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Superheated underground rocks reveal earthquake clues


Fluidlike rocks that reach temperatures above 160 degrees Celsius have been identified to cause tremors along part of California’s San Andreas Fault.  

A study conducted by University of Southern California (USC) scientists has found that the seismic waves caused by the friction of melting rocks deeper than the Earth’s crust can lead to earthquakes.

Rather than studying the movement of the Earth’s crust – which is common when studying the San Andreas Fault – the USC scientists looked at the problem from the bottom up to study underground rocks, friction and fluids that lead to tremors.

The study revealed that a segment of the San Andreas Fault near Parkfield, California has earthquake-causing friction from melting rocks far below where earthquake’s are usually monitored.

Assistant professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences Sylvain Barbot said tremors can occur much deeper than where most quakes are detected on the San Andreas Fault.

“Most of California seismicity originates from the first 10 miles (16 kilometres) of the crust, but some tremors on the San Andreas Fault take place much deeper,” he said.

“Why and how this happens is largely unknown. We show that a deep section of the San Andreas Fault breaks frequently and melts the host rocks, generating these anomalous seismic waves.”

By using mathematical models and laboratory experiments of rock, the scientists simulated fault activity in the deep Earth. They found that granite and quartz chunks that make up the Earth’s bedrock can produce heat due to friction after the tectonic plates move past each other.

When the rocks are heated above 160 degrees they become less solid and more slippery. This generates friction heat and fluids until the rocks slip past each other rapidly and trigger an earthquake.

“Just like rubbing our hands together in cold weather to heat them up, faults heat up when they slide. The fault movements can be caused by large changes in temperature,” Barbot said.

“This can create positive feedback that makes them slide even faster, eventually generating an earthquake.”

The study will continue to assist scientists in determining how and where earthquakes can occur and what triggers them.

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