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

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
There is one purely geometrical issue to be resolved. It involves specifying a focal depth for events in
each of the model source zones. There is little doubt that the focal depths of earthquakes in the WIPP
facility region should be considered shallow. Early instrumental locations were achieved using an arc
intersection method employing travel-time-distance curves calculated from a given crustal model, and the
assumption of focal depths of 5 km, 10 km, or for later calculations, 8 km. Good epicentral locations
could generally be obtained under these assumptions.
Within the range discussed, (that is, focal depths to 10 km) the issue of selecting a proper depth for the
probabilistic acceleration analysis at the WIPP site may be shown to be important only in the site source
zone itself. For example, the difference in hypocentral distance (the distance to be used in the
acceleration attenuation formula) for a closest approach event in the Central Basin Platform is only 1.05
km in this depth range, assuming that the closest approach of this source zone is 35 km as indicated by
Figures 2.5-25 and 2.5-26. This is clearly the greatest difference of this kind outside the WIPP facility
source zone. Within the WIPP facility source zone the selection of focal depth can be very important
simply because the form of the attenuation law used asymptotically approaches infinite acceleration at
very small distances. This is certainly not mechanically realistic and is not the intent of the empirical
fitting process to an attenuation law of this form. A focal depth of 5 km is used in all source zones of
this study including that of the site. For smaller hypocentral distances, the form of the attenuation law
adopted here severely exaggerates the importance of very small, very close shocks, in the estimation of
probabilistic acceleration at the WIPP site (Figure 2.5-22).
2.5.4.2 Source Zone Recurrence Formulas and Maximum Magnitudes
The risk calculation procedure used in this study requires that earthquake recurrence rates for each
seismic source zone be specified. This is done formally by computing the constants "a" and "b" in the
equation,
log N = a - b M
where N is the number of earthquakes of magnitude greater than or equal to M within a specified area
occurring during a specified period.
For the WIPP facility region, three formulas of this type are needed&one for the active province west and
southwest of the site (the Basin and Range subregion or Rio Grande rift source zone), another for the
inactive province of the WIPP facility exclusive of the Central Basin Platform (the Permian Basin
subregion or background source zone), and a final one for the Central Basin Platform. In practice, the
difficulties in finding meaningful recurrence formulas for such small areas in a region of low historical
earthquake activity are formidable.
Several estimates of recurrence rates in the WIPP facility region have been published. 1,14,21 For
earthquakes within 180 mi (290 km) of the WIPP facility, exclusive of shocks from the Central Basin
Platform and aftershocks of the 1931 Valentine, Texas earthquake, Sanford and Toppozada 1 find
recurrence formulas of the form:
using instrumental data only, and
log N O = 1.65 - 0.6 M L
log N O = 1.27 - 0.6 M L
using both historical and instrumental data. In these and following recurrence formulas in this section,
M L is the Richter local magnitude and N O is the number of earthquakes in the area of interest normalized
to a time period of one year and an area of 3.6x10 4 mi 2 (9.3 x 10 4 km 2 ).
2.5-11 January 24, 2003