Fact or Friction: Slow Earthquakes

Earthquakes are once again in the news, this time in Mexico. Although it is only the biggest quakes that make international headlines, we might take a minute to contemplate other quakes - the ones you'll never feel. So-called "slow" or "silent" earthquakes slip so softly they don't even show up on regular seismographic equipment.

As the name implies, slow quakes release the energy built up along the fault over hours or even days, as opposed to mere seconds for a fast, shaking quake. So why should we care about what happens in earthquakes that even scientists have barely noticed? For one thing, says the Institute's Dr. Eran Bouchbinder, slow quakes are likely to be a part of the larger seismic picture, possibly releasing stresses from one part of a fault by adding to the stress on another. There is some evidence that certain slow earthquakes might precede the big, fast ones.

Are slow earthquakes really different from fast ones? Bouchbinder thinks that the answer may possibly be: yes. He and his team have developed a new model for sliding friction - friction between two moving plates, for instance - that suggests an explanation for the physics of slow quakes.

Composite satellite image reveals a slow earthquake on the Hayward fault in California (Image: NASA)

Here it is in a tiny nutshell: Earthquakes - of any type - occur when the shear forces pushing the tectonic plates past each other surmount the force of friction that is holding them in place. In a big quake, the frictional interface fails very quickly, releasing huge amounts of energy in a short period of time. The speed of failure is limited by the speed of sound - the very same speed at which waves propagate through the earth's crust, shaking everything above. But the slow quakes seem to be different - the speed of sound may not influence them, says Bouchbinder. The model he and his team propose for movement at frictional interfaces might explain the underlying mechanics without sound wave physics. Instead, it implies that friction and sliding speed have a more complex relationship than previously thought, so that friction might increase at speeds where it was thought to decrease. For more on the model, go to our online article or check out the paper in Geophysical Research Letters.

The new model, by the way, might also help explain the dynamics of other frictional interfaces, e.g. between car tires and roads, or lend insight into nano-mechanics, among other things.