Unsteady Crust Below Yucca Mountain?

The crust beneath Yucca Mountain--the site of a proposed U.S. repository for highly radioactive waste--appears to be deforming at least 10 times faster than expected. The satellite-based observations, reported in tomorrow's issue of Science, could imply higher chances of a disaster such as a volcanic eruption or major earthquake during the repository's projected 10,000-year lifetime.

Yucca Mountain, located in southern Nevada, has been so quiet for millennia that researchers have generally concluded that the risk of future earthquakes or volcanoes is low. Indeed, previous analyses had put the risk that, say, a new volcano would pierce the repository at 1 chance in 10,000 during the next 10,000 years. Because such cataclysms depend in part on movement of Earth's crust, a team of geologists--led by Brian Wernicke of the California Institute of Technology and James Davis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts--checked Yucca Mountain for deformation. By comparing radio signals from several Global Positioning System (GPS) satellites overhead, the researchers could determine the position of five benchmarks with millimeter accuracy.

Between 1991 and 1997, the survey line stretched 1.7 millimeters per year, on average. Although that's only one-quarter the rate of crustal deformation seen in active areas like the San Andreas fault, it's about 10 times faster than geologists had inferred for the Yucca Mountain region, based on how much nearby faults have slipped over hundreds of thousands of years. Wernicke and his colleagues suggest that the Yucca Mountain area may be undergoing a geologically brief episode of rapid crustal stretching, perhaps driven by rising magma.

Other researchers are intrigued but not alarmed by the findings. "It's an interesting and provocative idea," says geologist Bruce Crowe of Los Alamos National Laboratory in New Mexico, who has long worked on volcanic risks at Yucca Mountain. "It has to be looked at carefully." But he and others caution that the measurements are at the limit of what GPS can reliably detect.