Ancient precariously balanced rocks provide new method for assessing earthquake risks

Ancient formations of precariously balanced rocks can provide insight into the rates of rare large-magnitude earthquakes, scientists have said.

Formations of precariously balanced rocks (also known as PBRs) are found throughout the world, where boulders appear to be teetering on high pedestals due to natural geological processes.

These unusual formations can be due to blocks of rock preserved on cliffs, or when softer rocks erode, leaving harder rock behind. They can also form when landslides or retreating glaciers can leave large rocks in strange positions.

Examples include Brimham Rocks in Yorkshire, or Chiricahua National Monument in Arizona. Both of these formations have survived countless earthquakes occurring over millennia.

Therefore their survival can inform seismologists what the upper limit of earthquake magnitude has occurred since they were first formed - as any stronger shaking would have caused them to topple.

By examining precariously balanced rocks in California, a research team from Imperial College London has used this ancient geological recording method to inform a new technique to boost the precision of hazard estimates for large earthquakes by up to 49 per cent.

Lead author Anna Rood, from Imperial's Department of Civil and Environmental Engineering, 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 are used across the world to estimate the likelihood of future earthquakes in a given location.

They help engineers decide where bridges, dams, and buildings should be built and how robust they should be - as well as informing earthquake insurance prices in high-risk areas.

Current earthquake hazard estimates rely largely on observations like proximity to fault lines and how seismically active a region has been in the past, the researchers said. However, estimates for rarer earthquakes which have occurred over periods of 10,000 to 1 million years  ago are extremely uncertain due to both the complete lack of seismic data spanning those timescales, and previous reliance on “rocky assumptions”.

So, to tap into the seismology of the past, the researchers set out to determine the exact age and likelihood of toppling due to ground shaking of precariously balanced rock formations at a site near to the Diablo Canyon Nuclear Power Plant in coastal California.

They used a technique called cosmogenic surface exposure dating - counting the number of rare beryllium atoms formed within rocks by long-term exposure to cosmic rays - to determine how long the rocks had existed in their current formation.

They then used 3D modelling software to digitally recreate the rock formations and calculate how much earthquake ground shaking they could withstand before it would cause them to topple.

Both the age and fragility of the rock formations were then compared with current hazard estimates to help boost their certainty.

By combining their calculations with existing models they reduced the uncertainty of earthquake hazard estimates at the site by 49 per cent, and if they also removed the “worst-case-scenario” estimates, they found it reduced the average size of earthquakes estimated to happen once every 10,000 years by 27 per cent.

The research also revealed that precariously balanced rock formations can be preserved in the landscape for twice as long as previously thought.

The scientists concluded that this new method reduces the amount of assumptions, and therefore the uncertainty, used in estimating and extrapolating historic earthquake data for estimates of future risk.

Study co-author Dr Dylan Rood, of Imperial's Department of Earth Science and Engineering, said: “We're teetering on the edge of a breakthrough in the science of earthquake forecasting.”

He went on: “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 - specifically in coastal regions where the controlling seismic sources are offshore faults whose movements are inherently more difficult to investigate.”

The research team is now using their new assessment method to validate hazard estimates for southern California - one of the most quake-prone and densely populated regions of the United States.

The research is published in AGU Advances.