We’ve all seen them; We even took pictures of ourselves pretending to hold them or push them.
These are precariously balanced rocks on a mountain or coastal reef. This is the same as Gentle Nudges sending them tumbling.
In true sense, the disturbances required to destabilize the block are quite significant, and husband and wife geologists Dolce Dylan and Anna Rood are left wondering how these great stones can be used to understand the history of earthquakes .
Think about it: if an erratically balanced rock has maintained its position for 10,000 years, it means that the land around the stone does not experience tremors above a certain level in that time.
“Trying to change the phrase, is that these precariously balanced rocks, or PBRs, are an ‘inverted seismometer’,” Anna explains.
“A normal seismometer records an event that is Happened while our PBR is still standing there, and so it records the earthquake Is not happened. Specifically, a major earthquake, ”Imperial College London, UK, researcher tells BBC News.
This is really useful information if you want to build a nuclear power station or waste store; Or maybe a major dam or bridge.
Knowing how strong a structure must be, requires an understanding of seismic hazards that may occur during its lifetime.
Can it expect to move once every 100 years or once every 1,000 years, or indeed once every 10,000 years? The answer will be directly on the cost of safe construction as well as insurance risk.
Planners may be lucky enough to place the new power station where they already have a detailed, instrumented record of seismic behavior. But there will be places where this record is sparse – places where there may be major shocks known but where the history of the size and frequency of events is extremely sharp.
For these sites, geologists would normally conduct what is called a probabilistic seismic hazard analysis in which they would try to model the probabilities, taking into account all possible local sources of earthquakes, such as nearby fault lines.
Now what Dylan and Anna have shown is that any well-balanced rocks in the vicinity can be used to exclude those models from the most far-out possibilities.
As a proof of principle, he studied PBR near the Diablo Canyon Nuclear Power Plant in coastal Central California.
These are long chert blocks that have been freed from the surrounding rock platforms and may fall as a result given the correct amount of tremors. But exactly how much shaking would be required, and how long the blocks have been in this free state are two unknowns that the team had to resolve to use the PBR as an inverted seismometer.
The amount of shaking is done by taking lots of photos of PBRs and creating a 3D model. Various equations will then determine the strength of ground acceleration required to hit the rock.
Working the second part – the “age of fragility” – relies on a smart technique that tracks changes induced in rocks as they were exposed to energetic space particles through time.
The cosmic rays will produce the radioactive element beryllium-10 when they collide with the oxygen atoms in the quartz minerals of the chert. Calculating the amount of Be-10 in the block’s surfaces therefore estimates how long the stones have been in their precarious position.
Anna said that the results of the PBR investigation for the Diablo Canyon nuclear power plant should be reassuring.
“We all know that we are in an unstable place in California, but our results show that it is not as volatile as we had feared.
“Crucially, we have estimated earthquake tremors twice as definitively. We have shown that earthquakes with strong shaking to hit our precariously balanced rocks – and that it is most likely to crack the ground and walls of buildings Will do – has not been recorded. In the last about 21,000 years by our inverse seismometer. “
This type of assessment is so useful in the specific case of Diablo Canyon that the main earthquake “threat” comes from the so-called Hosagri Fault, which is located about 6 km underwater.
“So there are a lot of these sites of interest of critical infrastructure that are located on the coast or near the coast, because cold water is needed, and where the threats are offshore – as by the Fukushima nuclear plant and the 2011 Tohoku earthquake Proof, ”says Dylan.
“And [land faults] Where you can just go digging the ditch and rebuild the magnitude-frequency relationship and the amount of slip per time, etc. – which is incredibly difficult to do with these offshore faults. This is where PBR can deliver for us. ”
But how available are they? Can you find a sufficient number of them to make an inverse seismometer of a widely applied technique?
Anna says there is a rock type suit, where such features can be formed. “The set of vertical joints is all you need. And, once you know what you’re looking at and get your eyes on – they’re actually a lot more ubiquitous than you might think.”
Drs Dylan and Anna Rood, and their colleagues report their PBR work in the journal AGU Advance.