamic response analysis. The probabilistic approach is a relatively recent departure from the deterministic methodology previously used by Reclamation, which employed the concept of Maximum Credible Earthquakes. Since 1998, we have re-evaluated 69 dams located throughout the western U.S. The lowest hazard occurs in regions where active faults are generally absent or where they may have low slip rates e.g., eastern New Mexico and southern Arizona. Conversely, high-hazard regions include those near active faults such as western Washington, which is situated over the Cascadia subduction zone, and adjacent to the Wasatch fault in central Utah. Regions of moderate hazard include for example, western Idaho and central Washington. 14 pp., 16 references.