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MELISSA BLOCK, HOST:

They're calling it a rogue earthquake. Last month, one of the 10 biggest quakes ever recorded struck off the coast of Indonesia. It was felt from Bangladesh to Australia. But here in the U.S., the 8.6 magnitude quake barely made the news. That's because it did very little damage. Two people died, but there was no massive tsunami.

NPR's Richard Harris says surprising details of this quake explain why it wasn't catastrophic.

RICHARD HARRIS, BYLINE: To understand why this quake was so unusual, you need to know this: There are two kinds of major quakes in the world. One type happens when two plates of the Earth's crust slide past one another horizontally. That's what happens along California's San Andreas fault. The other kind of major quake occurs when two plates collide and one slips beneath the other with a jolt.

GREG BEROZA: We expect that the largest earthquakes occur in that kind of a setting, called a megathrust.

HARRIS: Greg Beroza at Stanford says every quake this huge in the modern record has been a megathrust quake. Or that was true until last month. The 8.6 magnitude quake off of Indonesia on April 11th was actually the side-slipping kind, called a strike slip. Fortunately, that kind of quake is much less likely to cause a tsunami because sideways motion doesn't generate big waves the way up and down motion does. That's why it caused relatively little damage.

But how could this sideways-moving quake be so huge? Beroza says the rupture was very deep, with an average depth of 30 miles. At that depth, the rock is very hot. And Beroza says the friction from a rupture would make it even hotter.

BEROZA: When slip starts to occur, it generates heat and that heat weakens the rock and in turn, allows more slip. It's called a thermal runaway

HARRIS: If that thermal runaway happened here, it could explain how this sideways moving quake got to be so big.

Beroza and a colleague write in Science magazine about another surprise. It was 60 miles away from the fault zone where two tectonic plates meet.

BEROZA: What made this earthquake so unusual is that it was not on a plate boundary, at least not on a commonly recognized plate boundary, and it was huge.

HARRIS: Beroza says that makes it the biggest quake ever recorded that was not on a fault. It dwarfed the best known example of quakes like that: the 1811 and 1812 quakes centered near New Madrid, Missouri.

Beroza suspects the quake is part of a gradual process that is making new faults and redefining the boundary between plates.

BEROZA: You have to start them somewhere.

HARRIS: That's debatable. Kerry Sieh, who runs the Earth Observatory of Singapore, suspects the quake was actually on an existing fault, just not an obvious one. He was jolted to attention on April 11th because he's been expecting another quake, akin to the 2004 mega-thrust quake off Indonesia that generated the devastating Christmastime tsunami. This wasn't the quake he was expecting.

But Sieh says it built up even more pressure off the coast of Indonesia. So he says it's primed the system for another dangerous tsunami in the years or decades to come.

Richard Harris, NPR News.

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