[Precariously Balanced Rocks] are more than just unusual geologic features—they’re a source of valuable scientific insight. They’re what are called “reverse seismometers” because their mere existence makes it possible to measure earthquakes that didn’t happen. If they’re still balanced, then the earth hasn’t moved enough to knock them over, at least in the last few thousand years, according to geologists David E. Haddad and J. Ramón Arrowsmith in their seminal 2011 report Geologic and Geomorphic Characterization of Precariously Balanced Rocks. So scientists study them to understand a region’s seismic history and, subsequently, predict what might come in the future.
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Brune’s approach to PBRs straddled all these fields, and that kind of triple-dipping in science often leads to skepticism, Stirling says. Other early PBR enthusiasts also struggled to get funding and resources for their work. But a good idea is a good idea and they persisted, Stirling says, with Brune leading the charge. By the early 2010s, PBR science had respect within the field and funding from sources such as PG&E and other energy companies, who wanted to understand the earthquake risk for their plants.
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To assess how long the rock has been precarious, Allam measures the concentrations of cosmogenic radionuclides on the underside of the rock, which offer a history of how long a rock has been exposed. The PBRs he studies tend to be approximately 30,000 years old. “Big earthquakes happen every 150 to 1,000 years, so 30,000 years is a statistically reliable data point,” he says.
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