Real culprit behind deep earthquakes found

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The mineral lawsonite undergoes brittle failure at high temperature and pressure, as evidenced by the cracks seen in the sample above. That brittleness could trigger earthquakes in subduction zones where lawsonite is present. – Hirth Lab / Brown University

Geologists may have uncovered what triggers earthquakes deep below the Earth’s surface – and in the process, explained why those quakes are common in some parts of the world but not others.

Keishi Okazako and Greg Hirth, from Brown University, recreated the crushing conditions of the deep Earth – 70 to 300 kilometres beneath the surface – in a lab for the first time.

They found as they wrung water from a mineral called lawsonite, it became brittle and cracked.

And while they were at it, Okazako and Hirth showed antigorite – a mineral long thought to cause the earthquakes when it cracked – was not to blame. 

“That’s one of the cool things about this,” Hirth said. “For 50 years everyone has assumed this is a process related to antigorite, despite the fact that there wasn’t much evidence for it.”

The most destructive earthquakes occur when a chunk of Earth’s crust suddenly slips under another, such as the one that devastated Japan in March 2011. But the same slipping movements can also happen deeper. These are called “intermediate depth earthquakes”.

Exactly how these deeper slips occur has been a geological mystery. The pressures within the Earth at depths of around 100 kilometres should be too great to allow the plates to slide fast enough to shake buildings on the surface.

So Okazako and Hirth decided to see how various minerals acted in deep Earth conditions, using what’s called a Grigg’s apparatus.

Pieces of lawsonite and antigorite were heated inside a chamber until the water trapped inside the mineral became unstable. Then the pressure was cranked up.

Lawsonite suddenly cracked – which, should it happen below the Earth, could trigger intermediate depth earthquakes. Antigorite, on the other hand, didn’t break abruptly – rather, it slowly squished. The results were published in Nature.

Should lawsonite be the culprit behind intermediate depth earthquakes, it would explain why such earthquakes are common in some areas and not others.

Lawsonite forms under high pressure but low temperature, and areas with old, cold crust sliding under another plate under high pressure – such as northwest Japan – tend to get more intermediate depth earthquakes than those with new, hot crust.

Hirth hopes the work might give geologists the tools they need to start forecasting earthquakes of all types, but admits there’s a fair way to go.

“We don’t really understand a lot of the earthquake cycle,” he said.

“Predictability is the ultimate goal, but we’re still at the stage of thinking about what’s the recipe for different kinds of earthquakes. This appears to be one of those recipes.”

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