Lawrence Livermore National Laboratory Draft New Production Reactor Probabilistic Seismic Hazard

DOE-STD-1024-92

difference between the seismicity parameters and upper magnitude

cutoffs.

The LLNL and EPRI attenuation models were compared to a set

of empirical Eastern United States data, for peak acceleration

and response spectral values. It was found that several

attenuation models fit the empirical set at frequencies less than

about 5 hertz. LLNL-AE5 fit the data particularly poorly at the low

frequencies.

2.2

Lawrence Livermore National Laboratory staff have undertaken a

probabilistic seismic hazard study for DOE at the Savannah River Site

(LLNL, 1992). Lawrence Livermore National Laboratory staff, recognizing

the technical issues related to their earlier work, have attempted to

ensure that seismic hazard uncertainties are properly identified and

quantified. Draft results from Savannah River when compared to earlier

LLNL estimates explicitly show reductions in uncertainty estimates. Such

results are likely to have generic implications. The following three issues

appear to have the most significant influence on the draft revised

Savannah River results.

LLNL-AE5 has modified the attenuation relationship he had

previously selected in the LLNL Eastern United States study. For

Savannah River this change is judged to be relatively minor

because of the deep soil site conditions. For rock site conditions

the revised attenuation relationships is likely to be more

significant.

The characterization of attenuation uncertainty is assessed

inconsistently between experts. Lawrence Livermore National

Laboratory has identified attenuation uncertainty as a key issue in

quantifying mean estimates of seismic hazard. Lawrence

Livermore National Laboratory has performed initial sensitivity

studies using modified estimates (narrower range) of attenuation

uncertainty which results in reduction of mean seismic hazard of

about 30 to 50 percent.

The assessment of uncertainty in earthquake occurrence

parameters is treated inconsistently by different experts.

Lawrence Livermore National Laboratory is developing diagnostic

tools to better determine if seismicity experts have properly

quantified earthquake occurrence uncertainty. Preliminary results

suggest that some LLNL seismicity experts may over-estimate the

range in earthquake occurrence parameters.