Unstablely balanced rocks, known in geology as the PBR, are ancient and subtle natural formations that can provide scientists with interesting clues about earthquake disasters in the region. By understanding the upper limits of the rock’s past shaking, researchers can get information about the region’s future seismic risk, and a cutting-edge new technology could greatly improve the accuracy of this modeling.
For decades, scientists have turned to PBRs to assess seismic activity and future seismic risks, long-standing rock formations that are evidence of the limits of earthquakes that have occurred locally for thousands of years. This information, together with other factors such as fault lines, provides the basis for seismic disaster modeling that engineers rely on to determine the safe location of bridge dams.
But there are many loopholes in this approach, one of the big problems is the lack of seismic data for larger earthquakes that were rare between 10,000 and 1 million years ago. A team at Imperial College London has come up with a way to fill these gaps by studying the ancient geological secrets of a group of PBR in California.
This involves using a technique called cosmic ray surface exposure age to count the number of rare radon atoms inside the rock, which is the result of long-term exposure to cosmic rays. This allowed the team to determine how long the BRRs had been in their current arrangement and to re-create the rocks using 3D modeling software and simulate how much ground vibration they would need to fall.
A team at Imperial College London found that PBRs, or unstablely balanced rocks, can exist in the landscape twice as long as previously assumed. The new data is combined and compared with the existing model to improve the accuracy of the model. According to the team, this helped them eliminate a series of assumptions in the modeling, ultimately reducing uncertainty about earthquake disaster estimation by 49 percent and reducing the average size of rare earthquakes estimated to occur every 10,000 years by 27 percent.
“We’re teetering on the brink of a scientific breakthrough in earthquake prediction, ” said study co-author Dr. Dylan Rood. “Our ‘rock clock’ technology has the potential to save huge costs in seismic engineering and can be used to test and update hazard estimates at specific locations in earthquake-prone areas — especially in coastal areas, where controlling seismic sources is offshore faults, and the movement itself is more difficult to investigate.”
Researchers are now turning their attention to improving risk estimates in southern California, a densely populated part of the United States and a particularly dangerous area for seismic activity.
“We are now studying PBR near major seismic faults such as the San Andreas Fault near Los Angeles,” said study lead author Anna Rood. “We’re also looking at how to determine which data — whether it’s fault slip rates or the choice of geodylom equations — are distorting the results of the original hazard model. In this way, we can further improve scientists’ understanding of large earthquakes. “
The study was published in the journal AGU Advances.