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Review of seismic-hazard issues associated with the Auburn Dam project, Sierra Nevada foothills, California

By USGS Auburn Project Review Team 1

U.S. Geological Survey Open File Report 96-0011

CONCLUSIONS AND RECOMMENDATIONS

Seventeen years have passed since the approval of the design values for the original Auburn Dam. These were based on the field data, the understanding of the earthquake process, and the seismic hazard methodology available at that time. In the interim, new information has been obtained on the behavior of faults in the western Sierran foothills, important advances have been made in understanding the causes of earthquakes and how faults work in time and space, and new methods to quantify seismic hazards and incorporate uncertainties into these analyses have been developed. We have reviewed and evaluated the 1979 seismic design values in light of these considerations and reach the following conclusions and recommendations:

  1. Neither the pre- nor post-1979 data precisely define the maximum displacement per event on a fault intersecting the dam foundation. In 1979 a single value was selected for the maximum displacement. In present seismic hazard practice a strong effort is made to identify sources of uncertainty and incorporate these into design values. Alternative interpretations of field observations and alternative tectonic and fault behavior models can be considered, weighted, and combined to derive a design value and the uncertainty associated with it. There is a basis for estimating a range of potential maximum displacements at the Auburn Dam site using available field data and our current understanding of surface faulting. This may require the consideration of a design value larger than 9 inches. We recommend that the design displacement be reevaluated.

  2. The maximum earthquake magnitude for ground motions at the dam site in 1979 was a single value of M 6.5. There are no data to indicate a significant change should be made in using M 6.5 to estimate design ground motions. However, the updated statistical relations available to calculate maximum magnitude are more robust, and their associated uncertainties are better quantified, than those used in 1979. Also, there are uncertainties in fault rupture parameters used for magnitude calculations, particularly fault length, that should be reexamined in light of present concepts of fault segmentation. There is a basis for estimating a range of potential maximum magnitudes, both higher and lower, using the recent field information and the new statistical fault relations. We recommend reevaluating the maximum earthquake magnitude using updated seismic hazard methodology.

  3. A large number of strong-motion records have been acquired and significant advances in understanding of ground motion have been achieved since the original evaluations. The design value for peak horizontal acceleration (0.64 g) is larger than the median of one recent study and smaller than the median value of another. The value for peak vertical acceleration (0.39 g) is somewhat smaller than median values of two recent studies. We recommend a reevaluation of the design ground motions that takes into account new ground motion data with particular attention to rock sites at small source distances.

  4. The potential for reservoir-induced seismicity must be considered for the Auburn Dam project. A reservoir-induced earthquake is not expected to be larger than the maximum naturally occurring earthquake. However, the probability of the occurrence of earthquakes up to the maximum magnitude event may be increased by reservoir impoundment. Both the flood-control-only dam and a permanent-water-storage dam entail some likelihood of reservoir-induced seismicity, although the flood-control-only version may involve a lower probability of significant induced seismicity than the multipurpose project. There is a clear need to improve our understanding of the probability of this hazard, and this issue will require additional analysis if the project continues. Methods should be developed to quantify the potential for reservoir-induced seismicity using seismicity data from the Sierran foothills, new worldwide observations of induced and triggered seismicity, and current understanding of the earthquake process.

  5. The reevaluation of the maximum displacement, maximum magnitude earthquake, and design ground motions can be made using available field observations from the Sierran foothills, updated worldwide statistical relations for faulting and ground motions, and current computational seismic hazard methods that incorporate uncertainty into the analysis. This does not require significant new geological field studies. If the Auburn Dam Project is authorized, a dense local seismic network should be installed around the Auburn Dam site to determine a baseline level of seismicity before impoundment.

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    1D.P. Schwartz, W.B. Joyner, R.S. Stein, R.D. Brown, A.F. McGarr, S.H. Hickman, and W.H. Bakun, all at 345 Middlefield Road, Menlo Park, CA 94025

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