DOE 2000. - Waste Isolation Pilot Plant - U.S. Department of Energy

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
This leaves only the Central Basin Platform, which is treated somewhat differently. Although the initial
formulas 14 above were developed for areas near 7.2 x 10 4 mi 2 (1.9 x 10 5 km 2 ) (with some increased
confidence in their validity because of the relatively large areas of data collection), this cannot be done
for the Central Basin Platform source zone because it is unique and of very limited area. Therefore, it
cannot be treated as a scaled-down version of some broader region. Although recent work using data
from the Kermit array 15 is available for this source zone, the recurrence formulation of Sanford et al. 2 is
used in this risk analysis primarily for consistency in approach. Based on the seismicity detected in the
Central Basin Platform since the installation of station CLN in April 1974, the cumulative number of
shocks versus magnitude may be expressed as,
log N O = 3.84 - 0.9 M L .
If the active portion of the Central Basin Platform is assumed to have an area of 2.9 x 10 3 mi 2 (7.5 x 10 3
km 2 ) during this period, 2 the proper recurrence relation for the Central Basin Platform source zone
becomes,
log N = 2.74 - 0.9 M L .
Because the Central Basin Platform seismicity is so really limited, this same recurrence formula is used
for all alternative geometric characterizations. This has the effect of maintaining a constant activity rate
for the Central Basin Platform as an entity.
These are the primary recurrence relationships used in the current risk analysis for the WIPP site.
However, whereas magnitudes as used in the site region attenuation law above, or in consideration of
maximum magnitude for a given source zone below, are by definition Richter local magnitudes, M L , the
earthquakes used to determine the recurrence formulas have measured magnitudes crucial to formula
development. Some apparent disagreement exists in how site region magnitudes should be computed,
with some suggestion 15 that the local magnitudes determined by Sanford et al. 2 may be, in some sense,
too low. In order to test the effect of this possibility, an alternate set of recurrence formulas is derived by
incrementing the M L values in the above relationships by 0.5, in general agreement with the suggested
relation between a "corrected" magnitude 15 and the local magnitude of Sanford et al. 2 The effect of this
process is clearly to increase the activity rate of all source zones.
The four formulas now become:
log N = 2.43 - M CORR
log N = 3.06 - M CORR
log N = 3.25 - M CORR
log N = 3.19 - 0.9 M CORR
Site source zone (background)
Algermissen & Perkins Rio Grande rift source zone
Basin & Range subregion
Central Basin Platform
2.5-13 January 24, 2003