Relieving stress at nuclear-waste site.

Contrary to earlier reports, a new land survey near Yucca Mountain, Nev., shows no appreciable ground warping at the proposed site for an underground nuclear-waste dump. If confirmed, the analysis would deflate concerns over geological hazards at Yucca Mountain.

The issue emerged last year, when Brian Wernicke of the California Institute of Technology in Pasadena and his colleagues reported significant ground movement--known as strain--around Yucca Mountain ($N: 4/18/98, p. 251). Measurements made by Global Positioning System (GPS) receivers at five sites showed that between 1991 and 1997, the region stretched at a rate much faster than expected.

James C. Savage and his colleagues at the U.S. Geological Survey in Menlo Park, Calif., performed a separate study by surveying 14 sites surrounding Yucca Mountain, using data from 1983 through 1998. The analysis combined conventional techniques with GPS data.

In the Aug. 10 JOURNAL OF GEOPHYSICAL RESEARCH, they report a rate of strain less than 20 percent of what Wernicke's group found. "There is essentially no strain accumulating at the proposed site for the high-level waste repository," says Savage.

One area of disagreement is how to treat a magnitude 5.4 earthquake that struck near Yucca Mountain in 1992. According to Savage, most of the motion detected in Wernicke's study represents a normal and temporary readjustment of the crust to the quake.

Wernicke doesn't discount this explanation, but he says that such an amount of postquake shifting would be unusually large. Another possibility is that some other source of stress near Yucca Mountain is causing the warping that his team measured.

An answer may emerge within 2 years. Wernicke and his colleagues have set up a network of GPS receivers that continuously collects data. The system should be able to detect ground movements as small as 0.1 millimeter per year, he says.