Seismic Hazard Position: Use of LLNL-AE5

DOE-STD-1024-92

4.2

Seismic Hazard Position: Use of LLNL-AE5

The final issue pertains to the use of LLNL-AE5. Figure 7 displays the

existing mean EPRI and LLNL (with and without LLNL-AE5) seismic

hazard results and the draft preliminary revised results from LLNL using

a reduced range of ground motion uncertainty for the Savannah River

Site. All estimates were made using a lower bound magnitude of 5.0. As

discussed previously, the revised LLNL mean results are significantly

lower than the earlier LLNL results. Also shown in Figure 7 are the

results of the LLNL/EPRI correction factor with (Choice 2) and without

(Choice 1) LLNL-AE5 taken from Appendix E applied to the Savannah

River Site.

The revised LLNL mean results reflect the following changes: (1) the

seismicity experts have revised earthquake recurrence parameters (a

and b values) to generally more narrow uncertainty distributions; (2) the

attenuation experts are explicitly addressing the definition and magnitude

of attenuation random uncertainty (one of the uncertainty terms); and (3)

LLNL-AE5 has altered his attenuation model. Thus, the revised

probabilistic results for Savannah River reflect increased attention to

uncertainty assessment for all seismic hazard parameters. The changes

made by LLNL-AE5 are expected to be more significant at rock sites

where the original LLNL-AE5 input had most impact (Bernreuter, et al,

1987). In general the above noted changes (trends in reduced mean)

are judged to be generic and applicable to all sites.

It is the S WGs judgement that Choice 1 more accurately reflects the

correction to be applied to the LLNL and EPRI results. Figure 7 shows

that for the Savannah River Site that the Choice 1 correction factor

(using the existing LLNL results without LLNL-AE5) more accurately

reflects the assessment of mean seismic hazard for the revised LLNL

results. While the specific degree of uncertainty assessment is likely to

change from site to site, the preliminary Savannah River results suggests

that the existing LLNL mean hazard results may substantially

overestimate the mean hazard, consistent with the previous assessment

that the mean hazard curves should not be directly used.

Based on the above, the S WG position is that the pseudo-mean

correction factor should be based on using the existing LLNL results

without LLNL-AE5. Thus for probabilities associated with low and

moderate hazard the correction factor is 1.80 and for high hazard the

correction factor is 1.65 (Appendix E).