Strong earthquakes can easily topple precariously balanced rocks (PBRs), which are naturally occurring geological features that provide various information about the seismic activity and hazards, according to the Southern California Earthquake Center (SCEC).

They are one of the most dramatic and eye-catching sights in nature that makes everyone wonder how it was formed. But more than its aesthetic beauty, experts also use PBRs to determine shaking or earthquake risk for an area.


PBRs Can Tell Earthquake Risks

Earthquakes in the past were only recorded in the 1800s so to track these, geologists trench along the known faults looking for the location of the slit layers which are offset by several feet that marks significant movements of the earth.

Geologists have used the PBRs to tell if a particular region has suffered an earthquake in the past. Strong movements from the ground can topple PBRs from the perches.

They determined the upper limit of earthquake intensity that occurred since the formation of the rock structures and used this data to decrease the uncertainty in the existing earthquake models for large earthquakes by 49%. Geologist Anna Rood from Imperial College London said that their study, published in AGU Advances, could help those areas frequently experiencing large earthquakes.

How Does Precariously Balanced Rocks Save Lives?
(Photo: Wikimedia Commons)
Balanced Rocks, Marble Canyon, Arizona.

She added that PBRs act like inverse seismometers as it has recorded previous seismic activity in the area in the past and tell experts the upper limit of past earthquake shakes simply by not falling off their perches.

"By tapping into this, we provide uniquely valuable data on the rates of rare, large-magnitude earthquakes," Rood said.

The study involved two stages, Science Alert reports. First, the researchers would try to determine the age of the PBR using cosmogenic surface exposure dating to count the beryllium atoms in the rock. Secondly, they used 3D modeling simulations to measure how much toppling did the PBRs endured before toppling over.

According to Rood, they are now looking at PBRs near major earthquake sites like the San Andreas and the fault line near Los Angeles. Also, they are looking at how to pinpoint the specific data that is responsible for the skewness of the result in the original hazard models.

Read Also: Large Earthquakes Found to Follow The Mathematical Pattern Called 'Devil's Staircase'


PBR Formation

According to Jon Noad, PhD. in Sedimentology at London University in the United Kingdom, PBRs are formed softer rocks are eroded which leaves the more resistant rocks behind.

Precariously balanced rocks can also be formed when receding glaciers leave behind boulders in unnatural positions, according to Science Alert. Some examples of PBRs include the Brimham Rocks, Chiricahua National Monument in Arizona, the Finger of God in Namibia, the Azure Window in Malta, and many more.

The more PBRs are, means that the researchers can input historical data about them into earthquake hazard models that help determine the risk of future earthquakes especially in residential areas that hold critical constructions, such as dam and nuclear power plants.
Knowing how long PBRs and how many earthquake hazards there are and how much shaking is needed to dislodge them would help determine the risk of future earthquakes.

Read More: California Desert Earthquake From a Year Ago Raised Risk of 'The Big One' Hitting Southern California, Study Reveals


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