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

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
The different physiographies of the two site subregions, as defined and briefly described above, are
closely related to their distinctive geologic histories and structural configurations. This is suggested by
Figure 2.5-20 which shows the boundary between the great Plains and Basin and Range physiographic
provinces. 48,49,50 For this reason, Figure 2.5-19 is a good approximation to the boundary between the
Permian Basin and Basin and Range subregions as suggested by the geologic evidence just outlined.
The results of a 1978 leveling survey between El Paso, Texas and Carlsbad, New Mexico, 29 are consistent
with this geologically suggested regional separation. Comparison of this survey with previous leveling
surveys along the same route carried out in 1934, 1943 and 1958, indicates that the Diablo Plateau region
of Trans-Pecos, Texas (in the Basin and Range subregion as defined above) has been uplifted
approximately 4 to 5 centimeters during this interval in archlike fashion in relation to the end points of
the survey. Extending east from El Paso, the leveling route traverses Basin and Range subregion-type
structures including the Hueco Basin, the Hueco Mountains, the Diablo Plateau, the Salt Basin and the
Guadalupe Mountains before terminating on the High Plains in the Permian Basin subregion near
Carlsbad. The observed relative uplift correlates well with the broad aspects of the tectonic evolution of
the Diablo Plateau. The observed elevation changes are most easily attributed to deep-seated tectonic
activity. 29
The observed movements along the El Paso - Carlsbad line are not the largest in the area. Movements
along the Roswell-Pecos line, which is entirely within and near the western margin of the Permian Basin
subregion, are larger (Figure 5 of Reference 42). However, the movements on this route, which runs
along a railroad near the Pecos River, are probably dominated by artificial water withdrawal. ?,? Carlsbad
appears to be relatively "inactive" with respect to Roswell, which is located well outside regions of
known neotectonic activity. 29
In summary, the WIPP facility region leveling data are consistent with the geologic evidence in that they
suggest current tectonic activity in the Basin and Range subregion and current stability in the Permian
Basin subregion. Because current tectonic activity implies crustal movement that in turn implies elastic
strain accumulation and release, earthquakes are often considered a barometer of tectonic activity. The
occurrence of more frequent and larger earthquakes is thus consistent with a higher level of tectonism.
Earthquakes occurring between 1923 and 1979 and between April 1974 and February 1979 are
superimposed on the suggested site subregions in Figures 2.5-19 and 2.5-21, respectively. From Figure
2.5-19 it may be seen that most pre-instrumental and a substantial proportion of 1962 to 1977
instrumental earthquakes are located in the Basin and Range subregion. In the Permian Basin subregion,
an important cluster of instrumental epicenters occurs on the Central Basin Platform, and a thin scattering
of both instrumental and pre-instrumental events appears throughout the rest of this subregion. In the
case of pre-instrumental events in the WIPP facility region, this distribution of shocks may be at least
partly controlled by a population density that has always been greatest along the Rio Grande rift (within
the Basin and Range subregion). A somewhat similar pattern appears in Figure 2.5-21, although in this
figure (for which the smaller magnitude events on the Central Basin Platform have been made recordable
by the inclusion of data from station CLN at the location of the WIPP facility) the recent predominance
of the Central Basin Platform in terms of the total number of recorded events is apparent. The largest
recorded earthquake in the Basin and Range subregion is the 1931 Valentine, Texas event whose
magnitude is estimated to be about 6.4. The largest event on the Central Basin Platform is of magnitude
3 to 4 depending upon precisely how magnitudes of events in these areas are calculated. The largest
event in the Permian Basin subregion but, not on or near the Central Basin Platform, was the 16 June
1978 event near Snyder, Texas, at the extreme eastern margin of the site region. This event was about
4.7 in magnitude.
2.5-7 January 24, 2003