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

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
Seismic Source Zones
Geologic, tectonic and seismic evidence indicates that three seismic source zones may be used to
adequately characterize the region. These are well approximated by the Basin and Range subregion, the
Permian Basin subregion exclusive of the Central Basin Platform, and the Central Basin Platform itself.
The seismic source zones are outlined in Figures 2.5-19 and 2.5-21. However, specific boundaries are
only intended to be simply defined approximations. For the purpose of earthquake risk analysis at the
WIPP facility, some measure of the effect of the likely uncertainty in these source zone boundaries is
desirable. Rather than allow the source zone boundaries to vary randomly by some amount, alternative
boundaries are used based on an independent analysis of the WIPP facility region. These are taken from
the study by Algermissen and Perkins of earthquake risks throughout the United States, 21 and were used
in a previous analysis of WIPP site seismic risk by SNL. 15 A detailed discussion of how this
characterization was developed and how it best fits recent estimates of site region seismic properties may
be found in that reference.
Site region seismic source zones after Algermissen and Perkins are shown in Figure 2.5-23. Superposed
on this figure are the earth-quake epicenters of Figure 2.5-1. It is clear from this superposition that the
zonation presented generally conforms with historical seismicity. The source zonation of Figure 2.5-23
has no explicit analog to the Permian Basin subregion exclusive of the Central Basin Platform. This is
considered part of the broad background region.
Another estimate of the appropriateness of the source zones as drawn in Figure 2.5-23 can be obtained
from a consideration of Quaternary faulting. As shown in Figure 2.5-24, evidence of Quaternary fault
offset is almost, but not quite completely, contained within the two western seismic source zones of
Algermissen and Perkins. These two zones may be combined under the name "Rio Grande rift" since
they include the parts of those provinces significant to the evaluation of probabilistic acceleration at the
WIPP facility.
The general Algermissen and Perkins model, then, consists of three sources:
Ä The Rio Grande rift zone drawn by combining the western source zones as discussed above.
Ä The Central Basin Platform zone as shown in Figure 2.5-26.
Ä A WIPP site source zone centered at the site to model background seismicity in the High Plains.
The manner in which the irregular Algermissen and Perkins source zones are adapted to the
quadrilateral source zone configuration, which is required for the application of the seismic risk
analysis method as discussed above, is straightforward (Figure 2.5-25).
For the purposes of this study, some minor modifications of the Algermissen and Perkins source zones
were made. Geologic and tectonic evidence suggests that the physiographic boundary between the Basin
and Range and Great Plains provinces provides a good and conservative approximation of the source
zones as discussed in Sections 2.5.2 and 2.5.3. In addition, refined information from the Kermit array 15
indicates that the geometry used to model the limits of the Central Basin Platform source zone may be
modified somewhat from the original preferred model for the WIPP site region seismic source zones in
this study. This model is preferred because it is based more completely on consideration of geologic and
tectonic information, as well as seismic data, and because it results in more conservative development of
risks at the WIPP facility.
2.5-10 January 24, 2003