Rock formations that look like they could topple at any moment, aptly named “precariously balanced rocks,’’ are effective earthquake barometers because they’re easily destroyed by ground shaking.
Now, researchers working in the Negev Desert of Israel have combined observations of precariously balanced rocks with models of seismic shaking to constrain the magnitudes of historical earthquakes known from archaeological records to have struck the region. They found that earthquakes within the past 1,300 years along some of the region’s faults were at most magnitude 5.0, weaker than previously thought.
These results, presented this week at the General Assembly of the European Geosciences Union in Vienna, Austria, shed light on the seismic history of southern Israel, an area crisscrossed by several fault lines, villages, and critical infrastructure such as hazardous material disposal sites and nuclear research facilities.
Precariously balanced rocks have been used to study seismically active areas across the United States and New Zealand. Their presence can help place maximum limits on estimates of earthquake shaking, said Lisa Grant Ludwig, a geologist at the University of California, Irvine not involved in the research. “Precariously balanced rocks are sort of like natural seismometers.”
Yaron Finzi, a geophysicist at the Dead Sea–Arava Science Center in Mitzpe Ramon, Israel, and his collaborators worked with citizen scientists to identify precariously balanced rocks. Tour guides working in the Negev Desert sent pictures of rock formations, and Finzi and his team followed up on likely candidates.
“It was a lot of hiking and field trips,” said Finzi.
Between 2015 and 2018, the researchers found roughly 80 limestone precariously balanced rocks in a 50- by 70-kilometer swath of desert. The tallest of these vertical pillars of rock, just barely connected at their base to cliffsides because of uneven weathering, measured over 40 meters.
The scientists investigated the maximum-size earthquakes that would have left the precariously balanced rocks standing.
To do this, they analyzed the stability of the rocks using digitized images of the formations. Finzi and his team used predictions of ground motion at varying distances from an earthquake’s rupture and calculations of how far the rocks could tip over before toppling. They also calculated the resonant frequencies of the rock formations—dictated by their dimensions—and found that that a handful of the pillars would have been susceptible to amplified shaking during an earthquake. These precariously balanced rocks would be particularly sensitive to ground shaking, Finzi and his team reasoned.
“They would topple from a relatively weak earthquake,” said Finzi.
To determine how long the precariously balanced rocks had been standing, the scientists age dated some of them using a technique called optically stimulated luminescence. This method involves determining how long ago quartz grains, blown by the wind into the crevasse between a pillar and the adjoining cliffside, were exposed to the Sun. This age correlates with when the grains were buried and therefore the age of the pillar.
Finzi and his team concluded that some of the faults in the Negev Desert hadn’t produced any earthquakes stronger than magnitude 5.0 within the past roughly 1,300 years. That’s weaker than archaeological records suggest, which is “a pleasant surprise,” said Finzi.