Deepest earthquake ever detected struck 467 miles beneath Japan - National Geographic

Most of Earth's temblors strike within a few dozen miles of the surface, but these quakes stirred at depths where temperatures and pressures grow so intense that rocks tend to bend rather than break.

The first jolt, which struck off the coasts of Japan's remote Bonin Islands, was recorded at magnitude 7.9 and up to 680 kilometers (423 miles) underground, making it one of the deepest quakes of its size.

The ultradeep quake, described recently in the journal Geophysical Research Letters, is estimated to have struck some 751 kilometers (467 miles) beneath the surface in the layer of our planet known as the lower mantle, where scientists have long thought earthquakes unlikely, if not impossible.

"This is by far the best evidence for an earthquake in the lower mantle," says Douglas Wiens, a seismologist specializing in deep quakes at Washington University in St.

Some scientists caution that more research is needed to confirm the quake is real and did indeed strike in the lower mantle.

Under Japan, the lower mantle is believed to start about 700 kilometers (435 miles) down.

Rare lower mantle quakes may be possible in particular conditions, says Heidi Houston, a geophysicist and deep quake expert at the University of Southern California who was not part of the study team.

Add in the sizzling temperatures deep underground, and rocks act more like putty than solid chunks, says Magali Billen, a geodynamicist at the University of California, Davis, who was not part of the new study.

Sure enough, four aftershocks rumbled between 695 and 715 kilometers deep, and another stood apart from the pack: a quake 751 kilometers underground.

The dance of seismic waves around this boundary suggests the rocks below are much denser than those above—the beginning of the lower mantle.

The tiny aftershocks following the magnitude 7.9 quake seem to have occurred near the base of a torn slab of subducted Pacific seafloor that pierced the top of the lower mantle.

That small shift might have been enough to concentrate stresses at the base of the slab as it plunged into the denser lower mantle rocks.

In his past work, Zhang and his colleagues saw hints that the magnitude 7.9 quake near the Bonin Islands, sitting some 680 kilometers (422 miles) deep, may have also struck within this layer of the Earth.

But John Vidale, a seismologist at the University of Southern California, says that the seismic signals appear to come from a true quake at least as deep as the study authors suggest.

Seismic imaging hints that as the slab sinks into the dense rocks of the lower mantle, it starts folding back and forth "like a wet noodle," Houston says.

The accumulation of cold seafloor rocks could cool the surrounding rocks, pushing the lower mantle boundary to greater depths, and making the system much more complex to interpret, she says