Balanced Rocks Defy Quake Hazard Maps

THE GIST:

• Balancing rocks are amazing natural art forms, but they are useful too.
• Earthquake hazard maps overestimate potential shaking in places where balanced rocks have stood for millennia.
• A new dating method allows the rocks to be dated -- and new upper quake limits to be set.

Precariously balanced rocks have been found in places where they shouldn't stand a chance: All around the San Andreas Fault zone in southern California and along other faults as well.

Two new studies have looked into dating delicately balanced rocks as well as how earthquake hazard maps might need to be revised based on the way some of these rocks have survived thousands of years in what has been thought of as high-risk quakes zones.

"None of these rocks would be standing if the earthquake hazard maps were right," said geologist James Brune of the University of Nevada at Reno.

The problem with the earthquake hazard maps, says Brune is that they are based on two things: 1.) the recorded earthquake ground accelerations (the speed at which the ground shakes back and forth during a quake) and 2.) some estimates from engineers to explain damages to buildings from real earthquakes.

With those two things alone an estimate is made projecting the probability of maximum shaking some number of years into the future.

WATCH VIDEO: Building designs are put to the test on the University of California-San Diego shake table, where engineers recreate destructive earthquakes to help make our structures safer and survivable.

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To build a wood-frame house, for instance, engineers want to know maximum ground accelerations that can be expected in 500 years. A nuclear power plant might require the same thing but for 10,000 years.

"The hazard comes from the biggest earthquakes," said Brune. "But there's very little data to predict into the future."

Waiting around for 1,000 years to see what happens isn't really practical, says Brune, who is scheduled to present his work at the Seismological Society of America meeting in Portland, Ore., on April 22.

But looking at precariously balanced rocks that have managed not to be shaken off their pedestals in 10,000 years can be very revealing.

Brune and his colleagues have gone so far as to test rocks on "shake tables" used by engineers to study buildings in earthquakes. They have also conducted careful tests of real balanced rocks near fault zones.

"We go out in the field and tip them up" to measure balancing rocks' stability and see just what kind of shaking would be required to topple them, said Brune. "So we've worked it all out."

As for figuring out how long the rocks have been standing, that's a tough nut that another researcher, Dylan Rood of Lawrence Livermore National Lab, has just cracked.

"What's most important about dating a precarious rock is dating when it became precarious," Rood explained.

One technique that he has used successfully is sampling minerals on the rocks for beryllium-10, which is created in quartz crystals when they are struck by cosmic rays. In other words, the longer a rock surface is exposed to the sky, the more cosmic rays the rock's minerals are exposed to and the more beryllium-10 they accumulate.

To translate that beryllium-10 measurement into a date, Rood and his colleagues have developed a model that gives them a good idea what to expect for minerals of different ages.

For one such case which Rood will be presenting at the same meeting, a balancing rock atop the Cleghorn Fault in the Mojave Desert has proven to be about 10,000 years old. Yet the hazard maps say the area can expect a 7.0 magnitude quake every 1,000 years.

That's why Brune trusts the rocks: "The rocks basically said that that fault is dead."