Precariously balanced rocks, or PBRs, can be found throughout the world, identified by slender rocks balanced on a pedestal boulder.
There are a few ways they can form. PBRs can be the result of landslides or retreating glaciers, which deposit the rocks in the unordinary formations. They can also manifest when softer blocks erode, leaving the stronger ones behind.
While they look delicate, some of them — like the Brimham Rocks in Yorkshire or Chiricahua National Monument in Arizona — have survived countless earthquakes over thousands of years. Now, researchers from Imperial College London are taking a closer look at PBRs, because they are showing promise in helping scientists determine earthquake hazards more accurately.
The study’s authors say the construction of PBRs provides insight on the upper limit of earthquake shaking that has occurred in the area.
It’s this information that can help create better estimate models.
“Rare earthquakes can be extremely destructive and costly, but existing hazard estimates for rare earthquake shaking are highly uncertain because observations are limited to historical records,” reads an excerpt from the study.
Anna Rood collecting a sample for cosmogenic surface exposure dating,which was used to deduce the fragility age of the PBR beneath her front foot. Courtesy: Anna Rood & Dylan Rood, Imperial College London
“At a site in Central California, we characterize the probability of toppling of such precariously balanced rocks and determine their age of formation. The precariously balanced rock constraints are used to directly eliminate estimates in the hazard model that are inconsistent with the preservation and antiquity of the rocks. These results dramatically improve the hazard model and significantly reduce uncertainties in the estimates.”
The findings are important because earthquake hazard models are needed to estimate the likelihood of