Recommended Interim Procedure.

DOE-STD-1024-92

especially from PGA down to 2.5 hertz, where (at the amplitude levels listed

above) the ratio is (7/3.5) = 2.0, (6/2.5) = 2.4, (9/4.5) = 2.0, (18/8) = 2.25, and

(38/18) = 2.1 for PGA, 25 hertz, 10 hertz, 5 hertz, and 2.5 hertz respectively3.

We shall use these consistent ratios in the recommendations below.

Next look at the most positive part of the LLNL versus EPRI comparison, the central or

median estimates of hazard. These are graphed (for 0.2g PGA) in Figure 4 of the Interim

Position Report, and statistics are reported in Table 1 of that document. The conclusion is that

these results are remarkably consistent. (A very few sites are shown with a ratio of median

hazards of 5 or more; these are predominantly very low hazard Gulf Coast sites). The typical

ratio of 2 is, recall, between hazards in the 10-3 to 10-4 range; this is a difference of only 0.3 in the

exponent. (Or, as we shall see below, a factor of only about 20% in the associated PGA.) the

implication is that the average (simple or geometric) median hazard at a given PGA or the

average PGA at a given (median) hazard is a good representation of both of the studies. In

different words, the study-to-study variability in the results is small, particularly in comparison to

the uncertainty (as measured by, say, the ratio of 85 percentile to median hazard, where typical

values are 4 to 8, as we have seen above). Further, the site-to-site variability in this ratio is

relatively small (Table 1, Interim Position Report), imply that this simple conclusion can be

accepted for all sites.

Recommended Interim Procedure. Based on the observations above the following

procedure is recommended for developing from the two studies, a ground motion spectrum

associated with a specified mean hazard level.

Focus on the PGA as the ground motion level parameter.

Step (1)

As seen above the 85/median hazard ratios are consistent at the highest

frequencies for the amplitudes of prime interest. There are strong technical

reasons to prefer a spectral ordinate for this level or scaling parameter (rather

than the PGA), but residual concerns discussed above about the effect of major

differences in the ground motion estimation schemes for spectral ordinates, plus

tradition, lead us to adopting here the PGA. (The 10 hertz spectral velocity would

appear to work equally well and with the same numerical conclusions.)

Enter the two studies' median hazard curves for the PGA at

Step (2)

the site, at the target hazard level (e.g., 2x10-4). Take the (geometric4) average of

the two PGA levels. This represents the "dual-study composite" PGA associated

with a median hazard equal to the target level. This will be multiplied a factor to

For the 5GX, the ratios are very similar (about 2) for 5 and 2.5 hertz; they are 1

(surprisingly) for lower frequencies, but they are unfortunately considerably higher (13/3.5)

= 3.7 and (16/4.5) = 3.6 for higher frequency cases: PGA and 10 hertz, respectively. The

25 hertz case appears anomalously high.

The two values will virtually always be close enough that the simple average will give

almost the same answer.

D-3