Deepest earthquake ever detected hits 467 miles under Japan

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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 struck off the coast of Japan’s remote Bonin Islands, was recorded at 7.9 on the Richter scale and as far as 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 tremor 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. 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 size of this shiver shook the earth both 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). Even less, about four percent, hit less than 300 kilometers (186 miles), the depth commonly used as a cutoff 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 the roaring temperatures deep underground, and the rocks act more like putty than hard bits, says Magali Belin, 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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