Geophysicists Discover Link Between Seismic Waves Called PKP Precursors and Strange Anomalies in Earth’s Mantle

Since their discovery, seismic signals known as PKP precursors have posed a challenge to scientists. Regions of the Earth’s lower mantle scatter incoming seismic waves, which return to the surface as PKP waves at different speeds.

The origin of the precursor signals, which arrive before the main seismic waves that pass through the Earth’s core, remains unclear, but research by geophysicists at the University of Utah is shedding new light on this mysterious seismic energy.

PKP precursors appear to be spreading from locations deep beneath North America and the western Pacific and may be associated with “ultra-low velocity zones,” thin layers of the mantle where seismic waves slow dramatically, according to a study published in AGU Progress.

“These are some of the most extreme features found on the planet. We don’t really know what they are,” said lead author Michael Thorne, an associate professor of geology and geophysics at the university. “But one thing is for sure, they seem to eventually accumulate beneath hotspot volcanoes. They seem to be the source of entire mantle plumes that give rise to hotspot volcanoes.”

These plumes are responsible for the volcanism observed in Yellowstone, the Hawaiian Islands, Samoa, Iceland and the Galapagos Islands.

“These very large volcanoes appear to persist for hundreds of millions of years in roughly the same location,” Thorne said. In previous work, he also discovered one of the largest known very low-velocity zones in the world.

“It’s just below Samoa, and Samoa is one of the largest hot spot volcanoes,” Thorne noted.

For nearly a century, geoscientists have used seismic waves to probe the Earth’s interior, leading to many discoveries that would not have been possible otherwise. Other researchers at the University, for example, have characterized the structure of the Earth’s solid inner core and tracked its motion by analyzing seismic waves.

When an earthquake shakes the Earth’s surface, seismic waves travel through the Earth’s mantle, the 2,900-kilometer-thick, dynamic layer of hot rock between the Earth’s crust and its metallic core. Thorne’s team is interested in these waves “scattering” as they travel through irregular structures that cause changes in the material composition of the mantle. Some of these scattered waves become precursors to PKPs.

Thorne sought to determine exactly where this scattering occurs, especially since the waves pass through the Earth’s mantle twice, that is, before and after passing through the Earth’s liquid outer core. Because of this double journey through the mantle, it was nearly impossible to distinguish whether the precursors came from the source or the receiver side of the ray path.

Thorne’s team, which included research assistant professor Surya Pachhai, devised a way to model the waveforms to detect crucial effects that previously went unnoticed.

Using a state-of-the-art seismic network method and new theoretical observations from earthquake simulations, the researchers analyzed data from 58 earthquakes that occurred around New Guinea and were recorded in North America after crossing the planet.

“I can put virtual receivers anywhere on the surface of the Earth, and it tells me what the seismogram of an earthquake should look like at that location. And we can compare that to the actual recordings that we have,” Thorne said. “Now we’re able to project where that energy is coming from.”

Their new method allowed them to pinpoint where the diffusion occurred along the boundary between the liquid metal outer core and the mantle, known as the core-mantle boundary, located 2,900 kilometers below Earth’s surface.

Their results indicate that PKP precursors likely originate in regions that harbor ultralow velocity zones. Thorne suspects that these layers, which are only 20 to 40 kilometers thick, form where subducted tectonic plates collide with the core-mantle boundary in the oceanic crust.

“We’ve now discovered that these very low velocity zones don’t just exist beneath hotspots. They’re distributed all over the core-mantle boundary beneath North America,” Thorne said. “It really does appear that these very low velocity zones are actively generating. We don’t know how. But because we’re observing them near subduction, we think that the mid-ocean ridge basalts are melting, and that’s how they’re generating. And then the dynamics push these things all over the Earth, and they’re eventually going to accumulate beneath hotspots.”

The dynamics push these things all over the Earth, and eventually they’ll accumulate against the boundaries of large, low-velocity provinces, which are compositionally distinct continent-wide features beneath the Pacific and Africa, Thorne said.

“It is possible that they also accumulate under hot spots, but it is not known whether these ULVZs are generated by the same process,” he said. Future research will have to wait to determine the consequences of such a process.