Weird mystery waves that baffle scientists may be 'everywhere' inside Earth's mantle

Mysterious zones in the deep mantle where earthquake waves slow to a crawl may actually be everywhere, new research finds.

Scientists already knew that ultra-low velocity zones (ULVZs), hover near hotspots — regions of the mantle where hot rock moves upward, forming volcanic island chains such as Hawaii. But mysterious earthquake waves suggest that these features might be widespread.

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

"Here's one of the most extreme features that we see anywhere inside the planet," Thorne told Live Science. "And we don't know what they are, where they're coming from, what they're made of, [or] what role they play inside the Earth."

Thorne wasn't thinking of 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 quakes, like those that occur at subduction zones where one tectonic plate slides under 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 mystery features in Earth's lower mantle. To identify these features, Thorne and his colleagues modeled PKP waves Traveling through a computer model of Earth's mantle, into which they added areas that changed the waves' velocity. They found predictable patterns in how PKP waves varied in speed.

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