Featured articles in December issue of BSSA

December 04, 2007

Yucca Mountain: Putting a Limit on Risk

Looking ahead 100 million years, new research puts a maximum limit of 3.6 meters per second on potential ground movement caused by earthquakes at Yucca Mountain, Nevada, the site of the proposed high-level radioactive waste repository. Yucca Mountain has unique characteristics that make it arguably the best location to store hazardous waste, chiefly a water table so low that it is possible to store steel canisters of waste 1000 feet below ground and 1000 feet above the water table. Two questions form the current debate: how dry will the site remain, and what is the risk from earthquakes"

Seismic hazard assessments usually look at the risk over 500 to 1000 years. The Nuclear Regulatory Agency is requiring a much more cautious evaluation that exams what would happen with odds as low as 1 in 10,000 over 10,000 years, which would be equivalent to something that happens only once every 100 million years. Scientists study the past to help predict the future, but Yucca Mountain was formed only 10 million years ago, limiting the value of the historical record. While the relative stability of the area is clear, some seismic hazard evaluations assessed potential movement at rates larger than experienced anywhere on earth. Researchers turned their attention instead to quantifying the maximum possible movement from any earthquake at Yucca Mountain, given its unique geological composition. Was there a limit to ground motion"

D. J. Andrews and colleagues at USGS looked at the worst-case scenario to find that the ground can move a maximum of 3.6 meters per second, which is near the most intense ground motion ever recorded anywhere, but is within the range of feasible engineering mitigation. Andrews, et al., used a numerical method to calculate ground motion related to stress changes at the source of an earthquake and throughout the surrounding area to establish physical limits on extreme ground motion.

The authors suggest this new finding adds significantly to the body of evidence that supports a long-term stable seismic environment for Yucca Mountain and provides an opportunity to shift discussion to the large question of the comparable merits of available options for hazardous waste storage.

D. J. Andrews, Thomas C. Hanks, and John W. Whitney work at the U.S. Geological Survey - Menlo Park.




Small Earthquakes are Useful Predictor of Large Ones

Conventional wisdom in seismology says slip on faults that rupture during the largest continental strike-slip earthquakes is generally limited to the seismogenic layer, the upper 15 km or so of the earth's crust to which aftershocks extend and background seismicity is limited. That idea when coupled with theory predicts that the amount of slip on faults in large earthquakes should not continue to increase once the dimensions of a rupture have surpassed about 15 km. The prediction is not supported by observation. Slip does continue to increase as the length of earthquake ruptures increase. The contradiction between the prediction and observation has long been recognized to suggest that the physics of large earthquakes is different than small. This interpretation has been unsettling because it lends uncertainty to the idea that observations from small earthquakes, which are far more abundant, can be scaled upward to make predictions about much less frequent but far more damaging large earthquakes that will occur in the future, a common practice in seismology. New work by seismologists Geoffrey C. P. King and Steven G. Wesnousky shows how the conundrum may be resolved if slip during the largest earthquakes is allowed to extend modestly below the seismogenic layer and that the base of the seismogenic layer is marked by a transition to stable sliding rather than viscous relaxation.

Geoffrey C. P. King is the Director of the Laboratoire Tectonique at the Centre Nationale de la Recherche Scientifique's Institut de Physique du Globe de Paris. Steven G. Wesnousky is Director of the Center for Neotectonic Studies and Department of Geological Sciences University of Nevada-Reno.
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Note: Please cite the Bulletin of the Seismological Society of America as the source of this information.

Seismological Society of America

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