What happens inside an earthquake?

Normal friction
Above ground, friction is a steady, stubborn force that opposes motion. Friction generates heat, as people with cold hands know, and increases with the stress you put on objects. So the heat on faults during sliding should increase with depth in the Earth. The rocks should definitely melt where they meet.

But underground, during earthquakes, two huge, hard, weight-pressurized rock slabs slip past or under each other. And nothing melts. Usually.

That's weird. It could be because the friction and thus the heat are much lower than you'd expect from rocks above ground, Goldsby said.

Earthquake friction works like this, Heaton said: It starts out high when there is little to no movement; then friction plummets to zero as the rocks move fast; then friction goes to high again when the rocks slow down.

That weird behavior of friction during an earthquake might be the reason there is little to no melting, Goldsby said. If friction is low when the rocks move fast, then much less heat is generated and no detectable melting occurs.

Maybe some other mechanism kicks in before the rocks get to their melting phase, Heaton said.
One explanation is "flash heating." Faults are stuck in place by very high forces. Once faults start sliding, if they slide fast enough, they become extra slippery at microscopic contact points, like skaters on ice. Heat is generated, but the result is a zero-friction, high-temperature cushiony flash of light or superheated gas called plasma that yields no detectable melted material, Heaton said. When the faults slow down, they stick tight again.

Another idea is that pressurized water in the rocks during a slip could decrease the stress on the fault and therefore the friction, Goldsby said. Faults might ride on a cushion of steam, allowing the fault to slide at low friction and the rock heat would not reach the melting point.

Ripple in the rug
The key to understanding earthquakes is actually not where they start but how the fracture spreads, and that has a lot to do with the weird behavior of underground friction, Heaton says.

The highest sliding velocities happen at the leading edge of a pulse of slip that runs through the Earth like a ripple in a rug, says Heaton, who described this fault behavior in a landmark paper 17 years ago.

Think of a fault as a rug that you want to move, he said. You can just pull the rug from the edge. That's the hard way to move it. The easy way to move a rug is to "put a little bump in it and move the bump and when you're done, you've offset the rug," he explained.

Friction is in a yin-yang arrangement with those slip-pulses, it turns out, Heaton said. "The slip in the pulse depends on the friction, but it turns out the friction turns on how fast the slip is happening," he said. "That's a math problem, a positive feedback system. They are notoriously unstable."

If you knew how big a pulse would be, you could predict an earthquake's magnitude, but the exotic behavior of friction underground botches all that up, Heaton said.

Still, the revolution in the field of earthquake physics has brought new insights, Goldsby said.

"I am not only hopeful but certain that we will learn even more about how earthquakes occur in the coming decade," he said. "This knowledge will help us understand how to mitigate the damaging effects of earthquakes and help prevent the loss of life, and may someday allow us to detect earthquake precursors."

MORE FROM TECHNOLOGY & MONEY

Technology & Money Section Front

Dad lost his job, so this family lives on the road Living on the road full-time: Is it right for you? Do you need a mini money makeover?   Insider shares shopping secrets   Ford unveils inflatable seat belts   Experts offer financial advice Recession over? Readers don't believe it   How to deal with rising credit card rates   U.K. gets tough on distracted drivers   College dorms get an upgrade Technology & Money Section Front   Add Technology & Money headlines to your news reader: