Scientists Say The Deepest Earthquake Ever Detected Should’ve Been ‘Impossible’ Which Can Only Mean One Thing

BY CONNOR TOOLENOVEMBER 9, 2021

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• Scientists are baffled by an earthquake that set a record for the deepest seismic disturbance ever recorded
• Experts say the earthquake—which was detected off of Japan—was theoretically impossible based on widely-accepted research

It’s been almost two years since most of us got a crash course in what it’s like to be in a movie that kicks off with the bulk of the world opting to ignore murmurings concerning a mysterious airborne disease responsible for sparking a health crisis in the Chinese city that was placed on lockdown in the hopes of stopping the spread (a strategy that—as we know all too well by now—didn’t exactly pan out as hoped).

Based on what we learned from that situation, I feel like it no longer hurts to err on the side of caution in the hopes we aren’t doomed to repeat a similarly catastrophic scenario—which is why I feel like we might want to pay attention to a new report from Live Science concerning an earthquake that occurred off the coast of Japan in 2015.

In June of that year, researchers who were monitoring a 7.9-magnitude seismic event that hit the Bonin Islands detected a minor aftershock that originated 467 miles below the surface of the planet—a depth that set a new record for the deepest earthquake ever observed. While it may not seem like an incredibly newsworthy development, it came as a bit of a surprise to experts who say the disturbance was theoretically impossible based on everything that was previously known about the conditions required for such a disturbance to occur

I’d recommend checking out the aforementioned article if you’re looking for an in-depth explanation of their reasoning, which centers around the long-held belief that the nature of rocks located in Earth’s lower mantle (where the earthquake originated) makes them impervious to the effects of water that triggers the quakes that occur closer to the surface by leaking into more porous objects and weakening their structure.

As things currently stand, there are a couple of operating theories. One is that researchers incorrectly estimated the depth of the boundary between the upper and lower mantle, while the other rests upon the assumption that certain minerals found at the depth were exposed to unexpected conditions that made them susceptible to cracking.

Of course, there’s also the possibility that the initial earthquake roused one of the Great Old Ones from their eternal slumber and it’s only a matter of time until a vast, indescribable horror beyond the comprehension of all but those unfortunate enough to witness its unbridled wrath and might rises from the depths of the Pacific Ocean, but I guess we’ll just have to wait and see.

The Deepest Earthquake Ever Recorded Happened 467 Miles Underground, Surprising Scientists

Because of intense heat and pressure, quakes are rare beyond 186 miles deep beneath Earth’s crust

Rasha Aridi

SMITHSONIAN
Daily Correspondent

November 8, 2021

In 2015, a 7.9 magnitude earthquake struck beneath Japan's Bonin Islands.

 Lee Render via Flickr

Between 1976 and 2020, nearly 57,000 earthquakes rattled our planet. The bulk of them were shallow, and only a mere four percent occurred beyond 186 miles deep, which was thought to be the maximum depth for what scientists call "deep earthquakes," reports Maya Wei-Haas for National Geographic.

Now, a team of researchers has zeroed in on what could be the deepest earthquake ever detected, shaking up scientists' understanding of them. In 2015, a 7.9 magnitude earthquake struck beneath Japan's Bonin Islands. One of the aftershocks occurred deeper than the original earthquake itself, at 467 miles. It's so deep that it nears the layer of Earth known as the lower mantle, reports Andrei Ionescu for Earth.com.

"This is by far the best evidence for an earthquake in the lower mantle," Douglas Wiens, a seismologist at Washington University in St. Louis who was not involved in the study, tells National Geographic.

The study, published in the journal Geophysical Research Letters, used measurements collected by the High Sensitivity Seismograph Network, a string of stations across Japan that record seismic data. They were able to trace the origin of the seismic waves produced by the 7.9 magnitude earthquake and its aftershocks, according to a press release.

But what puzzled this team is that the shock erupted in the lower mantle, closer to Earth's core. There, temperatures can exceed 6,000 degrees Fahrenheit and the pressure is 1.3 million times the atmospheric pressure.

Deep earthquakes occur at subduction zones, where two tectonic plates collide and one is forced below the other, sending shockwaves through the Earth, National Geographic reports. But in such intense elements, rock tends to bend instead of break, begging the question: How did this earthquake even happen?

