Delicately balanced boulders have been used to create a ‘rock clock’ that can improve the accuracy of earthquake severity predictions by up 50 per cent.
Imperial College London researchers created a new severe earthquake forecasting model using information from formations of precariously balanced rocks (PBRs).
PBRs, rock formations where a ‘slender boulder’ is delicately perched on a pedestal boulder, are formed when softer rocks are eroded, leaving the harder ones behind.
The team started by studying balanced boulders at the Diablo Canyon Nuclear Power Plant in California – this involved dating the rocks and creating a 3D model.
They then used the 3D model to ‘throw Earthquakes at the boulders’ to see how strong it needed to be in order to make them topple over.
This allowed them to then create a ‘hazard mode’ that determines the risk of future major earthquakes in the area around the rocks from past information.
Combining the ‘rock clock’ with existing models improved hazard estimates by around 50 per cent, the researchers found.
This image shows a precariously balanced rock preserved on a tectonically uplifted marine terrace near the Diablo Canyon nuclear Power Plant in coastal Central California
Anna Rood collecting samples for cosmogenic surface exposure dating, which will be used to model the history of the PBR being exhumed from the surrounding softer weathered rock
As part of the study to create a ‘rock clock’ the team determined the age of PBRs near the Diablo Canyon Nuclear Power Plant in coastal California.
To do this they used a technique called ‘cosmogenic surface exposure dating’ that counts the number of beryllium atoms formed within the rocks by the long-term exposure to cosmic rays.
3D modelling was then used to digitally recreate the PBRs and calculate how much shake was needed to topple the rocks.
Having survived for thousands of years, they can be used to calculate the ‘upper limit of earthquaking shaking’, the point at which the boulder would topple.
This is where the ‘rock clock’ label comes in – by understanding how long the rocks have stood, the team can determine how long it has been since a major earthquake struck – and its maximum size – in the area surrounding that rock.
Anna Rood measures a number of balancing rocks as part of the study into their age. This work allows the team to predict future large earthquakes and their estimated size
Anna Rood sat next to a precariously balanced rock that will be used to validate how earthquakes rupture along the San Andreas fault
Study lead author Anna Rood, of Imperial College, London, said: ‘This new approach could help us work out which areas are most likely to experience a major earthquake.
‘PBRs act like inverse seismometers by capturing regional seismic history that we weren’t around to see, and tell us the upper limit of past earthquake shakes simply by not toppling.
‘By tapping into this, we provide uniquely valuable data on the rates of rare, large-magnitude earthquakes.’
Earthquake hazard models help engineers decide where to build buildings, bridges and dams by calculating the chances that an earthquake will strike.
Current models however rely on observations like how close an area is to a fault line and how seismically active it has been in the past.
This makes rarer earthquakes, which may have occurred over 10,000 or sometimes a million years, ‘extremely’ hard to predict.
‘By counting rare cosmic ray-generated atoms in PBRs, we have created a new method of earthquake hazard validation that could be built into existing models to finetune their precision,’ Rood explained.
Co-author Dr Dylan Rood, also at Imperial College London, said they were ‘teetering on the edge of a breakthrough’ in the science of earthquake forecasting.
‘Our ‘rock clock’ techniques have the potential to save huge costs in seismic engineering, and we see them being used broadly to test and update site-specific hazard estimates for earthquake-prone areas,’ he said.
This has worked specifically in coastal regions where the controlling seismic sources are offshore faults whose movements are inherently more difficult to investigate.’
The PBR overhangs its pedestal, which makes it teeter on the edge of toppling. Brightly coloured tape is used to aid the construction of 3D models of the PBR and surrounding outcrop
Anna Rood measuring the scale of an excavated section into the marine terrace. The two rock outcrops extending above the terrace surface in the background are abandoned sea stack on which the PBRs have formed
Removing the worst-case-scenario from their model, reduced the average size of earthquakes expected to happen every 10,000 years by around 30 per cent.
Ms Rood added: ‘We’re now looking at PBRs near major earthquake faults like the San Andreas fault near Los Angeles.
‘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 models.
‘This way we can improve scientists’ understanding of big earthquakes even more.’
The findings were published in the journal AGU advances.