03/26/1998 08:00:00

Yucca Mountain Is Possibly More Seismically Active Than Once Believed, Geologists Discover

PASADENA—Recent geodetic measurements using Global Positioning System (GPS) satellites show that the Yucca Mountain area in southern Nevada is straining roughly 10 to 100 times faster than expected on the basis of the geologic history of the area. And for the moment at least, geologists are at a loss to explain the anomaly.

In the March 28 issue of the journal Science, Brian Wernicke of the California Institute of Technology (Caltech) and his colleagues at the [Smithsonian Astrophysical Observatory] in Cambridge, Massachusetts, report on Global Positioning System surveys they conducted from 1991 to 1997. Those surveys show that the Yucca Mountain area is stretching apart at about one millimeter per year east-southeastward.

"The question is, why are the predicted geological rates of stretching so much lower than what we are measuring?" asks Wernicke. "That's something we need to think through and understand."

The answer is likely to be of interest to quite a few people, because Yucca Mountain has been proposed as a site for the permanent disposal of high-level radioactive waste. Experts believe that the waste-disposal site can accommodate a certain amount of seismic activity, but they nonetheless would like for any site to have a certain amount of stability over the next 10,000 to 100,000 years.

Yucca Mountain was already known to have both seismic and volcanic activity, Wernicke says. An example of the former is the 5.4-magnitude "Little Skull Mountain" earthquake that occurred in 1992. And an example of the latter is the 80,000-year-old volcano to the south of the mountain. The volcano is inactive, but still must be studied according to Department of Energy regulations.

The problem the new study poses is that the strain is building up in the crust at a rate about one-fourth that of the most rapidly straining areas of the earth's crust, such as near the San Andreas fault, Wernicke says. But there could be other factors at work.

"There are three possibilities that we outline in the paper as to why the satellite data doesn't agree with the average predicted by the geological record," he says. "Either the average is wrong, or we are wrong, or there's some kind of pulse of activity going on and we just happened to take our data during the pulse."

The latter scenario, Wernicke believes, could turn out to be the case. But if Yucca Mountain is really as seismically active as the current data indicate at face value, the likelihood of magmatic and tectonic events could be 10 times higher than once believed.

 

Written by Robert Tindol