Cocooned within the bowels of the Earth, one mineral’s metamorphosis into another may trigger some of the deepest earthquakes ever detected.
These cryptic tremors — known as deep-focus earthquakes — are a seismic conundrum. They rupture at depths exceeding 300 kilometers. Extreme temperatures and pressures can cause rocks to flow freely. Now, experiments suggest that those same hellish conditions might also sometimes transform olivine — the primary mineral in Earth’s mantle — into the mineral wadsleyite. This mineral switch-up is possible Stabilize the rock surrounding youReporting September 15, mineral physicist Tomohiro Ohuchi with colleagues reports that it has enabled earthquakes at previously impossible depths. Nature Communications.
“It’s been a real puzzle for many scientists because earthquakes shouldn’t occur deeper than 300 kilometers,” says Ohuchi, of Ehime University in Matsuyama, Japan.
Subduction zones are often the location of deep-focus earthquakes. Tectonic plates made of oceanic crust — rich in olivine — plunge toward the mantle (SN: 1/13/21). Since the quakes’ seismic waves lose strength during their long ascent to the surface, they aren’t typically dangerous. But that doesn’t mean the quakes aren’t sometimes powerful. In 2013, a magnitude 8.3 deep-focus quake struck around 609 kilometers below the Sea of Okhotsk, just off Russia’s eastern coast.
Studies in the past suggested that Quakes could be triggered by unstable olivine crystallines. But those studies tested other minerals that were similar in composition to olivine but deform at lower pressures, Ohuchi says, or the experiments didn’t strain samples enough to form faults.
His team and he decided to test olivine. To replicate conditions deep underground, the researchers heated and squeezed olivine crystals up to nearly 1100° Celsius and 17 gigapascals. Next, the researchers used a mechanical press and watched the deformation of the olivine.
From 11 to 17 gigapascals and about 800° to 900° C, the olivine recrystallized into thin layers containing new wadsleyite and smaller olivine grains. The researchers also found tiny faults and recorded bursts of sound waves — indicative of miniature earthquakes. Researchers suggest that many of these thin layers can grow along subducting plates and link to form weak areas in the rock. This is where faults and earthquakes may start.
“The transformation really wreaks havoc with the [rock’s] mechanical stability,” says geophysicist Pamela Burnley of the University of Nevada, Las Vegas, who was not involved in the research. These findings prove that olivine transformations can enable deep-focus earthquakes.
Next, Ohuchi’s team plans to experiment on olivine at even higher pressures to gain insights into the mineral’s deformation at greater depths.
