CLOSE-UP : Seismometers of the Gods

When seismologist Jim Brune sees a big rock perched on a hillside, balanced like a beach ball on a seal snout, he doesn’t smile and see a potential one-ton disaster right out of the Road Runner cartoons. He sees an ancient seismometer--a beautiful instrument crafted by nature--that is actually a record of strong ground motion going back into the wordless, machineless ages.

“Precarious rocks are a direct measure of ground-shaking. I see them as low-resolution seismoscopes that have been in operation for thousands of years,” says Brune. While most earthquake-hazard indicators are based on theoretical models, he says these rocks provide empirical evidence that an area has been seismologically stable for eons and is likely to stay so, a valuable bit of information for anyone looking for a safe place to, say, dump a bunch of radioactive waste.

Brune, a seismologist who has been on the California Earthquake Prediction Evaluation Council for 20 years, is a professor of geophysics at University of Nevada, Reno, and directs the seismological laboratory there. He first realized how to use these rocks as earthquake recording devices a few years ago while studying the site proposed for America’s first permanent high-level nuclear waste facility at Yucca Mountain in Nevada. “I was at a Geological Society of America meeting in San Diego, and a geomorphologist was showing pictures of Yucca Mountain, and he said, ‘See this dark varnish on the rocks along the slope of that hill? That shows that they’ve been sitting there, close to the air for about 10,000 to 100,000 years!’ I said, ‘Wow, there’s the time scale!’ ” The varnish meant that Brune could tell with some reliability how long Yucca Mountain’s precarious rocks had been standind undisturbed by earthquakes.

Another area Brune studies is Victorville, where there are loads of precarious rocks only about 20 miles from the San Andreas fault. “There’ve been almost a dozen big earthquakes on the fault in the last few thousand years. Each one of them could easily have been felt here, no question. Felt strongly. But it wouldn’t have been enough to knock these rocks down.” So, says Brune, residents out here don’t need to “sweat the effects of the Big One” as much as folks closer to the fault.

As for Yucca Mountain, the final decision on whether to bury 77,000 tons of spent nucelar fuel rods there is still several years away. But the mountain’s rocks, says Brune, “make me feel a whole lot better about the seismic hazard there.”