Balancing Rocks Indicate History and Strength of Quakes



The riddle of why the balancing rock collection near the San Andreas fault has never been toppled by earthquakes has been solved, claims US scientists.

Balancing rocks are not seen within 15km of major faults.  Ten years ago Prof Brune and his colleagues found two sizeable collections of such stones just 7-10 km from the San Andreas and San Jacinto faults, in the San Bernardino mountains of California.  The teetering rocks sit less than 10km from two major faults.

According to researchers, these rocks are a check on seismic hazard maps,  giving long-term indications of ground shaking.  Different measures can tell exactly how long the stones have perched in their places – and it is millennia, not centuries.

For example, age study of one of the rocks reveals that it was in that position for about 18,000 years.

They are a kind of natural seismoscopes – but they didn’t tell when an earthquake happened but tell when ‘an earthquake strong enough to knock them down did not occur.

Their decade-long study that began in the 1990s, collecting measurements of balancing rocks concludes that quakes can stop or “jump” due to interactions between the San Andreas and the neighbouring San Jacinto fault.

Importantly, the team calculated how much force it would take to tip each of the rocks over using two models.  One method is by actually trying to tip the rock physically.  The other method is by creating a 3D computer model using images from multiple angles of the balanced stone and calculate its centre of gravity, mass and so on.

The famous San Andreas fault stretches 1,300km across California.  The San Andreas and San Jacinto faults are only about 2km apart. Such a rupture can cause an earthquake to jump across or step over.

In the long-term, seismic activity is gradually shifting from the southern stretch of the San Andreas fault across to the younger San Jacinto fault.  The connected nature of the faults has implications for quake planning.

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