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New research suggests that mysterious zones in the Earth’s deep mantle where earthquake waves travel slowly may be everywhere.

Scientists already knew that ultra low speed zones (ULVZs), hover near hot spots — regions of the mantle where hot rocks rise to form volcanic archipelagos like Hawaii. But mysterious earthquake waves suggest these features could be widespread.

ULVZs, which are located in the lower mantle near the core-mantle boundary, can slow seismic waves by up to 50 percent. That’s remarkable, said Michael Thornegeologist and geophysicist at the University of Utah.

“This is one of the most extreme features we see anywhere on the planet,” Thorne told Live Science. “And we don’t know what they are, where they came from, what they’re made of, (or) what role they play in the Earth.”

Thorne wasn’t thinking about ULVZs when he launched the new research, which was published Aug. 10 in the journal AGU progress. Instead, he was intrigued by another mantle mystery. Very large earthquakes, such as those that occur in subduction zones where one tectonic plate slides under another, releasing powerful waves. Some of these so-called PKP waves travel through the mantle, the liquid outer core, and then back through the mantle on their way to the opposite side of the planet from where they came. These waves are sometimes preceded by another strange type of wave, called a precursor PKP wave.

Precursor PKP waves arrive before the main wave after being scattered by mysterious features in the Earth’s lower mantle. To identify these features, Thorne and his colleagues modeled PKP waves traveling through a computer model of the Earth’s mantle, to which they added regions that changed the speed of the waves. They found predictable patterns in how the PKP waves varied in speed.

So the team set out to look for similar patterns in real earthquake data. The researchers used data from 58 deep earthquakes with magnitudes greater than 5.8 near New Guinea that occurred between 2008 and 2022. Waves from these quakes traveled through the core and up into North America, where they were recorded by EarthScope, a project that deployed portable seismic monitors across the U.S. between 2003 and 2018.

The findings suggested that something was dramatically slowing the earthquake waves to dissipate their energy, Thorne said. The two likely candidates were valleys and ridges along the core-mantle boundary where the waves traveled, or ULVZs. The core-mantle boundary beneath the western Pacific Ocean where the waves passed is thought to be smooth. But previous research found a large ULVZ beneath the western Pacific Ocean, east of the Philippines, that overlaps with the area studied.

And the researchers also found signatures of ULVZs when they looked elsewhere. The study found smaller patches of what appear to be more ULVZs under North America. And other research has found signs of ULVZs under North Africa, East Asia, Papua New Guinea and the Pacific Northwest, Thorne said.

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Some researchers have theorized that ULVZs could be the cause remains of gigantic impact bodies of the early phase of meteor bombardment of Earth. But if ULVZs are widespread, that suggests they are being actively generated today, Thorne said. He suspects these zones could be areas of volcanic basalt rock formed at mid-ocean ridges where the seafloor spreads out. When this mid-ocean basalt is eventually pulled into the mantle by subduction, it melts easily and can form pockets where seismic waves slow down. These pockets can then be pushed through the mantle by other slabs of subducting crust, which poke into Earth’s interior like stirrers in a smoothie.

A better understanding of these ULVZs could give geologists more insight into volcanic hotspots and the way the mantle moves.

“There are still a lot of open questions that we don’t have answers to yet,” Thorne said.