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

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
Based on 11 years of instrumental data (1962 - 1972 inclusive), analysis of earthquakes throughout New
Mexico of magnitude greater than or equal to 2.5 (which are believed to have been uniformly located
during this interval) indicates a roughly comparable level of earthquake activity in the inactive and in the
active physiographic provinces. 2,18 This result must further qualify the confidence with which the modest
differences in historical seismicity levels (in terms of number of events) in the (inactive) Permian Basin
and (active) Basin and Range subregions can be argued to be significant.
Thus, in light of geologic evidence and consistent recent leveling survey data, the Basin and Range
subregion, as shown in Figures 2.5-19 or 2.5-21, exhibits a higher level of recent tectonism than the
Permian Basin subregion. This is supported by the maximum magnitude earthquakes occurring in these
subregions during historical time. The distribution of all known site region earthquakes shows that, with
the exception of the Central Basin Platform area, the Permian Basin subregion has experienced
marginally fewer events than the Basin and Range subregion. A significant cluster of small events is
located along the Central Basin Platform.
2.5.3 Correlation of Earthquake Activity with Geologic Structures or Tectonic Provinces
The best available evidence does not suggest that recorded earthquakes have been well correlated with
faults anywhere in the WIPP facility region. This is true for both the surface faults of the Basin and
Range subregion (a number of which show evidence of Quaternary movement) and for the geologically
older subsurface faults in the Permian Basin subregion.
Although no earthquakes in the WIPP facility region are known to be correlated to specific faults, a
substantial cluster of seismic activity has occurred on and near the Central Basin Platform since about the
mid-1960s. This suggests division of the Permian Basin subregion into a Central Basin Platform portion
and a background portion. The seismicity pattern leading to this suggestion is made fairly explicit in
Figures 2.5-19 and 2.5-21. There is no known evidence of any differences since late Permian time in the
geologic histories of the Central Basin Platform and surrounding portions of the Permian Basin (Sections
2.5.2 ). In addition, there does not appear to be enough data at present to convincingly determine the
direction of tectonic forces and the type of faulting on the Central Basin Platform; 4 therefore, this
information could not be used to distinguish the Central Basin Platform.
First Shurbet, 13 and later Sanford and Toppozada 1 and Rogers and Malkiel 15 suggested that Central Basin
platform earthquakes are not tectonic but are instead related to water injection and withdrawal for
secondary recovery operations in oil fields in the Central Basin Platform area. Such a mechanism for the
Central Basin Platform seismic activity could provide a reason why the Central Basin Platform is
separable from the rest of the Permian Basin on the basis of seismicity data but not by using other
common indicators of tectonic character. Both the spatial and temporal association of Central Basin
Platform seismicity with secondary recovery projects at oil fields in the area are suggestive of some cause
and effect relationship of this type. 15
In summary, the best available evidence does not suggest that known earthquakes are well correlated with
faults in the WIPP facility region. A substantial number of earthquakes have occurred on and near the
Central Basin Platform since about the mid-1960s. The cause of the spatial coincidence of recent
seismicity with this buried large-scale Paleozoic structure is not known. With this exception, WIPP
facility region earthquakes may be correlated with two tectonic provinces for the purposes of
this study. The first is a relatively inactive province made up of the eastern and northeastern
two-thirds (approximately) of the WIPP facility region (and encompassing the WIPP facility). The other
WIPP facility region tectonic province is a relatively inactive province made up of the rest of the WIPP
facility region. A simple and reasonable model of these two general WIPP facility region tectonic
provinces is furnished by the Permian Basin/Basin and Range subregion characterization of Section .5.2.
2.5-8 January 24, 2003