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
SUMMARY
The U.S. Geological Survey was requested by the U.S. Department of the Interior
to review the design values and the issue of reservoir-induced seismicity for a
concrete gravity dam near the site of the previously-proposed Auburn Dam in the
western foothills of the Sierra Nevada, central California. The dam is being
planned as a flood-control-only dam with the possibility of conversion to a
permanent water-storage facility. As a basis for planning studies the U.S. Army
Corps of Engineers is using the same design values approved by the Secretary of
the Interior in 1979 for the original Auburn Dam. These values were a maximum
displacement of 9 inches on a fault intersecting the dam foundation, a maximum
earthquake at the site of magnitude 6.5, a peak horizontal acceleration of
0.64 g, and a peak vertical acceleration of 0.39 g. In light of geological and
seismological investigations conducted in the western Sierran foothills since
1979 and advances in the understanding of how earthquakes are caused and how
faults behave, we have developed the following conclusions and recommendations:
- Maximum Displacement. Neither the pre-1979 nor the recent observations
of faults in the Sierran foothills precisely define the maximum
displacement per event on a fault intersecting the dam foundation.
Available field data and our current understanding of surface faulting
indicate a range of values for the maximum displacement. This may
require the consideration of a design value larger than 9 inches. We
recommend reevaluation of the design displacement using current seismic
hazard methods that incorporate uncertainty into the estimate of this
design value.
- Maximum Earthquake Magnitude. There are no data to indicate that a
significant change is necessary in the use of an M 6.5 maximum earthquake
to estimate design ground motions at the dam site. However, there is a
basis for estimating a range of maximum magnitudes using recent field
information and new statistical fault relations. We recommend
reevaluating the maximum earthquake magnitude using current seismic
hazard methodology.
- Design Ground Motions. 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.
- Reservoir-Induced Seismicity. The potential for reservoir-induced
seismicity must be considered for the Auburn Darn project. A
reservoir-induced earthquake is not expected to be larger than the
maximum naturally occurring earthquake. However, the probability of
an earthquake may be enhanced by reservoir impoundment. A
flood-control-only project may involve a lower probability of significant
induced seismicity than a multipurpose water-storage dam. There is a need
to better understand and quantify the likelihood of this hazard. A
methodology 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.
- Reevaluation of Design Parameters. 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 statistical relations for faulting and ground motions, and
current computational seismic hazard methodologies that incorporate
uncertainty into the analysis. The reevaluation does not require
significant new geological field studies.
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^{1}D.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