Approach Selected - Continued

DOE-STD-1024-92

As shown on Figures 2 through 5 and in Appendix A, the absolute

value of the ratio of the 85th/median is drastically different between

the two studies. The representative value at a peak ground

acceleration of 0.20g is about 3.5 for the EPRI study, and about 7.2

for the LLNL study without LLNL-AE5 and about 11.2 with LLNL-

AE5 included. In general, if LLNL-AE5 is excluded, the ratio for the

LLNL results decreases by about 20 to 40 percent;

The value of the ratio of the 85th/median is dependent on the

response spectral frequency. The ratio value increases as the

spectral frequency decreases, with a more drastic trend observed

for the EPRI data. The use of the lower frequency information is

complicated by the observation that the attenuation models selected

by the LLNL and EPRI studies result in different spectral shapes.

As discussed above, some attenuation models provide direct

spectral estimates while others are associated with standard

spectral shapes such as the Newmark/Hall spectral shape. These

two different approaches can be one of the factors which result in

larger ratio at the lower frequency; and

The difference between the LLNL and EPRI median hazard curves

is generally less than a factor of 2. This result is displayed on

Figure 6 for a peak ground acceleration of 0.20g for the LLNL

results including attenuation expert 5. Table 1 also provides

summary statistics for the ratio of the medians, broken down by

rock and soil site conditions, with and without LLNL-AE5. The

largest difference between the LLNL and EPRI medians is for soil

sites where the uncertainty in ratios is relatively large. This is

thought to reflect differences between the two studies regarding how

soil conditions could impact ground motion estimates.

Based on the review of this information, the approach selected for the

seismic hazard position is as follows: The trends in the ratio of the

LLNL/EPRI medians and individual LLNL or EPRI 85th/median can be

used to derive a pseudo-mean correction factor. Trends observed are

relatively stable, which result in the advantage that a specific correction

factor can be derived to result in consistent hazard estimates from site to

site. This is thought to be superior to the other approaches in that specific

reliance on the LLNL or EPRI 85th percentile hazard curves may not be

warranted given their extreme differences, and it is not necessary to derive

complex weighing methods to combine the two studies.

The derivation of the specific correction factor is built around the most

stable seismic hazard curves, the LLNL and EPRI median curves. A

decision was also required regarding the specific spectral frequency and

level of ground motion to base the correction factor on. The preferred

frequency would be one that shows a high degree of relative stability and

be of engineering significance, such as between 2.5 and 10 hertz.