Deepest earthquake ever detected hits 467 miles under Japan

One spring evening six years ago, our planet began to pound from a series of strange earthquakes, hundreds of miles underground. Most earthquakes strike within a few dozen miles of the surface, but these earthquakes moved at depths where temperatures and pressures increase so severely that rocks tend to bend rather than break.

The first tremor, which hit the coast of Japan’s remote Bonin Islands, was recorded at 7.9 on the Richter scale and up to 680 kilometers (423 miles) underground, making it one of the deepest earthquakes of its size. Then another strange case appeared in the series of aftershocks that followed: a small earthquake that, if confirmed, would be the deepest earthquake ever detected.

The very high-altitude earthquake, recently described in Geophysical Research Letters, is estimated to have struck about 751 kilometers (467 miles) below the surface of our planet known as the lower mantle, where scientists have long believed earthquakes unlikely, if not so. impossible. While there have been hints of earthquakes in the lower mantle before, researchers have struggled to locate them within this layer of Earth.

“This is by far the best evidence for an earthquake in the lower mantle,” says Douglas Wiens, a seismologist specializing in deep earthquakes at Washington University in St. Louis, who was not part of the study team.

Some scientists warn that more research is needed to confirm that the earthquake is real and that it has indeed hit the lower mantle. While the boundaries lie on average 660 kilometers (410 miles) underground, they can vary around the world. Under Japan, the lower mantle is believed to begin about 700 kilometers (435 mi) away. The team detected several aftershocks around this depth – but one particular earthquake persisted much later.

While deep earthquakes don’t cause the same kind of devastation as their shallow counterparts, studying these events can help scientists decipher the mysterious ways our planet is moving away from our feet. Seismic tremors are among the few windows into our planet’s inner workings—and every unexpected event, like an earthquake in the lower mantle, can provide new views of the underworld.

Rare earthquakes in the lower mantle may be possible under certain conditions, says Heidi Houston, a geophysicist and deep seismologist at the University of Southern California who was not part of the study team. “It can’t be ruled out,” she says. “That’s one of the things that makes this interesting and exciting and important to look at.”

gurgle from the depths

The 7.9 magnitude earthquake was strange in itself. The great depth and sheer magnitude of this earthquake shook the earth near and far. Residents in all 47 prefectures of Japan reported feeling the quake, the first in more than 130 years that records were kept.

In contrast, the vast majority of earthquakes are shallow. Of the 56,832 moderate to large earthquakes recorded between 1976 and 2020, only about 18 percent were deeper than 70 kilometers (43 miles). And it struck 300 kilometers (186 miles) less than that, about four percent, the depth commonly used as cutoffs to define “deep earthquakes.”

For nearly a century—since English astronomer and seismologist Herbert Hall Turner discovered the first deep earthquake in 1922—scientists have been baffled as to how such quakes ever occurred.

Near the surface, slow motion battles of tectonic plates build up stress until the ground breaks and moves, triggering tremors from the earthquake. However, deep in the Earth’s interior, high pressures prevent similar tremors from occurring. “Everything is compressed very hard in all directions,” Houston says.

Add to that the roaring temperatures deep underground, and the rocks will act more like putty than hard bits, says Magali Belen, a geodynamic scientist at the University of California, Davis, who was not part of the new study. I demonstrated it during a video interview using a bubble gum pink block of silly paste. As you slowly draw it, it stretches and flows into the ruby ​​threads. But if it deforms quickly, “then it breaks,” says Belin. It rushes quickly on the pink dot, and with a faint crackle, it splits in two

“What makes that happen?” Bellin asks.

To explore this question, University of Arizona seismologist Eric Kayser and colleagues took a closer look at the large earthquake beneath the Bonin Islands, which has illuminated seismometers around the world, including Japan’s dense network known as Hi-Net.

The team examined the Hi-Net data set for tremors following the large earthquake. Such a large event would send feedback energy through the Earth’s interior, which could obscure the earthquake’s small aftershocks. To amplify small signals amid all the noise, the researchers used a method known as back-projection, which allows them to stack data from multiple seismographs. Sure enough, four aftershocks deepened between 695 and 715 kilometers, and another occurred far from the group: an earthquake 751 kilometers underground.

mysterious origins

All deep earthquakes strike near recent or ancient subduction zones, where the collision of tectonic plates results in one plate under the other. It is possible that changes in the sunken plates as they dive deep underground will push the vibrations far below the surface.

