Balancing rocks and the earthquake detectives

Scientists have solved a mystery as to why precariously balanced rocks near the San Andreas Fault have never been toppled over by earthquakes.

During large earthquakes shaking of the ground causes unstable structures, both natural and man-made, to collapse in a wide region around the source of the quake. This high shaking zone can be anything from a few kilometres to hundreds of kilometres depending on the size of the earthquake.

Nick Hinze / Nevada Bureau of Mines & Geology
Nick Hinze / Nevada Bureau of Mines & Geology

It’s long been an intriguing mystery as to why many so called “precariously balanced rocks” are found in close proximity to one of the most active earthquake generating structures in the United States: the San Andreas Fault. Some of these rocks have been balanced for thousands of years. During which there might have 50 -100 earthquakes in the area.

A decade long study by US scientists, measuring and cataloguing balancing rocks along with computer modelling reveal that interactions between the San Andreas Fault and the neighbouring San Jacinto Fault may be the answer to the mystery.

The researchers believe that precariously balanced rocks have survived because interaction between the two faults has weakened earthquake ground shaking near them.

“These faults influence each other, and it looks like sometimes they have probably ruptured together in the past,” said lead author Lisa Grant Ludwig. “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 scientists have realised that these rocks could provide a check for seismic hazard maps, and give long-term indications of ground shaking.

“”They are kind of natural seismoscopes – but you have to read them indirectly.”

More information:
[1] Press release – http://news.uci.edu/press-releases/precariously-balanced-rocks-provide-clues-for-unearthing-underground-fault-connections
[2] Journal article – http://srl.geoscienceworld.org/content/early/2015/07/31/0220140239.extract

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