Throughout the past decade, scientists have been trying to work out how the so-called "precariously balanced rocks" (PBRs) that occur in California have managed to stay put without toppling, an impressive feat considering they are found between two seismically active fault lines. Finally, researchers think that they’ve managed to figure this mystery out, and their answer could have implications for how the region plans for future earthquakes.

“It was a real scientific puzzle, a real head-scratcher,” says Lisa Grant Ludwig, who co-authored the study published in Seismological Research Letters. "How can you have these rocks right next to the San Andreas Fault? It's an interesting scientific question, but it also has practical implications, because we want our seismic hazard maps to be as good as possible."

The researchers estimate that the rocks have been balanced for at least 10,000 years. During this period, there have been many large, surface-rupturing earthquakes. Therefore, the researchers first wanted to check that the rocks would indeed normally topple under such a degree of ground-shaking.

To do this, they examined 36 of the balancing rocks, all of which lay within 10 kilometers (6 miles) of either the San Andreas or the San Jacinto faults. They then measured the fragility of the rock positions by putting a pulley on them and tugging until the point at which if they let go, the rocks would tumble. This allowed them to then calculate what effect different magnitude earthquakes would have on the rocks, and at what point they’d fall. Their results suggested that the boulders should have toppled long ago, adding to the mystery.

After investigating all the possible explanations for this conundrum, they’ve finally settled on a solution. They think that there is an interaction between the two fault lines that run on either side of the rocks, and that the earthquakes “jump” between the two, skipping the rocks in the middle.

“These faults influence each other, and it looks like sometimes they have probably ruptured together in the past,” said Grant Ludwig from the University of California, Irvine. “We can't say so for sure, but that's what our data point toward, and it's an important possibility that we should think about in doing our earthquake planning.”

The worry is that an earthquake caused by the San Jacinto Fault, which has been very seismically active, could trigger an earthquake on the southern portion of the San Andreas Fault, which comparatively has been fairly quiet. 

^{Center image: A precariously balanced rock in Nevada, which should have been toppled by earthquakes, and yet still stands. Credit: Nick Hinze/Nevada Bureau of Mines & Geology}