The researchers introduced a few possibilities. First, the molecular structure of minerals becomes unstable as pressure increases further into the mantle. That deformation could leave weak spots in the rock, causing earthquakes. Another theory is that the larger earthquake caused a torn slab of the seafloor to shift, and even a miniscule shift is enough to cause an earthquake, reports National Geographic.

This discovery throws a wrench in what geologists thought they knew about earthquakes in the lower mantle. They were surprised that one could occur so deep in the Earth, raising questions about the mechanisms at play beneath our feet.

Deepest earthquake ever detected should have been impossible


By Stephanie Pappas 

The quake occurred in the lower mantle, well deeper than previous quakes.

The Bonin Islands are part of a geologic arc called Izu-Bonin-Mariana Arc. The arc sits above the subduction zone, where the Pacific plate is slowly diving beneath the Philippine Sea Plate. (Image credit: pianoman555 via Getty Images)


Scientists have detected the deepest earthquake ever, a staggering 467 miles (751 kilometers) below the Earth's surface.

That depth puts the quake in the lower mantle, where seismologists expected earthquakes to be impossible. That's because under extreme pressures, rocks are more likely to bend and deform than they are to break with a sudden release of energy. But minerals don't always behave precisely as expected, said Pamela Burnley, a professor of geomaterials at the University of Nevada, Las Vegas, who was not involved in the research. Even at pressures where they should transform into different, less quake-prone states, they may linger in old configurations.

"Just because they ought to change doesn't mean they will," Burnley told Live Science. What the earthquake may reveal, then, is that the boundaries within Earth are fuzzier than they're often given credit for.

Crossing the boundary

The quake, first reported in June in the journal Geophysical Research Letters, was a minor aftershock to a 7.9-magnitude quake that shook the Bonin Islands off mainland Japan in 2015. Researchers led by University of Arizona seismologist Eric Kiser detected the quake using Japan's Hi-net array of seismic stations. The array is the most powerful system for detecting earthquakes in current use, said John Vidale, a seismologist at the University of Southern California who was not involved in the study. The quake was small and couldn't be felt at the surface, so sensitive instruments were needed to find it.

The depth of the earthquake still needs to be confirmed by other researchers, Vidale told Live Science, but the finding looks reliable. "They did a good job, so I tend to think it's probably right," Vidale said.

The deepest earthquake ever, which occurred off Japan in 2015, reached into Earth's lower mantle.

This makes the quake something of a head-scratcher. The vast majority of earthquakes are shallow, originating within the Earth's crust and upper mantle within the first 62 miles (100 km) under the surface. In the crust, which extends down only about 12 miles (20 km) on average, the rocks are cold and brittle. When these rocks undergo stress, Burnley said, they can only bend a little before breaking, releasing energy like a coiled spring. Deeper in the crust and lower mantle, the rocks are hotter and under higher pressures, which makes them less prone to break. But at this depth, earthquakes can happen when high pressures push on fluid-filled pores in the rocks, forcing the fluids out. Under these conditions, rocks are also prone to brittle breakage, Burnley said.

These kinds of dynamics can explain quakes as far down as 249 miles (400 km), which is still in the upper mantle. But even before the 2015 Bonin aftershock, quakes have been observed in the lower mantle, down to about 420 miles (670 km). Those quakes have long been mysterious, Burnley said. The pores in the rocks that hold water have been squeezed shut, so fluids are no longer a trigger.

"At that depth, we think all of the water should be driven off, and we're definitely far, far away from where we would see classic brittle behavior," she said. "This has always been a dilemma."

Changing minerals

The problem with earthquakes deeper than around 249 miles has to do with the ways the minerals behave under pressure. Much of the planet's mantle is made up of a mineral called olivine, which is shiny and green. Around 249 miles down, the pressures caused olivine's atoms to rearrange into a different structure, a blue-ish mineral called wadsleyite. Another 62 miles (100 km) deeper, wadsleyite rearranges again into ringwoodite. Finally, around 423 miles (680 km) deep into the mantle, ringwoodite breaks down into two minerals, bridgmanite and periclase. Geoscientists can't probe that far into the Earth directly, of course, but they can use lab equipment to recreate extreme pressures and create these changes at the surface. And because seismic waves move differently through different mineral phases, geophysicists can see signs of these changes by looking at vibrations caused by large earthquakes.

