World's deepest earthquake beneath Japan

If confirmed, tremors at a depth of 751km could surprise geologists, who think the mantle is virtually earthquake-proof.

On a spring evening six years ago, hundreds of kilometers underground began to vibrate due to a series of strange earthquakes. Most earthquakes on Earth occur within a few tens of kilometers below the planet's surface, but these earthquakes occur at depths where the temperature and pressure are so great that rocks bend instead of cracking. broken. The first earthquake was recorded off the remote Bonin Islands of Japan with a magnitude of 7.9 and located at a depth of 680 km. This was followed by the deepest small earthquake ever detected.

The quake struck hundreds of kilometers below the Bonin Islands. (Photo: Alamy)

The super-deep quake, recently described in the journal Geophysical Research Letters, is estimated to have occurred at a depth of 751km below ground level in the mantle. Researchers have long thought earthquakes are unlikely or unlikely there. Although there is some evidence of earthquakes in the middle mantle before, researchers are still having difficulty pinpointing their locations. "This is the best evidence of a mid-mantle earthquake ever," said Douglas Wiens, a seismologist who studies deep earthquakes at Washington University in St. Louis said.

Some scientists say more research is needed to confirm this quake is real and occurred in the mantle. Although the average boundary of the mantle is 660 km below ground level, there are many variations across the globe. Below Japan, the middle mantle is thought to begin at a depth of 700 km. The team has detected several aftershocks around this depth.

While deep quakes don't cause as much damage as shallow earthquakes, studying these events could help scientists identify movements below the ground, revealing the Earth's interior. Mid-mantle earthquakes are rare, and can occur under specific conditions, said Heidi Houston, a geophysicist and expert on deep earthquakes at the University of Southern California.

The 7.9 magnitude earthquake itself is very strange. People in all 47 prefectures of Japan reported feeling the earthquake for the first time in more than 130 years. The vast majority of earthquakes occur quite shallowly. Of the 56,832 moderate to large earthquakes recorded between 1976 and 2020, only 18% occurred at depths greater than 70km. Only 4% are concentrated at depths below 300km.

For nearly a century, since British astronomer and seismologist Herbert Hall Turner discovered the first deep quake in 1922, researchers have wondered how such earthquakes occur. Near the ground, the slow motion of the tectonic plates builds up pressure until the ground cracks and shifts, generating vibrations. However, deep inside the Earth, high pressure blocks similar vibrations. Combined with extreme temperatures, rock behaves more like mortar than solid mass, says Magali Billen, a geodynamicist at the University of California, Davis.

To explore this issue, seismologist Eric Kiser of the University of Arizona and his colleagues took a closer look at the massive earthquake beneath the Bonin Islands. Seismometers around the world, including Japan's Hi-Net network , recorded the earthquake. The team examined Hi-Net data to look for tremors after the earthquake. Such a large event would send energy successively through the near-surface soil, overpowering small aftershocks. To amplify the small signal amid all the noise, the researchers use a method called back-projection , which allows them to superimpose data from multiple seismometers. They detected four aftershocks at a depth of 695-715 km and one earthquake 751 km from the planet's surface.

All deep earthquakes occur near modern or ancient subduction zones, where tectonic plates collide causing one plate to sink under another. Changes in subducted tectonic plates as they plunge to great depths are more likely to drive vibrations deep below the ground. But scientists are still uncertain about how pressure builds up high enough to cause deep earthquakes. The answer may be the phenomenon that causes the coating to layer.

The upper mantle is filled with the sparkling green mineral olivine. But at great depths, the crystal structure of the mineral is no longer stable. Starting at 410 km, the atoms can rearrange into the minerals wadsleyite or ringwoodite. Modification of oblivin within the mantle can create many weak spots in rocks. They can quickly deform, causing deep earthquakes.

But from a depth of 660km, the system changes markedly. Seismic waves around this boundary show that the rock below is much denser than the one above. Here, the earth-colored bridgmanite mineral dominates. The earthquake-inducing transformation of oblivin is no longer as ongoing as in the upper layer. So earthquakes in this layer must have been caused by something else.

One possibility is the alteration of another mineral within the submerged tectonic plate such as the reddish-brown mineral enstatite. But Kiser and his colleagues discovered another possible cause from the shifting of the tectonic plate. Small aftershocks after the 7.9 magnitude earthquake occurred near the base of the tectonic plate at the bottom of the Pacific Ocean. The team thinks that large earthquakes can cause part of the tectonic plate to shift. That very small displacement is enough to concentrate pressure at the base of the tectonic plate as it sinks deeper into the denser middle mantle. Scientists will need to further analyze and model the subducted plate structure and the location of the aftershocks of the 7.9 magnitude quake to determine the mechanism of the deep quake.