What happens inside an earthquake?
'There are many reasons to believe that something exotic is happening'
 Robert E. Wallace, USGS Aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo. |
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Greatest Mysteries |
Several scientists from various fields were asked what they thought were the greatest mysteries today, and then LiveScience added a few, too. This article is part of LiveScience's "Greatest Mysteries" series: |
When a sizable earthquake strikes, experts can explain exactly where it started and what type of fault is involved and maybe even predict how long aftershocks will last. But the strange truth is that seismologists and geophysicists are quite unsure of what happens inside the planet during a quake.
Earthquake physics has undergone a revolution during the past decade, thanks to new insights from lab experiments, field studies of exhumed faults and better theories.
But the nature and behavior of the forces that keep faults from moving and then suddenly fail are still unknown.
And when faults do move, something is missing — there is little to no evidence of the extremely high levels of friction and melting that would be expected to follow above ground when two giant rocks slid against each other.
"There are many reasons to believe that something exotic is happening," said Caltech geophysicist Tom Heaton.
"The problem of frictional sliding in earthquakes is one of the most fundamental problems in all of Earth science," Heaton said. "It has been a 30-year mystery story of figuring out the basic physics of the earthquake problem."
Gentle earthquakes
Most earthquakes happen where tectonic plates meet and glide against each other. Quakes occur when the frictional stress of the movement exceeds the strength of the rocks, causing a failure at a fault line. Violent displacement of the Earth's crust follows, leading to a release of elastic strain energy. This energy takes the form of shock waves that radiate and constitute an earthquake.
One of the strangest things about earthquakes is how gentle they are, Heaton said.
For instance, some scientists thought they had figured out how to simulate mini-earthquakes in the lab. But when they scale up the energies observed in the lab to the size of real faults, the model would predict extensive melting on faults. And such models predict devastation far beyond what killed more than 500 people this week in Peru, more than 80,000 people in the 2005 Pakistan quake or more than a quarter of a million people in the 2004 earthquake off the coast of Sumatra.
"Earthquakes would be so violent that no living thing could survive the shaking," Heaton said.
Therefore, no one has actually simulated anything close to a real earthquake yet.
A machine design problem
The simulation problem lies partly in the fact that it's very difficult to make lab machines generate all the environmental conditions that occur miles below ground during an earthquake — including high stress, high pressure, elevated temperatures and a slip rate of about a yard per second (about the pace at which we walk).
David Goldsby and his rock mechanics colleagues at Brown University have designed machines that can apply the high stresses of temblors to rock specimens so the geophysicists can study friction at depth.
"We can apply normal stresses as high as occur throughout the entire seismogenic zone of the Earth's crust, about 10 kilometers [6 miles] in depth," he said.
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That's incredibly impressive and important for earthquake science, but it still leaves a lot of questions unanswered, because what happens inside the Earth is so strange in magnitude and physics.
"No apparatus in the world is yet capable of meeting all of these criteria," Goldsby said.
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