New research finds that mysterious zones in the deep mantle where seismic waves move slowly might actually be everywhere.
Scientists already knew that ultra-low velocity zones (ULVZs) are found near hot spots — regions of the mantle where hot rock moves upward and forms volcanic island chains like Hawaii. But the mysterious seismic waves suggest these features could be widespread.
ULVZs, which are found in the lower mantle near the core-mantle boundary, can slow seismic waves by as much as 50%. That's remarkable, said Michael Thorne, a geologist 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 do they play within the Earth.”
Thorne wasn’t thinking about ULVZs when he launched the new research, published Aug. 10 in the journal AGU Advances. Instead, he was intrigued by another mantle mystery. Very large earthquakes — such as those that occur at subduction zones where one tectonic plate slides beneath another — release powerful waves. Some of these so-called PKP waves travel through the mantle, the liquid outer core, and then the mantle again on their way to the opposite side of the planet from where they originated. 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 scattering off mysterious features in Earth's lower mantle. To identify these features, Thorne and his colleagues modeled the PKP waves traveling through a computer model of Earth's mantle, adding areas that changed the speed of the waves. They found predictable patterns in the way the PKP waves varied in speed.
So the team looked 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 earthquakes traveled through the core and all the way to North America, where they were recorded by EarthScope, a project that deployed portable seismic monitors across the United States between 2003 and 2018.
The findings suggested something was dramatically slowing the seismic waves to disperse their energy, Thorne said. The two likely candidates were valleys and ridges along the core-mantle boundary where the waves traveled, or ULVZ. The core-mantle boundary beneath the western Pacific, where the waves passed, is thought to be smooth. But previous research found a large ULVZ beneath the western Pacific, east of the Philippines, overlying the area studied.
In addition, researchers also found signs of ULVZs when they looked elsewhere. The study found smaller patches of what appear to be larger ULVZs beneath North America. And other research has found signs of ULVZs beneath North Africa, East Asia, Papua New Guinea and the Pacific Northwest, Thorne said.
Some researchers have theorized that ULVZs could be the remnants of giant impacts from the early stage of Earth's meteorite bombardment. However, if ULVZs are widespread, this suggests they are being actively generated today, Thorne said. He suspects these zones may be areas of volcanic rock basalt, formed at mid-ocean ridges where the seafloor is pulled apart. When this mid-ocean basalt is eventually dragged into the mantle by subduction, it melts easily and could form pockets where seismic waves are slowed. These pockets could then be pushed around in the mantle by other subducting slabs of crust, which are pushed into Earth's interior like stir sticks in a milkshake.
A better understanding of these ULVZs could improve geologists' understanding of volcanic hotspots as well as how the mantle moves.
“There are a lot of open questions that we don't have answers to yet,” Thorne said.