Geophysicists decode mysterious deep seismic signals

Geophysicists have discovered a link between seismic waves called PKP precursors and anomalies in the Earth's mantle.

A new study reveals that the PKP’s precursor seismic signals, which have puzzled scientists for years, originate in extremely low-velocity zones deep in North America and the western Pacific Ocean. The findings, by University of Utah researchers, link these zones to important geological features such as hotspot volcanoes, using advanced seismic techniques to trace their origins to the core-mantle boundary.

Introduction to PKP and Seismic Puzzles

Over the decades since their discovery, seismic signals known as PKP progenitors have been a challenge for scientists. Regions in 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 travel through the Earth's core, has remained unclear, but research led by geophysicists at the University of Utah has shed new light on this mysterious seismic energy.

The PKP ancestors appear to be spreading from deep beneath North America and the western Pacific Ocean and are probably associated with “ultra-low velocity zones,” thin layers in the mantle where seismic waves slow down dramatically, according to research published Aug. 10 in AGU Advancesthe leading journal of the American Geophysical Union.

Linking PKP ancestors to geological features

“These are some of the most extreme features ever discovered on this planet,” said lead author Michael Thorne, an associate professor of geology and geophysics at the University of Utah. “We don’t really know what they are, but the one thing we do know is that they seem to be accumulating beneath hot volcanoes. They seem to be the root of entire mantle plumes that give rise to hot volcanoes.”

These plumes are responsible for volcanic activity observed in Yellowstone National Park, the Hawaiian Islands, Samoa, Iceland, and the Galapagos Islands.

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

“The area is directly under the island of Samoa, and Samoa is one of the largest hotspots,” Thorne noted.

Developments in seismic wave analysis

For nearly a century, geologists have used seismic waves to explore the Earth’s interior, leading to many discoveries that would not have been possible otherwise. For example, other researchers at the University of Utah 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 mantle — the dynamic, 2,900-kilometer-thick layer of hot rock between the Earth’s crust and its metallic core. Thorne’s team is interested in those waves that “scatter” as they pass through irregular features that force changes in the physical composition of the mantle. Some of these scattered waves become the precursor to the PKP.

Thorne sought to pinpoint exactly where this scattering occurred, since waves travel through the Earth's mantle twice, before and after passing through the Earth's liquid outer core. Because of this double journey through the mantle, it became almost impossible to distinguish whether earlier waves originated on the source side or the receiver side of the ray path.

Innovative research techniques in seismology

Thorne's team, which included research assistant professor Surya Pachai, designed a way to model the waveforms to reveal crucial effects that had not been observed before.

Using an advanced seismic ensemble method and new theoretical observations from earthquake simulations, the researchers were able to analyze data from 58 earthquakes that occurred around New Guinea and were recorded in North America after passing through the planet.

“I can put virtual receivers anywhere on the Earth’s surface, and that tells me what the seismogram should look like from an earthquake at that location. We can compare that to the real recordings we have,” Thorne said. “We’re now able to predict where that energy is coming from.”

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

Core-mantle interactions and very low-lying rocky regions

Their findings suggest that the PKP’s ancestors probably came from areas that are home to extremely low-velocity zones. Thorne suspects that these layers, which are only 20 to 40 kilometers thick, form where subducting tectonic plates collide at the core-mantle boundary in the oceanic crust.

“What we’ve found now is that these very low-velocity regions aren’t just under hotspots. They’re all over the core-mantle boundary under North America,” Thorne said. “It really looks like these very low-velocity regions are being actively generated. We don’t know how. But because we see them near subduction, we think they’re actually in the atmosphere.” mid ocean basalt “This material melts, and that's how it's generated. Then the dynamics push this material through the Earth, and eventually it will accumulate under the hot spots.”

The dynamics are pushing these things around the Earth, and eventually, they will pile up against the boundaries of large, low-velocity provinces, which are distinct continental features in terms of composition under the Pacific and Africa, according to Thorne.

“It may also accumulate under hot spots, but it is not clear whether these very low-emission areas are generated by the same process,” he said. We will have to wait for future research to determine the consequences of such a process.

Reference: “Investigating ultra-low velocity zones as a source of PKP dispersion beneath North America and the western Pacific: possible links to subducting oceanic crust” by Michael S. Thorne, Surya Pachai, Mingming Li, Jamie Ward, and Sebastian Rust, August 10, 2024, AGU Advances.
Digital Document Number: 10.1029/2024AV001265

This research, funded by the National Science Foundation, was conducted in collaboration with geologists at Arizona State University and the University of Leeds in the United Kingdom.