Ground Water at Yucca Mountain How High Can It Rise? (1992) / Chapter Skim
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Could a Nearby Earthquake Cause Flooding of the Proposed Repository?
Could a Nearby Earthquake Cause Flooding of the Proposed Repository?
Pages 104-127
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From page 104... ...
strain are unlikely to cause flooding of the repository for a broad range of crustal strain rates. Nonetheless, the contemporary strain field is one of the few parameters in most models of tectonically induced water table fluctuations that can be measured, and both the number and size of potential excursions of the water table from its present level induced by earthquakes over the next 10 ka are clepencient in part on contemporary crustal motions.
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From page 105... ...
Savage, 1983) and estimates from slip rates on Quaternary faults (summarized in Eddington et al., 1987~.
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From page 106... ...
Such subsidence could occur as a result of slip on clipping normal faults and hence would require extension rates at least two orders of magnitude greater than the accumulated slip rates on Quaternary faults on Yucca Mountain estimated by Scott (19901. The greater rate, 3 mm/yr, is consistent with the rate determined for the southern Great Basin excluding the Owens Valley event.
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From page 107... ...
Although the impact of this information on increased understanding of the hydrologic conditions around Yucca Mountain is uncertain, knowledge of the rate of crustal strain is clearly important to nearly any seismic risk assessment related to the proposed repository site. In the panel's view, this information used as an accessory to, or confirmatory of, more standard information on rates of crustal deformation, can provide additional confidence in the prediction of future tectonic behavior and the probabilities of seismic events in the area.
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From page 108... ...
of the occurrence of an earthquake of a Even size in a specific location is an important part of that evaluation. Thus, in evaluating the range of possible responses of the water table to an earthquake that could reasonably be expected to occur in the Yucca Mountain vicinity, the panel has considered some examples from the historical record of water table responses to significant recent earthquakes; some modeling results, with the data and assumptions used in the calculations; and the probabilities of significant earthquakes occurring in the area.
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From page 109... ...
The best documented of hydrologic responses are for the Alaskan Good Friday earthquake; the Loma Prieta earthquake; and the Borah Peak earthquake. In some instances it is difficult to distinguish between the hydrologic effects produced by volume strain and those produced by an increase in permeability.
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From page 110... ...
The best explanation of these observations appears to be that the ground motion, which was amplified along Skyline Riclge as the surface wave passed through, increased the permeability of the shallow aquifers. The increase in permeability caused the water table to drop as the ground water drained clownwarcl through the rocks and discharged to the streams.
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From page 111... ...
RECOMMEN DATION The information included here by no means reflects an exhaustive search of the water level changes associated with earthquakes. The panel recommends that DOE conduct a detailed literature search to determine the hydrologic effects of over historic earthquakes, locat and worldwide, to evaluate the potential for significantly large water table rises by the coupling of the seismic and hydrologic systems.
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From page 112... ...
Both modeling approaches (the dislocation and the regional stress change) , have been used to analyze the effects of an earthquake on the water table level.
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From page 113... ...
One has to invoke an improbable permeability distribution with this mo~lel to produce a large water table rise. Regional Stress Change Models Cook and Kemeny (1991)
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From page 114... ...
This suggests that the water table rise will be less than 40 meters (see Table 5.1~. Table 5.1 also indicates that knowledge of the properties of the carbonate rocks at depth at Yucca Mountain is essential to predicting water table behavior in response to changes in the regional crustal stresses produced by earthquakes.
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From page 115... ...
The analyses of both the panel and Cook and Kemeny indicate that the calculated water table rise in the regional stress change model is especially sensitive to: (1) the amount of the stress change, (2)
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From page 116... ...
Although the models are based on very limited data, me panel concludes that stress/strain changes resulting from an earthquake are inadequate to cause more than a few tens of meters rise in the water table based on the convergence of the results by a variety of models and assumptions, especially if the deep carbonate aquifer is as incompressible as the limited data suggest. A Possible Alternate Modeling Approach: Discontinuum Analysis of Ground-Water Motion Another approach to prediction of seismically induced groundwater motion is based on explicit recognition of the discontinuous state of a seismically active rock mass.
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From page 117... ...
of fractures at depth can provide sufficient fluid which, on subsequent clisplacement, can have a substantial effect on the water table in the near-subsurface. The states of stress in the shallower part of the crust are typically low, resulting in open fractures, and consequent high storativity.
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From page 118... ...
Additional data are needed on the hydrologic and elastic properties of the deeper carbonate rocks and the chemical and isotopic composition of fluids in the saturated zone below the water table that underlies the proposecl repository site. As the panel's independent analysis indicates, data are needed on rock compressibil;ly' porosity, and permeability of the deep aquifer.
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From page 119... ...
that might affect crustal stress conditions and the water table near Yucca Mountain will occur on one of the identified faults or faulted zones. The most critical of these assumptions is the first; it says that the slip rate on the Windy Wash fault is typical of slip rates on other faults in the region.
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From page 120... ...
The proposed repository boundary is shown as a dotted line; rock units in the area are shown hatched.
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From page 121... ...
Again, the purpose here is to make reasonable estimates of slip rates that are not low or high by an order of magrutucle. If alternative slip rates are preferred, their effects on the probabilities reported here would be proportional, at least at the lower probability levels.
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From page 122... ...
First, the vertical offsets of the Topopah Spring Member shown on Table 5.2 total 1880 m; dividing this by the 13 Ma age of this unit gives a total vertical slip rate for the faults in Figure 5.1 of about 0.15 mm/yr. Assuming oblique slip with equal components of horizontal and vertical movement gives a total slip rate of 0.21 mm/yr.
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From page 123... ...
, with the understanding that further fault-specific site investigations are likely to improve or refine those estimates. CONCLUSIONS A range of uncertainties necessarily accompany any current assessment of the effects of earthquakes on the water table in the Yucca Mountain area, because of the absence of sufficient data to constrain the moclels, to determine slip rates, and to predict earthquake magnitudes.
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From page 124... ...
All of these conclusions have significant uncertainty as a result of the uncertainties in fault slip rates. Therefore, while there are uncertainties in current interpretalions because specific site data are not available, the panel concludes that there is nevertheless sufficient confidence In the aseismicity of the site and In the inability of earthquakes to generate large water table changes at the site, based on the historical evidence and modeling results, to warrant further characterization of the site to determine its suitability for a MODS.
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From page 125... ...
1991. Potential for water table excursions induced by seismic events at Yucca Mountain, Nevada.
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From page 126... ...
1991. Hydrologic changes associated with the Loma Prieta earthquake in the San Lorenzo and Pescadero drainage basins.
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From page 127... ...
1991. Observations and subsequent history of spectacular ground water flows and aquifer pressure increases: 1083 Borah Peak and 1957 Hebgen Lalce earthquakes (abs.)
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