Here’s how oily earthquakes can cause deep earthquakes

Cocooned within Earth’s bowels, the transformation of one mineral into another may trigger some of the deepest earthquakes ever detected.

These subtle tremors – known as high-focal earthquakes – are a seismic mystery. They rupture violently at depths of more than 300 km, where extreme temperatures and pressures are believed to force the rocks to flow smoothly. Now, experiments indicate that these same infernal conditions can sometimes turn olivine – the primary mineral in the Earth’s mantle – into a wadislite mineral. This mineral shift can destabilize surrounding rock, causing earthquakes of impossible depths, mineral physicist Tomohiro Ohuchi and colleagues report Sept. 15 in Nature Communications.

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“It has been a real mystery to many scientists because earthquakes shouldn’t occur at depths greater than 300 kilometres,” says Uchi, of Ehime University in Matsuyama, Japan.

Deep-focus earthquakes typically occur in subduction zones where tectonic plates made of ocean crust – rich in olivine – sink into the mantle (SN: 1/13/21). Since seismic waves of earthquakes lose strength during their long ascent to the surface, they are usually not dangerous. But this does not mean that earthquakes are sometimes not strong. In 2013, an earthquake measuring 8.3 on the Richter scale struck a depth of about 609 kilometers under the Sea of ​​Okhotsk, off the eastern coast of Russia.

Previous studies have hinted that unstable olivine crystals can spawn deep earthquakes. But these studies tested other minerals that were similar in composition to olivine but deformed at lower pressures, Occhi says, or that the experiments did not stress the samples enough to form the defects.

He and his team decided to put olivine itself to the test. To replicate conditions deep in the Earth, the researchers heated and compressed olivine crystals to approximately 1,100 degrees Celsius and 17 gigapascals. Then the team used a mechanical press to slowly press the olivine and watched for the deformation.

From 11 to 17 GPa and about 800 ° to 900 ° C, the olivine was recrystallized into thin layers containing new wadslite and smaller olivine grains. The researchers also found small flaws and recorded bursts of sound waves – indicative of mini-quakes. Along the subducting tectonic plates, the researchers suggest that many of these thinner layers grow and connect to form weak regions in the rock, in which faults and earthquakes can begin.

“The transformation is really wreaking havoc on [rock’s] “Mechanical stability,” says geophysicist Pamela Burnley of the University of Nevada, Las Vegas, who was not involved in the research. The results help confirm that olivine shifts enable deep-focus earthquakes, she says.

Next, Ohuchi’s team plans to experiment with olivine at higher pressures to gain insight into the mineral’s deformation at greater depths.

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