Locked away in the bowels of the Earth, the metamorphosis of one mineral into another can trigger some of the deepest earthquakes ever detected.
These cryptic tremors, known as deep-focus earthquakes, are a seismic conundrum. They rupture violently at depths greater than 300 kilometers, where intense temperatures and pressures are thought to force the rocks to flow smoothly. Now, experiments suggest that those same hellish conditions could also sometimes transform olivine, the main mineral in Earth’s mantle, into the mineral wadsleyite. This mineral change can destabilize surrounding rockallowing earthquakes at otherwise impossible depths, mineral physicist Tomohiro Ohuchi and colleagues report September 15 in Nature Communications.
“It’s been a real puzzle for many scientists because earthquakes shouldn’t happen more than 300 kilometers away,” says Ohuchi of Ehime University in Matsuyama, Japan.
Deep-focus earthquakes generally occur at subduction zones where tectonic plates made of oceanic crust—rich in olivine—precipitates into the mantle (Serial number: 01/13/21). Since seismic waves from earthquakes lose strength during their long ascent to the surface, they are generally not dangerous. But that doesn’t mean earthquakes aren’t sometimes powerful. In 2013, a magnitude 8.3 deep-focus earthquake struck about 609 kilometers below the Sea of Okhotsk, off the east coast of Russia.
Previous studies suggested that Unstable olivine crystals could trigger deep earthquakes. But those studies either tested for other minerals that were similar in composition to olivine but deform at lower pressures, Ohuchi says, or the experiments didn’t filter the samples enough to form faults.
He and his team decided to put olivine itself to the test. To replicate conditions underground, the researchers heated and squeezed olivine crystals up to nearly 1,100° Celsius and 17 gigapascals. The team then used a mechanical press to further compress the olivine slowly and monitored the deformation.
At 11 to 17 gigapascals and around 800° to 900°C, the olivine recrystallized in thin layers containing new wadsleyite and smaller olivine grains. The researchers also found small faults and recorded bursts of sound waves, indicative of miniature earthquakes. Along subducting tectonic plates, many of these thin layers grow and coalesce to form weak regions in the rock, over which faulting and earthquakes can initiate, the researchers suggest.
“The transformation really wreaks havoc on the [rock’s] mechanical stability,” says geophysicist Pamela Burnley of the University of Nevada, Las Vegas, who was not involved in the research. The findings help confirm that olivine transformations are enabling deep-focus earthquakes, she says.
Next, Ohuchi’s team plans to experiment with olivine at even higher pressures to gain insight into the mineral’s deformation at greater depths.