Part A- Comments - Continued

DOE-STD-1024-92

The assumption of lognormality does not necessarily apply to all the sites in

the EUS. In particular, the divergence is maximum for some sites strongly

affected by seismic source areas with large upper magnitude cutoff. Ground

motion expert 5 tends to increase this effect in the LLNL study. Figure 1 (not

shown here), taken from the LLNL., 1989 study, shows an example of hazard

histogram for .25g at one site in the southeast, and for a single seismicity

expert. Fortunately, this bi-modality shown in Figure 1 for S-expert 1, occurs

at different places for the other experts, hence the mixed data, as shown in

Figure 2 (not shown here), (mixed over all the S-experts) does not exhibit

such a high bi-modality, for this site, and the assumption of lognormality is

adequate for the present purpose. (This brings up a side comment,

specifically that if one starts removing one or several S-experts from the

analysis, the assumption of lognormality becomes poorer and poorer.)

Although I have not had a chance to review the case of all EUS sites, I

cannot, at this point, discount the possibility that the lognormality assumption

could be grossly wrong in some locations of the EUS.

LLNL-3

The values chosen for rmm are representative of most of the sites in the EUS.

There are a few cases, as shown in Figure 4 of the proposed document, for

which the LLNL values are drastically different from the selected values. It

could be, as it is mentioned in several places in the attachment, that those

anomalous values correspond to sites in the Gulf Coast area, where the

estimated hazard is very low anyway and leads to large uncertainties. It may

also be that the parameter should be region dependent.

LLNL-4

With the selected slope b = 3.5 the variation in the composite mean ground

motion aCEL at 2 10-4 could vary from 1.45 times the EPRI-LLNL composite

median aC to as much as 2.17 times (see table below).

(using aCEL = aC . rmm1/b)

rmm = (log [composite85/median])2

CEL/aC

85th/median

rmm

EPRI

LLNL

3.5

3.65

1.45

3.5

23.2

11.2

2.00

3.5

30.0

15.0

2.17

A-5