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

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
Because the numbers of shocks used to establish the linear portions of these curves are very small (16
and 25, respectively), and the total time intervals over which data were collected are very short (11 and
50 years, respectively), an error in the slope (or b value) is quite possible. In fact, a certain
dissatisfaction with these results on the part of Sanford and Toppozada 1 is indicated by their development
of alternative curves defined to have a slope of 1.0 instead of 0.6. To the problems imposed by the
spatially and temporally restricted data set available must be added the fundamental uncertainty
associated with the definition of magnitude in the WIPP facility region. However, Sanford et al. 3
indicate that data collected since the Sanford and Toppozada 1 study of 1974 do not change any of the
original conclusions regarding the magnitude, location, and recurrence intervals of major earthquakes
within 180 mi (290 km) of the WIPP facility.
Recent work 14 allows a preliminary treatment of the data. This work is based on 11 years of instrumental
seismicity data which have been reinterpreted with respect to magnitude. In addition, recurrence
formulas are computed for broad physiographic regions of New Mexico vastly increasing the data base.
For example, Sanford et al. 14 find
log N O = 2.4 - 1.0 M L
for the High Plains physiographic province of the Permian Basin subregion or background source zone,
and
log N O = 2.5 - 1.0 M L
for the Basin and Range - Rio Grande rift region. The b value in these equations is further substantiated
by very recent work 44 in which all instrumental data on New Mexico earthquakes from 1962 through
1977 has been considered. The general criterion used in this earthquake risk analysis for the Rio Grande
rift/Basin and Range subregion and Permian Basin/background source zones is the Sanford et al. 14
recurrence formula for the physiographic province. For this recurrence formula, an individual source
zone occurs with the "a" value scaled to reflect area difference. The area of the High Plains province of
interest for this analysis is approximately a 60 mi (97 km) radius [1.2 x 10 4 mi 2 (3.1 x 10 4 km 2 )]
surrounding the WIPP facility, but exclusive of part of the Central Basin Platform. Thus, the proper
recurrence formula for site area background seismicity becomes,
log N O = 1.93 - M L Site source zone.
(background)
Similarly, the part of the Southern Basin and Range - Rio Grande rift region of interest has been referred
to in the above discussion as the Algermissen and Perkins 22 Rio Grande rift source zone and has an area
of about 4.1 x 10 4 mi 2 (1.1 x 10 5 km 2 ). The proper recurrence formula for the Algermissen and Perkins
Rio Grande rift source zone becomes,
log N = 2.56 - 1.0 M L .
The Basin and Range subregion as shown in Figure 2.5-19 has an area of about 6.4 x 10 4 mi 2 (6.4 x 10 5
km 2 ). Thus, the proper recurrence formula for the Basin and Range Subregion becomes,
log N = 2.75 - 1.0 M L .
2.5-12 January 24, 2003