That last transition marks the end of the upper mantle and the beginning of the lower mantle. What's important about these mineral phases is not their names, but that each behaves differently. It's similar to graphite and diamonds, said Burnley. Both are made of carbon, but in different arrangements. Graphite is the form that's stable at Earth's surface, while diamonds are the form that's stable deep in the mantle. And both behave very differently: Graphite is soft, gray and slippery, while diamonds are extremely hard and clear. As olivine transforms into its higher-pressure phrases, it becomes more likely to bend and less likely to break in a way that triggers earthquakes.

Geologists were puzzled by earthquakes in the upper mantle until the 1980s, and still don't all agree on why they occur there. Burnley and her doctoral advisor, mineralogist Harry Green, were the ones to come up with a potential explanation. In experiments in the 1980s, the pair found that olivine mineral phases were not so neat and clean. In some conditions, for example, olivine can skip the wadsleyite phase and head straight to ringwoodite. And right at the transition from olivine to ringwoodite, under enough pressure, the mineral could actually break instead of bending.

"If there was no transformation happening in my sample, it wouldn't break," Burnley said. "But the minute I had transformation happening and I was squishing it at the same time, it would break."

Burnley and Green reported their finding in 1989 in the journal Nature, suggesting that this pressure in the transition zone could explain earthquakes below 249 miles.

Much of Earth's mantle is made up of the mineral olivine. (Image credit: underworld111/Getty Images)

Going deeper

The new Bonin earthquake is deeper than this transition zone, however. At 467 miles down, it originated in a spot that should be squarely in the lower mantle.

One possibility is that the boundary between the upper and lower mantle is just not exactly where seismologists expect it to be in the Bonin region, said Heidi Houston, a geophysicist at the University of Southern California who was not involved in the work. The area off the Bonin island is a subduction zone where a slab of oceanic crust is diving beneath a slab of continental crust. This sort of thing tends to have a warping effect.

"It's a complicated place, we don't know exactly where this boundary between the upper and lower mantle is," Houston told Live Science.

The paper's authors argue that the subducting slab of crust may have essentially settled onto the lower mantle firmly enough to put the rocks there under a tremendous amount of stress, generating enough heat and pressure to cause a very unusual break. Burnley, however, suspects the most likely explanation has to do with minerals behaving badly — or at least oddly. The continental crust that plunges toward the center of the Earth is much cooler than the surrounding materials, she said, and that means that the minerals in the area might not be warm enough to complete the phase changes they are supposed to at a given pressure.

Again, diamonds and graphite are a good example, Burnley said. Diamonds aren't stable at Earth's surface, meaning they wouldn’t form spontaneously, but they don't degrade into graphite when you stick them into engagement rings. That's because there's a certain amount of energy the carbon atoms need to rearrange, and at Earth's surface temperatures, that energy isn't available. (Unless someone zaps the diamond with an X-ray laser.)

Something similar may happen at depth with olivine, Burnley said. The mineral might be under enough pressure to transform into a non-brittle phase, but if it's too cold — say, because of a giant slab of chilly continental crust all around it — it might stay olivine. This could explain why an earthquake could originate in the lower crust: It's just not as hot down there as scientists expect it to be.
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"My general thinking is that if the material is cold enough to build up enough stress to release it suddenly in an earthquake, it's also cold enough for the olivine to have been stuck in its olivine structure," Burnley said.

Whatever the cause of the quake, it's not likely to be repeated often, Houston said. Only about half of subduction zones around the world even experience deep earthquakes, and the kind of large quake that preceded this ultra-deep one only occurs every two to five years, on average.

"This is a pretty darn rare occurrence," she said.


Originally published on Live Science.

Stephanie Pappas Live Science Contributor

Stephanie Pappas is a contributing writer for Live Science covering topics from geoscience to archaeology to the human brain and behavior. A freelancer based in Denver, Colorado, she also regularly contributes to Scientific American and The Monitor, the monthly magazine 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.