Determination of earthquake risk through unstable rocks in California.

The research on earthquake hazards has taken into account such balanced, unstable rocks, but this is the first study to use them for …

Significantly improved risk assessment entirely

In an earthquake zone near the Diablo Canyon Nuclear Power Plant in Southern California, stands a precariously balanced rock that Anna Rudd dubbed Damaris, after her best friend. “The joke is that it’s incredibly fragile,” says Rudd, PhD. Student in Geology at Imperial College London.

She believes Damaris has stood in place for 21,000 years, which would seem very powerful. The fact that it was still standing meant that an earthquake strong enough to strike it didn’t happen the entire time it was dangerously floating. Now, by analyzing unstable rock eras such as Damaris and knowing the size of an earthquake that could knock them down, Rudd and her colleagues have constructed a more accurate picture of the behavior of past earthquakes in the region – and thus what might happen in the future. It contrasts with the relatively pessimistic official forecast for this area, called the Hosgary Fault Zone, which is located off the coast of California midway between San Francisco and Los Angeles.

Glenn Biasi, a US Geological Survey seismologist who was not involved in the study, says. “This will be an important case study of how to deal with the use of fragile geological features,” he says.

Seismologists have been studying unstable rocks for nearly half a century. These ancient rock formations provide important data on earthquakes that would not otherwise be available, since modern seismometers, which detect Earth’s shaking, have been around for 100 years or so. This lack of data is a problem for vital installations like dams or nuclear power plants that need to withstand the type of catastrophic earthquake that may only come once every 10,000 years.

To get a better idea of ​​previous earthquake behavior in this part of California, Rudd and colleagues selected seven risky rocks from a site near the Diablo Canyon Power Station, west of San Luis Obispo. They dug the exposed rock chunks and ground them into small pieces. To find out the age of the rocks, colleagues in Australia used mass spectrometry to analyze their chemical composition. By comparing the ratio of beryllium-10, a rare atomic isotope resulting from exposure to cosmic rays that rain continuously on Earth, with the more common and stable beryllium-9, they can estimate how long the surface of rocks has been exposed to the atmosphere. It’s “like looking at a sunburn in a rock,” says Rudd.

This tells researchers about the age of the rocks, but not the types of earthquakes they have survived. To achieve this, the researchers took pictures of the rocks and made 3D models so that they could simulate the earthquake’s vibration needed to bring it down. The results indicate that ancient, high-strength earthquakes were less frequent than previously assumed. This allowed the researchers to predict a less frightening future: the average shaking from a single earthquake in 10,000 years is 27% lower than the official estimate for the Pacific Gas & Electric Company, they report today in AGU Advances.

There are some reasons for caution, Biasi says. He notes that the continuously eroded rock may not have been risky the whole time the “sunburn” has accumulated. He adds that the team’s drastic reduction in risks to the region could be controversial, as it conflicts with the established estimates. These estimates are based on a large body of research, which includes not only studies of seismic faults and ground motion in historical earthquakes, but also analyzes of evidence of past earthquake behavior such as stagnation and carving marks along faults that indicate ancient ruptures. The new work will open the formal earthquake risk assessment for further scrutiny, he says, but given the high stakes, the evidence is not yet sufficient to bring down the status quo.

This news was originally published on sciencemag.org

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