But scientists are still not sure how the stresses build up large enough to trigger an earthquake deep within the Earth. One popular idea involves the same phenomenon that divides the mantle into layers.

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The upper mantle is full of bright green olivine, but at greater depths, the crystalline structure of the mineral is no longer stable. Starting at about 410 kilometers (255 miles), the atoms can rearrange into the wadslite or ringwoodite minerals, which become more common with depth. The shift of olivine within the slab can create weak points in the rock where it can rapidly deform, creating a deep earthquake.

But about 660 kilometers (410 miles) away, the system changes abruptly. The dance of seismic waves around this boundary indicates that the rocks below are denser than those above – the beginning of the lower mantle.

In this layer, the earthy-colored mineral pergamanite dominates, and the seismic transitions of olivine that could occur above no longer occur. So if an earthquake hit this layer of the planet, something else must have caused it.

One possibility is to transform a different mineral within the sinking slab, such as the dark brown mineral enstatite. But Kiser and his colleagues also discovered another potential catalyst in the plate’s movements.

The small aftershocks that followed the 7.9 magnitude earthquake appear to have occurred near the base of a torn slab of the sloping Pacific floor that pierced the upper part of the lower mantle. The team suggests that the large earthquake may have set part of the worn-out slab a little bit — “we’re talking very, very slightly,” Kaiser says. This small shift may be enough to focus stresses at the base of the plate as it sinks into the denser lower mantle rock.

One of the ways such an increase in stress can cause a deep earthquake is to deform the rocks slightly, which can generate heat and weaken it. The process could have set off a feedback loop, causing the rocks to deform faster and faster as it gets hotter and weaker until the blocks shift quickly in an earthquake. Bellin says the building’s heat may have generated a melt that served as a slip lubricant.

Further analysis and modeling of sunken plate structures and aftershock locations of the 7.9 magnitude earthquake can help decipher the mechanism of not only this event, but other deep earthquakes. “It probably cannot be explained by a single mechanism,” says Hai Jiang Zhang, a seismologist at the University of Science and Technology of China (USTC) who was not part of the study team.

bouncing energy

Zhang, for example, “was not surprised” that an aftershock might hit the lower mantle. In his earlier work, Zhang and colleagues have seen hints that the 7.9-magnitude earthquake near the Bonin Islands, at a depth of 680 kilometers (422 miles), may have also struck within this layer of Earth.

But a low-mantle earthquake would upend long-term thinking about the inner workings of our planet — and not everyone is convinced by the new study’s claims. In some cases, methods used to amplify the signals of an earthquake like this can “issue false alarms,” ​​University of Houston seismologist Yingcai Zheng wrote by email. “Evil is in the details.”

In the study, the team noted that a false alarm can occur, for example, if waves from a different earthquake bounce off Earth’s internal structures, and are then picked up by the seismic array. But John Vidal, a seismologist at the University of Southern California, says the seismic signals appear to come from an actual earthquake at least at the depth the study authors suggest. “It seems unmistakable to me,” he says.

Additional confirmation of the event could come from searching for another type of seismic wave, known as shear or S wave. The new study identifies earthquakes using pressure waves, or P waves, that travel quickly through the Earth like earthquakes that are pulled back and forth. S waves move slower and shake the ground side to side or up and down. If the S and P waves arrive at the expected time based on the location the team believes this highly dangerous earthquake has occurred, Vidale says, “that kind of spikes.”

However, Vidale points out that even if the depth can be confirmed, the lower mantle boundary at this location is still an open question. Seismic imaging suggests that when the plate sinks into the dense rock of the lower mantle, it begins to flex back and forth “like wet pasta,” Houston says. Accumulation of cold seafloor rock could cool the surrounding rock, push the lower mantle boundary to greater depths, and make the interpretation of the system more complex, she says.

Studying the Earth’s interior is never easy. “We can’t go down there,” Houston says. “We only see what the earthquake waves show us.” But as techniques for identifying and studying earthquakes continue to improve, scientists have a better chance of unraveling the mysterious rumble at depth — an effort that is sure to reveal new surprises about our quivering planet.

“The data, it always makes us think about the Earth differently,” says Belen.

Editor’s note: The scientific journal in which the new study was published has been corrected to Geophysical Research Letters.

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