In the western part of the San Bernardino Mountains, near the highway that connects Los Angeles and Las Vegas, scientists recently stumbled upon a geological mystery: huge boulders frozen in a strange position just above the San Andreas Fault.
It's not the balancing stones themselves that are baffling, according to Lisa Grant Ludwig, a researcher who presented the puzzle to her colleagues at the annual meeting of the American Seismological Society. What is most surprising is that the stones remain in place, despite powerful earthquakes, the epicenter of which is located just a few kilometers away.
Stones with acrobatic balance can be found all over the world. The meteorological and geological forces of nature wash away the material around them, leaving giant rocks in unusual positions, balancing on other rocks. They even came up with a name - unstable hanging blocks.
As you find yourself in more and more seismically active areas, you see fewer and fewer of these hanging blocks, says Ludwig, a professor at the University of California, Irvine. "You don't expect to see a similar phenomenon near active faults."
They are still hanging
In fact, the absence of dangling blocks may indicate that this area is seismically hazardous. “Some time ago, it was recognized that the balancing stones indicate that there are no strong earthquakes in these places, because otherwise such structures would have simply collapsed,” Ludwig said.
However, the stone structures in San Bernandino are very surprising. There are two rock clusters with more than a dozen hanging blocks, some of which are located just 7 kilometers from the fault.
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Ludwig said that one of her colleagues brought her photographs of these stones and asked how they could withstand earthquakes and not fall. To answer his question, she had to go to the site and examine the stones.
“I have spent a lot of time researching the large earthquakes in the San Andreas Fault,” she said. "I could swing some of the hanging blocks with my own hands."
Seismic riddle
Subsequent research showed that the stones remained in their position for thousands of years, and some for 18 thousand years! “This is too long a period of time, especially considering the proximity of a large fault,” says Ludwig.
It is not yet clear why the strong shaking bypassed the small accumulations of hanging blocks that remained intact. While researchers do not have any clear explanation of this phenomenon, they are tirelessly trying to solve the riddle.