In the decades since their discovery, seismic signals known as PKP precursors have challenged scientists. The Earth’s lower mantle scatters incoming seismic waves, which return to the surface as PKP waves at varying speeds. The origin of the precursor signals, which precede the main seismic waves that pass through the Earth’s core, remains unclear, but research led by University of Utah geophysicists is shedding new light on this mysterious seismic energy.

According to a study published in AGU AdvancesThe ancestors of the PKP extend from deep within North America and the western Pacific Ocean and may be associated with “ultra-low-velocity zones,” thin layers in the mantle where seismic waves slow significantly.

“These are some of the most extraordinary features ever discovered on the planet. We don’t know what they are,” said lead author Michael Thorne, an associate professor in the Department of Geology and Geophysics at the University of California, San Francisco. “But what we do know is that they appear to be accreting beneath hot volcanoes. It looks like these could be the roots of all the mantle plumes that are producing hot volcanoes.”

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

“These really big volcanoes appear to have been in roughly the same place for hundreds of millions of years,” Thorne said, adding that his previous work had also found one of the world’s largest known ultralow-velocity zones.

“It’s right under Samoa, and Samoa is one of the largest hot volcanoes,” Thorne said.

For nearly a century, geologists have used seismic waves to explore the Earth’s interior, leading to numerous discoveries that would not have been possible otherwise. For example, other researchers in the United States have identified the structure of the Earth’s solid inner core and tracked its motion by analyzing seismic waves.

When an earthquake cracks the Earth’s surface, seismic waves travel through the mantle, the dynamic, hot rock layer 2,900 kilometers thick between the Earth’s crust and metallic core. Thorne’s team is interested in what “scatters” as it passes through irregular structures that cause changes in the composition of the material in the mantle. Some of these scattered waves become precursors to the PKP.

Thorne tried to determine exactly where this scattering occurred, particularly because the waves passed through the Earth’s mantle twice, before and after passing through the Earth’s liquid outer core. Because of this dual journey through the mantle, it was nearly impossible to distinguish whether the ancestral ray’s path originated from the source or the receiver.

Thorne’s team, which included assistant professor Surya Pachai, invented a way to model the waves to reveal important effects that had not been noticed before.

Using an advanced seismic array method developed by the researchers and new theoretical observations for earthquake modeling, they analyzed data from 58 earthquakes that occurred around New Guinea and were recorded in North America after passing through the planet.

“I can place virtual sensors anywhere in the world, and it will tell me what the seismogram of the earthquake at that location should look like. And we can compare that to the actual recordings that we have,” Thorne said. “Now we can determine where this energy is coming from.”

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

Their findings suggest that the progenitors of the PKP most likely came from regions where ultralow-velocity zones are located. Thorne suspects that these layers, which are only 20 to 40 kilometers thick, formed where tectonic plates in the oceanic crust meet the core-mantle boundary.

“What we’re discovering now is that these ultralow-velocity zones are not just found under hot spots. They’re distributed along the core-mantle boundary beneath North America, Thorne said. “It really looks like these ULVZs are being actively produced. We don’t know how. But because we see them close to subduction, we think they’re forming as mid-ocean ridge basalts are melting. And then the dynamics are pushing these things all the way around the Earth, and eventually they’re going to accumulate under hot spots.”

According to Thorne, dynamics are spreading these things around the Earth, and they will eventually accumulate near the boundaries of the Large Low-Velocity Zones, which are compositionally distinct continent-scale objects beneath the Pacific Ocean and Africa.

“They may also accumulate under hot spots, but it is not clear whether these ULVZs are produced by the same process,” he said. We will have to wait for future studies to determine the consequences of such a process.