Precarious rocks, like Balanced Rock in Manitou Springs, Colorado, can be used to estimate ancient earthquakes that didn’t occur.
In an earthquake zone near the Diablo Canyon nuclear power plant in Southern California stands a precariously balanced rock that Anna Rood has nicknamed Damaris, after her best friend. “The joke is that she’s incredibly fragile,” says Rood, a Ph.D. student in geology at Imperial College London.
She thinks Damaris has stood in place for 21,000 years, which might seem pretty sturdy. The fact that it’s still standing means an earthquake strong enough to fell it hasn’t come along in all the time it’s been so perilously perched. Now, by analyzing the ages of unstable rocks like Damaris and working out just how big of an earthquake would topple them, Rood and her colleagues have built a more precise picture of past earthquake behavior in the region—and therefore what could happen in the future. And it’s at odds with the comparatively pessimistic official forecasts for this region, called the Hosgri fault zone, which sits off the coast of California about halfway between San Francisco and Los Angeles.
Previous research on earthquake hazards has taken into account such precariously balanced rocks, but this is the first study to use them to dramatically refine an entire hazard assessment, says Glenn Biasi, a seismologist at the U.S. Geological Survey who was not involved in the study. “This is going to be an important case study for how to approach the use of fragile geologic features,” he says.
Earthquake scientists have been studying precarious rocks for nearly half a century. These ancient rock formations offer crucial data on earthquakes that otherwise aren’t available, as modern seismometers, which detect ground shaking, have only been around for the past 100 years or so. That lack of data is a problem for critical facilities like dams or nuclear power plants that need to withstand the kind of catastrophic earthquake that might only come along once in 10,000 years.
To get a better idea of past earthquake behavior in this part of California, Rood and her colleagues selected seven precarious rocks from a site near Diablo Canyon Power Plant, just west of San Luis Obispo. They chiseled off exposed chunks of the rocks and ground them down into bits. To find out how old the rocks were, colleagues in Australia used a mass spectrometer to analyze their chemical composition. By comparing the ratio of beryllium-10, a rare atomic isotope produced by exposure to the cosmic rays that rain steadily down on Earth, with the more common and stable beryllium-9, they could estimate how long the surface of the rock had been exposed to the atmosphere. It’s “like looking at the sunburn of the rock,” Rood says.
That told the researchers how old the rocks were, but not what kinds of earthquakes they survived. To work that out, the researchers took photos of the rocks and built 3D models so they could simulate the earthquake shaking needed to topple them. The results suggest ancient high-magnitude earthquakes were less frequent than previously supposed. That allowed the researchers to forecast a less fearsome future: The average shaking caused by a one-in-10,000-year earthquake is 27% smaller than official estimates from the Pacific Gas & Electric Company, they report today in AGU Advances.
There are some reasons to be cautious, Biasi says. He points out that the constantly eroding rocks may not have been precarious the entire time they were accumulating “sunburn.” He adds that the team’s dramatic reduction in hazard for the region may be controversial, because it flies in the face of established estimates. Those estimates rely on a large body of research, which includes not just studies of seismic faults and ground motion in historic earthquakes, but also analyses of clues to past earthquake behavior like slumps and scour marks along faults that mark ancient ruptures. The new work will open up the official earthquake hazard assessment to more scrutiny, he says, but given the high stakes, the evidence isn’t enough just yet to topple the status quo.
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October 01, 2020 at 10:40PM
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Precarious rocks help refine earthquake hazard in California - Science Magazine
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