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DOE-STD-1024-92
An Interim Recommendation
for DOE Use of the
LLNL and EPRI Hazard Curves
C. Allin Cornell
September, 1991
A Report to
J. K. Kimball
For reasons well described in the DOE Seismic Working Group document1. "Use of the
LLNL and EPRI Probabilistic Seismic Hazard Curves: Interim Position", it is desirable to
establish a straight-forward procedure to use the most dependable, stable information provided
by both of the two large-scale, state-of-the-art regional seismic hazard analyses conducted by
Lawrence Livermore National Laboratory for the Nuclear Regulatory Commission and by the
Electric Power Research Institute for the Seismicity Owners Group, a set of electric utilities
operating nuclear power plants, in the U. S.
Based on a comparison of results at some 70 EUS sites it is clear that the two studies
produce hazard results that are quite similar in terms of their central or median estimates, but
quite different in their estimates of the uncertainty about this central estimate, as evidenced by
major differences in their 15 percentile, 85 percentile, and mean2 estimates. Nonetheless there
are certain stable trends in these differences. These will be discussed below. This stability
suggests that it is feasible to establish a simple procedure for combining the results of the two
studies that can be considered applicable at all sites.
For use with UCRL-15910 and DOE's Interim Position document, the objective here is
to provide a ground motion measure associated with a prescribed target mean hazard level,
e.g., 2 x 10-4. The procedure will make use of the more consistent, stable parts of the LLNL
and EPRI studies, primarily their median hazard estimates. In addition, in order to reflect the
uncertainty-induced difference between the median estimate and the higher mean hazard
estimate, an additional factor will be developed to modify the median-based estimate. This
factor is based on the observed consistencies within and between the studies (e.g., site-to-site,
frequency-to-frequency, etc.), coupled with a simple averaging approach.
Some Observed Differences and Consistencies. The "data" used for this study were
primarily median and 85 percentile hazard estimates for 70 sites produced both by LLNL and
by EPRI for PGA and spectral velocities (at 25, 10, 5, 2.5, and 1 hertz) at a range of
1
This report is designed to serve as an attachment to that report, avoiding duplication
of text, figures, and tables.
2
In this case the mean is a measure of the degree of uncertainty. The reason is
simple. In analyses of rare events, e.g., in the 10-3 to 10-5 range, it is common that the
mean estimate is much larger than the median, e.g., of the order of the 85 percentile,
because the uncertainty band spreads over two or more decades and, although often
quite symmetrical with respect to the log probability, it is skewed strongly right (upward)
on an arithmetic scale.
D-1


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