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

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
The histogram in Figure 2.5-5 for events with M3.0 (upper left) suggests a complete data set of this
magnitude level. The greatest number of events (6) occurred during the second interval (from February
4, 1965 through March 9, 1968), a period when no seismograph was operating within 135 mi (217 km) of
the location of the WIPP facility except station FOTX during the first 67 days of the interval. (Station
FOTX was located 72 mi (116 km) southeast of the WIPP facility). The least number of earthquakes
occurred in the first, third, and eighth intervals. The WIPP seismographic network was fully operational
during the eighth interval, but no seismic instrumentation within 135 mi (217 km) of the location of the
WIPP facility existed during the first and third intervals except station FOTX (in operation the last 228
days of the first interval). Because the number of observed quakes with M3.0 does not correlate with the
presence or absence of instrumentation at or near the WIPP facility, the data set is believed to be
complete at that strength level. If the data set is complete, then the variations in activity observed in the
histogram represent true temporal changes in the activity rate for earthquakes with M3.0.
In the lower two histograms of Figure 2.5-5, the period of maximum instrumentation is even more clearly
defined by the increase in numbers of earthquakes during the fifth and sixth time intervals. In summary,
the general shape of the histograms relative to temporal changes in instrumentation indicates the data set
is probably complete above magnitude 2.7, and that it becomes progressively less complete at lower
magnitudes.
2.5.1.2.3 Recurrence Interval Formulas
Many studies have demonstrated a linear relation between the logarithm of the cumulative number of
earthquakes (N) and the magnitude (M), i.e.,
log N = a - bM.
The values of the constants "a" and "b" are derived from existing earthquake data by plotting log N
versus M and performing linear regression on those points that fall above the minimum magnitude where
the data set is complete. The formulas obtained in this manner can be extrapolated to determine the
recurrence interval for the maximum probable earthquake in the region. Section 2.5.4.2 describes in
some detail how these relations can be used in establishing risk and ultimately the DBE.
Shown in Figures 2.5-6 and 2.5-7 is a log N versus M plot for the combined time periods from
January 1, 1962 through September 30, 1986. Seismographs were not in operation near the WIPP facility
from July 24, 1980 to August 29, 1983. Linear regression for data points greater than magnitude 1.9
yields the recurrence equation,
log N = 4.05 - 1.01 M.
The value of "b," 1.01, is three percent less than that obtained by Sanford et al. (1.04) using data for the
3 1/4 year period, April 1974 through June 1977. The "a" values cannot be compared because
(1) the magnitudes in Table 2.5-3 are on the average approximately 0.4 less than those listed in Sanford
et al., 45 (2) the time period is approximately three times greater here than in Sanford et al, 3 and (3) the
degree of activity at the M2.0 strength level was not as great in later periods as it was from April 1974
through June 1977 (see histograms in Figure 2.5-5).
2.5-4 January 24, 2003