Why Don’t Balancing Boulders Fall During Earthquakes?

The interaction of nearby fault lines may lessen ground shaking around some balancing rocks

Balanced Rocks
Left:Nick Hinze / Nevada Bureau of Mines & Geology; Right: Lisa Grant Ludwig

Some boulders defy gravity. Despite balancing on other rocks in the midst of fault lines, they stay precariously perched — thanks in part to those fault lines brushing up against each other, geologists report in the journal Seismological Research Letters.

“Precariously balanced rocks” show up around the world and get their unpredictable looks through different mechanisms. Many are remnants of bigger rocks that eroded away, while others are the products of melted glaciers, avalanches or rock falls. When hit with an earthquake, they topple — but not in California’s San Bernardino Mountains.

To figure out how these rocks maintain their balance, researchers analyzed 36 rocks in the Silverwood Lake and Grass Valley areas of California. Some are 10,000 years old, and each is a stone’s throw (4 to 6 miles) from the San Andreas and San Jacinto fault lines. Theoretically, all of these rocks should have hit the ground during earthquakes — obviously, they haven’t.

Here’s what researchers think is going on: Interaction between the two faults has weakened the ground near the balancing rocks. Because of this weakness, the ground doesn't shake as violently during an earthquake.

“These faults influence each other, and it looks like sometimes they have probably ruptured together in the past,” Lisa Grant Ludwig, a geologist at the University of California at Irvine and a co-author on the study, said in a statement.

If the research on why the rocks haven't fallen turns out to be right, it could shed new light on how the two faults influence one another. Ludwig notes that this could have major implications for area residents. “This brings up the question of whether we might have an earthquake on the San Jacinto that triggers one on the southern San Andreas, or vice versa,” she explained in the release. Though the future of the faults — and the rocks — is unclear, one thing is certain: As long as they stay standing, the seemingly-unstable boulders will continue to fascinate.

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