Seismic Working Group Meeting of March 11, 1991

DOE-STD-1024-92

process used to include attenuation models is different between

the two studies. The EPRI seismic hazard curves were based on

holding a ground motion workshop and then selecting three

attenuation models while the LLNL results are based on input from

several experts. In a relative sense, there is the potential that

attenuation uncertainty is underestimated in the EPRI study.

These issues degrade confidence in the fractiles below about the

15th percentile and above about the 85th percentile from the EPRI

study, which could impact the reliability of the mean;

There has been concern regarding the ground motion model

selected by LLNL-AE5. Lawrence Livermore National Laboratory

(1987) has recognized that the input from LLNL-AE5 can dominate

the upper fractiles of the hazard results, particularly for rock site

conditions. Both McCann (1991) and McGuire (1990a, 1990b and

1991) have expressed two concerns related to LLNL-AE5. The

development of the model has been questioned, regarding the

assumptions made and the methods used to develop an

appropriate intensity based attenuation model for the Eastern

United States based on Western ground motion data.

Additionally, the model has been compared to existing strong

motion data from the East, and questions have been raised

regarding how well the model fits the data compared to

attenuation models selected by other experts. These two factors

suggest that the hazard fractiles from the LLNL study which are

dominated by LLNL-AE5 should not be used to define the ground

motion. This supports the above assessment that the mean

hazard curve from the LLNL study may not be realistic;

The Uniform Hazard Spectra defined by the LLNL and EPRI studies

represents the combination of standard spectral shape models with

direct spectral ordinate models. The standard spectral shape is

typically based on statistical analysis of large (M>6) earthquakes

while the direct spectral ordinate method is based on a specified

magnitude and distance. The uncertainty distributions associated

with Uniform Hazard Spectra appear to be less stable than the peak

acceleration seismic hazard curves. These differences degrade

confidence that the Uniform Hazard Spectral shape actually

represents equal hazard spectra, and thus they should not be used;

and

The seismic hazard curves which appear to be most stable are the

median seismic hazard curves, from both the LLNL and EPRI studies.

Additionally, the uncertainty assessment in both studies regarding the

difference between the medians and the 15th percentile and 85th

percentile should be accounted for in the Standard.