The earthquake occurred in the lower mantle, much deeper than previous earthquakes.

Scientists detected the deepest earthquake in history, which occurred 467 miles (751 kilometers) below the surface.

This depth makes earthquakes occur in the lower mantle, and seismologists predict that earthquakes are unlikely to occur. This is because under extreme pressure, the rock is more likely to bend and deform, rather than bursting by the sudden release of energy. But Pamela Burnley, a professor of geological materials at the University of Nevada, Las Vegas, who was not involved in the study, said that the behavior of minerals is not always as accurate as expected. Even under the pressure that they should be transformed into a different, less earthquake-prone state, they may stay in the old configuration.

"Just because they should change doesn't mean they will change," Burnley told Live Science. So what the earthquake may reveal is that the boundaries of the earth's interior are much blurred than people usually think.

The earthquake was first published in the journal Geophysical Research Letters in June. It was a minor aftershock of the magnitude 7.9 earthquake that occurred in the Bonin Islands near the Japanese mainland in 2015. Researchers led by the University of Arizona seismologist Eric Kizer use Japan's Hi-Seismic Network Array. The University of Southern California seismologist John Vidale, who was not involved in the study, said that the array is the most powerful seismic detection system currently in use. The earthquake is small and cannot be felt on the surface, so sensitive instruments are needed to find it. 

Vidale told Live Science that the depth of the quake still needs to be confirmed by other researchers, but this finding looks reliable. "They did a good job, so I tend to think it might be right," Vidal said.

This makes the earthquake a bit of a headache. Most earthquakes are shallow earthquakes, originating in the crust and upper mantle within the first 62 miles (100 kilometers) below the surface. In the crust that extends down only about 12 miles (20 kilometers) on average, the rocks are cold and brittle. Burnley said that when these rocks are under pressure, they can only bend a little before breaking, releasing energy like spiral springs. Deep in the crust and lower mantle, rocks have higher temperatures and higher pressures, which makes them less likely to break. But at this depth, when high pressure pushes the fluid-filled pores in the rock, forcing the fluid to flow out, an earthquake will occur. Burnley said that under these conditions, rocks are also prone to brittle fracture.

These types of dynamics can explain earthquakes up to 249 miles (400 kilometers) deep, which are still in the upper mantle. But even before the Bonin aftershock in 2015, earthquakes were observed in the lower mantle at a distance of approximately 420 miles (670 kilometers). Burnley said these earthquakes have long been a mystery. The water-containing pores in the rock have been squeezed and closed, so fluid is no longer the trigger.

"At that depth, we believe that all water should be driven away, and we must be far, far away from where we will see the typical brittle behavior," she said. "This has always been a problem."

The problem of earthquakes deeper than 249 miles is related to the way minerals behave under pressure. Most of the planet’s mantle is made of a mineral called olivine, which is shiny and green. About 249 miles down, the pressure caused the atoms of the olivine to rearrange into a different structure, a blue mineral called wadsleyite. Deeper 62 miles (100 kilometers), the wadsleyite is rearranged into magnesite. Finally, about 423 miles (680 kilometers) deep in the mantle, magnesite decomposed into two minerals, Bridgmanite and periclase. Of course, geoscientists cannot directly probe the distance of the earth, but they can use laboratory equipment to reproduce extreme pressures and produce these changes on the surface. Because seismic waves move differently in different mineral phases, geophysicists can see the signs of these changes by observing the vibrations caused by large earthquakes. 

The last transition marks the end of the upper mantle and the beginning of the lower mantle. The important thing about these mineral phases is not their name, but the behavior of each mineral phase is different. Burnley said it is similar to graphite and diamond. Both are made of carbon, but they are arranged differently. Graphite is a stable form on the surface of the earth, while diamond is a stable form deep in the mantle. The performance of the two is very different: graphite is soft, gray and smooth, while diamonds are very hard and transparent. As the olivine transitions to a higher pressure stage, it becomes more likely to bend and less likely to break in a way that triggers an earthquake. 

Until the 1980s, geologists were puzzled by earthquakes in the upper mantle, but still could not agree on the cause of the earthquake. Burnley and her doctoral supervisor, mineralogist Harry Green, offered a possible explanation. In experiments in the 1980s, the two found that the mineral phase of olivine was not so clean. For example, in some cases, olivine can skip the wadsley stage and go directly to the magnesite stage. At the transition stage from olivine to magnesite, under sufficient pressure, this mineral may actually fracture rather than bend.

"If my sample didn't change, it wouldn't break," Burnley said. "But the moment I transform and squeeze it at the same time, it will break."

Burnley and Green reported their findings in Nature in 1989, showing that this kind of pressure in the transition zone can explain earthquakes under 249 miles. 

However, the new Bonin earthquake is deeper than this transition zone. At 467 miles down, it originated in a place that should be located just in the lower mantle.

Heidi Houston, a geophysicist at the University of Southern California, said that one possibility is that the boundary between the upper and lower mantle is not exactly the Bonin area that seismologists expected. The area near Bonin Island is a subduction zone, in which a piece of oceanic crust is diving under a piece of continental crust. This kind of thing tends to produce a warping effect.

"This is a complicated place, and we don't know where the boundary between the upper and lower mantles is," Houston told Live Science.

The author of the paper believes that the crustal subduction slab may have basically been firmly fixed to the lower mantle, enough to put the rocks there under tremendous pressure, generate enough heat and pressure, and cause very unusual fractures. However, Burnley suspects that the most likely explanation is related to the poor performance of the mineral — or at least strange. She said that the continental crust leaning toward the center of the earth is much colder than the surrounding material, which means that the minerals in the area may not be warm enough to complete the phase change they should have under a given pressure.

Burnley said that diamonds and graphite are also a good example. Diamonds are unstable on the surface of the earth, which means they do not form spontaneously, but when you insert them into an engagement ring, they do not degrade into graphite. That's because carbon atoms require a certain amount of energy to rearrange, and this energy is not available at the earth's surface temperature. (Unless someone irradiates the diamond with an X-ray laser.)  

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Burnley said that something similar might happen in the depths of the peridot. This mineral may transform into a non-brittle phase under sufficient pressure, but if it is too cold—for example, because there is a large cold continental crust around it—it may remain olivine. This may explain why the earthquake may have originated in the lower crust: it is just not as hot as scientists expected.

"My general thinking is that if the material is cold enough to generate enough pressure to release it suddenly in an earthquake, then it is also cold enough that the olivine gets stuck in its olivine structure," Burnley said.

Houston said that no matter what the cause of the earthquake, it is unlikely to recur frequently. Only about half of the subduction zones in the world will have deep earthquakes, and the type of large earthquakes before this ultra-deep earthquake will occur on average every two to five years.

"This is a very rare situation," she said. 

Originally published on Live Science.

Stephanie Pappas is a writer for Live Science, covering topics from earth sciences to archaeology to human brains and behavior. She is a freelancer in Denver, Colorado, and also regularly contributes to the monthly magazines Scientific American and Monitor of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz. 

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