Ground Water at Yucca Mountain How High Can It Rise? (1992) / Chapter Skim
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Appendix D: Response of the Ground-Water System at Yucca Mountain to an Earthquake
Appendix D: Response of the Ground-Water System at Yucca Mountain to an Earthquake
Pages 212-222
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From page 212... ...
Geological Survey Menlo Park, California Revised February 12, 1992 ABSTRACT The volume strain produced by a normal fault earthquake, of approximate magnitude 6, is computed using dislocation theory. The head change produced by the change in volume strain associated with the earthquake is also computed.
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EARTH TIDES AND ATMOSPHERIC LOADS Near the earth's surface rocks are subjected to earth tide strain and changes in atmospheric pressure. The response of the groundwater system to these effects can be used to investigate both the elastic and the hydrologic parameters of rocks in the area.
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analyzed the response of 4 water wells at Yucca Mountain to both atmospheric loading and earth tides. Figure 1, taken from Galloway and Rojstaczer (1988)
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indicate that the unsaturated pore space maybe approximately 5 percent. However, many tuffaceous rocks above the water table have moisture contents near 100 percent saturation; this is especially true of the less welded units.
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216 Ct ~ o ~ o X E :CQ~ ~ es ~ ~ o ~ ,~ ~ U)
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In the analysis presented here an elastic model is used to calculate the volume strain accompanying a magnitude 6+ earthquake. The volume strain causes an increase in pore pressure in the defo~mecl zone near the fault.
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One also can estimate the change in the average stress associated with the displacement. At depths of 5 kilometers and more, the volume strain ranges from -5 to -10 microstrain units (in the convention used here a - sign indicates compression)
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~ 9 odometers Figure 3 Initial change in hydraulic head resulting from a hypothetical earthquake. Head is plotted at various depths in the system.
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In the model I assume the fault plane to have the same hydraulic conductivity as the surrounding rocks. Both the upper luff layer and the lower carbonate rocks have isotropic hydraulic concluctivity.
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1 0000 Figure 5 Transient head plotted as ~ function of time for the dislocation model in which the tight confining layer is included. produced by the Volume strain decays rapidly from the system.
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ACKNOWLEDGMENT I thank Bob Simpson, USGS, Menlo Park, for calculating the volume strain for the dislocation model; and Bill Brace and Amos Nur for their discussions of the role of a regional change in stress following an earthquake. REFERENCES Bredehoeft, ]
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