We’ve all seen them; we even took pictures of ourselves pretending to hold them up or push them.
These are the precariously balanced rocks on a hill or coastal cliff. It is as if the slightest push made them tumble.
Indeed, the disturbance required to disrupt the blocks is quite significant, and this prompted geologists Dylan and Anna Rood to wonder how these large stones could be used to decipher the history of earthquakes.
Think about it: if a precariously balanced rock has held its position for 10,000 years without tipping over, it means that the ground around the stone has not been shaken above a certain level in all this time.
“The turn of phrase we are trying to coin is that these precariously balanced rocks, or PBRs, are an ‘inverse seismometer,'” explains Anna.
“A normal seismometer records an event that has it happened while our PBR is still there, and therefore it records an earthquake that He has not happened. Specifically, a major earthquake, “a researcher from Imperial College London, UK, told BBC News.
This is really useful information if you want to build a nuclear power plant or a waste yard; or perhaps a large dam or a bridge.
Knowing how robust that structure must be requires an understanding of the seismic hazards that could reasonably be expected during its life.
Can you expect a certain shaking threshold once every 100 years, or every 1,000 years, or even only once every 10,000 years? The answer will weigh directly on the cost of a safe construction as well as the insurance risk.
Designers may be in luck as the location where they want to install the new power plant already has a detailed and instrumented record of the seismic behavior. But there will be places where that record is scarce, places where great tremors are known to occur but where the history of the size and frequency of events is extremely fragmented.
For these sites, geologists will normally conduct what is called a probabilistic seismic risk analysis in which they will try to model the possibilities, taking into account all potential local sources of earthquakes, such as nearby faults.
What Dylan and Anna have now shown is that any precariously balanced rock in the vicinity can be used to limit those patterns by ruling out the more distant possibilities.
As a proof of principle, they studied PBRs near the Diablo Canyon nuclear power plant in coastal central California.
These are tall flint blocks that have eroded away from the surrounding rock platform and as a result could fall with just the right amount of shaking. But exactly how much shaking would be needed and how long the blocks remained in this independent state are the two unknowns the team had to solve in order to use the PBRs as inverse seismometers.
The amount of shaking is calculated by taking many photos of the PBRs and making a 3D model. Various equations will then produce the force of the ground accelerations needed to bring down the rock.
The elaboration of the second part – the “age of fragility” – is based on an intelligent technique that tracks the changes induced in the rocks when they have been exposed to energetic particles of space over time.
Cosmic rays when they strike the oxygen atoms in the quartz minerals of chert generate the radioactive element beryllium-10. By counting the quantity of Be-10 in the surfaces of the blocks, we therefore obtain an estimate of how long the stones have been in their precarious situation.
The results of the PBR investigation for the Diablo Canyon nuclear power plant should be reassuring, says Anna.
“We all know we’re on unstable ground in California, but our results show it’s not as shaky as we feared.
“Importantly, we have made earthquake shaking estimates twice as certain. We have shown that an earthquake with shocks strong enough to drop our precariously balanced rocks – and would most likely break ground and building walls – would not has been recorded by our inverse seismometers for the last 21,000 years or so. “
The reason this kind of assessment is so useful in the specific case of Diablo Canyon is that the main earthquake “threat” comes from the so-called Hosgri Fault, which is about 6km offshore, underwater.
“So there are many of these critical infrastructure sites of interest that are on or near the coast, due to the need for cooling water, and where the risks are offshore – as evidenced by the Fukushima Nuclear Power Plant and the Tohoku Earthquake. of 2011, “says Dylan.
“And differently [land faults] where you can just go digging trenches and reconstruct the magnitude-frequency relationship and the amount of slip over time, etc. – It is incredibly difficult to do with these offshore faults. That’s where PBRs can provide for us. “
But how available are they? Can you find enough of them to make inverse seismometers a widely applicable technique?
There is a number of rock types in which these features can form, Anna says. “The vertical joint set is all you need. And, once you know what you are looking for and have your eyes in it, they are actually a lot more ubiquitous than you might think.”
Doctors Dylan and Anna Rood and colleagues report their work in the PBR in AGU Advances magazine.