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Why 10,000-Year-Old Gravity-Defying Rocks Haven’t Toppled

Giant rocks stacked in seemingly gravity-defying poses could indicate that earthquakes on the San Andreas Fault can jump to another major fault in Southern California, which in turn could affect the severity of quakes in the region and how the state prepares for future temblors.

The delicately balanced rocks do not topple, despite being near active faults, which likely indicates that earthquake tremors generated by the San Andreas Fault — the 800-mile-long fault that cuts through California and marks the boundary between the North American Plate and the Pacific Plate — are able to transfer to the neighboring San Jacinto Fault, weakening overall shaking in the areas where the rocks sit, the researchers said.

precariously balanced rock
A precariously balanced rock (PBR) near Searchlight, Nevada. Nick Hinze / Nevada Bureau of Mines and Geology

The granite rocks, called Precariously Balanced Rocks, or PBRs, form when tectonic processes allow a big chunk of rock, tattooed with patterns of fractures, to ascend from beneath the surface of the Earth. As the rock rises, wind, water and other natural processes erode bits and pieces of it away, eventually chiseling out the remaining delicately balanced rocks, said study co-author Julian Lozos, a postdoctoral researcher at Stanford University.

PBRs typically develop over thousands or tens of thousands of years, and they can be found around the globe, including in other earthquake-prone spots in the United States and New Zealand, the researchers said. There are more than 1,200 PBRs just in California and western Nevada, they added.

In the new study, the researchers focused on PBRs that are approximately 10,000 years old, located in the middle of the San Bernardino Mountains, about 90 miles east of Los Angeles.

Locating PBRs near active faults could help officials plan for future earthquakes. To prepare infrastructure — including the water supply, telecommunications and energy systems — in areas prone to shaking, "you would really like to have an idea of what to expect," said Lisa Grant Ludwig, lead author of the study and an expert on the San Andreas Fault at the University of California, Irvine.

The PBRs close to the faults "suggest that the San Andreas and San Jacinto can go off together, which would be a huge earthquake with a lot of impact for inland Southern California," Lozos told Live Science.

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The researchers used 3-D modeling to simulate how different kinds of shaking would affect various shapes and sizes of rock. Although some rocks are easily knocked over by fast, small shakes, other rocks topple to slow, big shakes, Lozos said.

The presence of PBRs close to the San Andreas Fault suggests that nearby earthquakes didn't happen very often, or happened with little energy, Lozos said.

A ruptured fault can either continue on the same fault, or "jump" to another one nearby, Lozos said. It's "like the rupture taking a different freeway exit," he added. Maybe the rupture is "charging down the San Andreas and decides to keep going or decides to take the exit onto the San Jacinto."

"If you had a rupture start on the San Jacinto and jump onto the San Andreas," less energy would transfer to the San Andreas Fault because the jump would absorb energy, Lozos said. When more than one fault is involved in an earthquake, it's called a "multi-fault" earthquake.

PBRs near the San Andreas and San Jacinto faults are also puzzling, Lozos said, because the rocks are roughly 10,000 years old and earthquakes there typically occur every 200 to 300 years.

This is a condensed version of an article that appeared on Live Science. Read the original story here. Elizabeth Goldbaum is on Twitter. Follow Live Science @livescience, Facebook & Google+.

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