Precariously balanced rocks offer clues about future earthquake risk

Precariously balanced rocks, or PBRs as they’re known in geological circles, are ancient and delicately poised natural formations that can offer scientists interesting clues about earthquake hazards in the area. By understanding the upper limits of the wobbling these rocks have endured in the past, researchers can gain a picture of future earthquake risks in the area, and a cutting-edge new technique could greatly improve the accuracy of this modeling.

Scientists have turned to PBRs to assess seismic activity and future earthquake risk for decades, with the long-standing formations serving as evidence of the limits of tremors that have occurred locally over the preceding thousands of years. This information, along with other factors like fault lines, feeds into earthquake hazard modeling that engineers rely on when settling on safe locations to build dams of bridges.

But there are plenty of holes in the approach, and a big one concerns the lack of seismic data on rare, larger earthquakes between 10,000 and 1,000,000 years ago. A team from Imperial College London has come up with a way that could fill in these blanks, by studying the ancient geological secrets of a set of PBRs in California.

This involved using a technique called cosmogenic surface exposure dating to tally up the amount of rare beryllium atoms inside the rocks, which are the result of long-term exposure to cosmic rays. This enabled the team to determine how long these PBRs had existed in their current arrangement, and use 3D modeling software to recreate those rocks and simulate how much ground-shaking it would have taken for them to fall over.

An Imperial College London team has found that PBRs, or precariously balanced rocks, can exist in the landscape for twice as long as previously assumed
An Imperial College London team has found that PBRs, or precariously balanced rocks, can exist in the landscape for twice as long as previously assumed

Anna Rood & Dylan Rood, Imperial College London

The team found that PBRs can exist in the landscape for twice as long as previously assumed. The new data was combined and compared with existing models as a way of improving their accuracy. According to the team, it helped them remove a range of assumptions in the modeling and ultimately reduced the uncertainty of earthquake hazard estimates by 49 percent, and reduced the average size of rare earthquakes, estimated to occur every 10,000 years, by 27 percent.

“We’re teetering on the edge of a breakthrough in the science of earthquake forecasting,” says study co-author Dr Dylan Rood. “Our ‘rock clock’ techniques have the potential to save huge costs in seismic engineering, and could be used to test and update site-specific hazard estimates for earthquake-prone areas – specifically in coastal regions where the controlling seismic sources are offshore faults whose movements are inherently more difficult to investigate.”

The researchers are now turning their attention to improving hazard estimates in southern California, a heavily populated part of the US and one at particular risk of seismic activity.

“We’re now looking at PBRs near major earthquake faults like the San Andreas fault near Los Angeles,” says study lead author Anna Rood. “We’re also looking at how to pinpoint which data – whether it be fault slip rates or choice of ground shaking equations – are skewing the results in the original hazard models. This way we can improve scientists’ understanding of big earthquakes even more.”

The research was published in the journal AGU Advances.

Source: Imperial College London

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