Generation of Appropriate Enveloping Accelerograms

DOE-STD-1023-95

(1)

Magnitude

The historical earthquake magnitude will be a best estimate of the moment magnitude for the

earthquake. If instrumentally recorded, the magnitude will be either the recorded moment

magnitude or a derived moment magnitude from other estimated magnitudes using accepted

published magnitude conversion relationships. If the historical earthquake is a pre-

instrumental event, the moment magnitude should be estimated using information such as the

total felt area or other applicable intensity information found in the published literature and

authoritative unpublished records, diaries, scientists notes, etc. If the ground motion

attenuation relationship requires additional source parameters such as stress drop, these

parameters should also be defined as best estimates.

(2)

Distance

The distance should be based on a best estimate. For instrumentally recorded earthquakes, the

distance should be based on the best available location (including depth). For pre-instrumental

earthquakes, there is considerable uncertainty in the exact location of the event. In these cases,

a reasonably conservative estimate should be provided which considers factors such as the

highest intensity and the estimated rupture dimension for the magnitude being considered.

(3)

Ground Motion

Both median (50th percentile) and 84th percentile estimates of ground motion should be

completed for all frequencies comprising the response spectra. Methods for estimating ground

motion should be consistent with the approaches used to derive the spectral shapes as

discussed in Section A3.1.3.1.e. For PC 4 facilities, the DBE spectra shall be equal to or

greater than the 84th percentile estimate. For PC 3 facilities, the DBE spectra should be equal

to or greater than the median estimate. In general, the difference between the median and 84th

percentile is about a factor of 1.7 to 2 in ground motion, which approximates the ground

motion difference between PC 3 and PC 4 hazard probabilities coupled with typical hazard

curve slopes.

3.1.6 Generation of Appropriate Enveloping Accelerograms

For many seismic evaluations of structures and sites, the generation of accelerograms (time

histories) which envelope the developed DBE response spectrum must be developed. The guidelines

to be used for development of such time histories and acceptance criteria are present in McGuire, et.

al. (2001). The time histories need to have sufficient energy content at all frequencies of interest

and possess characteristics appropriate for the governing characteristic seismic events comprising

the seismic hazard of the site.