Geophysicists have established a connection between seismic waves known as PKP precursors and anomalies in the Earth’s mantle, which are linked to volcanic hotspots on the planet’s surface.
Since their discovery decades ago, PKP precursors, a type of seismic signal, have posed questions for scientists. The lower mantle of the Earth influences incoming seismic waves, causing them to return to the surface as PKP waves at various speeds.
The origin of these precursor signals, which arrive before the primary seismic waves traveling through the Earth’s core, has been a mystery. However, research from geophysicists at the University of Utah is starting to clarify this perplexing phenomenon.
According to findings published in AGU Advances, the leading journal from the American Geophysical Union, PKP precursors seem to originate from deep below North America and the western Pacific, potentially linked to “ultra-low velocity zones.” These zones are thin layers within the mantle where seismic waves slow down considerably. (The AGU featured this research in their magazine Eos.)
Lead author Michael Thorne, an associate professor of geology and geophysics at the University of Utah, remarked, “These are among the most extreme features identified on our planet. We truly do not understand their nature.” He added, “However, it appears they tend to be located beneath hotspot volcanoes, possibly serving as the source of entire mantle plumes that lead to the formation of these volcanoes.”
These mantle plumes contribute to the volcanic activity seen in locations like Yellowstone, the Hawaiian Islands, Samoa, Iceland, and the Galapagos Islands.
Thorne explained, “These massive volcanoes tend to remain in approximately the same location for hundreds of millions of years.” In earlier studies, he identified one of the largest known ultra-low velocity zones.
“It is situated directly beneath Samoa, which is recognized as one of the largest hotspot volcanoes,” Thorne highlighted.
For almost a century, geoscientists have utilized seismic waves to explore Earth’s interior, leading to numerous profound discoveries. Researchers at the University of Utah have also characterized the structure of the Earth’s solid inner core and monitored its movement through seismic wave analysis.
When an earthquake strikes the surface, seismic waves travel through the mantle—the dynamic, 2,900-kilometer-thick layer of molten rock situated between the Earth’s crust and metal core. Thorne’s research group focuses on waves that become “scattered” due to irregularities in the mantle that change material composition. Some of these scattered waves are recognized as PKP precursors.
Thorne aimed to identify where exactly this scattering takes place, particularly since the waves journey through the Earth’s mantle twice—before and after passing through the liquid outer core. This dual travel has made it challenging to determine if the precursors originate from the source side or the receiver side.
His team, which included research assistant professor Surya Pachhai, developed a method to model waveforms, thereby noting critical effects that had previously gone undetected.
By employing an advanced seismic array technique along with new theoretical insights from their earthquake simulations, the researchers scrutinized data from 58 earthquakes that originated around New Guinea and were recorded in North America as they traversed the Earth.
Thorne stated, “I can place virtual receivers anywhere on Earth’s surface, which shows me what the seismogram should resemble from an earthquake at that spot. We can then compare this with the actual recordings we possess.” He added, “This enables us to backtrack and identify where this energy is originating.”
Their innovative approach helped them ascertain where the scattering occurs along the core-mantle boundary—located 2,900 kilometers beneath the Earth’s surface, where the liquid outer core meets the mantle.
The research suggests that PKP precursors likely emerge from areas containing ultra-low velocity zones. Thorne theorizes that these layers, merely 20 to 40 kilometers thick, form where subducted tectonic plates collide with the core-mantle boundary, especially in oceanic regions.
“What we’ve discovered is that these ultra-low velocity zones aren’t limited to beneath hotspots; they are distributed across the core-mantle boundary beneath North America,” Thorne stated. “It seems that these ULVZs are being actively generated. We are still unsure how, but since we’re observing them near subduction zones, we suspect that melting of mid-ocean ridge basalts might contribute to their formation. The dynamics then facilitate their distribution throughout the Earth, ultimately leading to accumulation beneath hotspots.”
“What we’ve discovered indicates that these ultra-low velocity zones are not confined to beneath hotspots; they extend across the core-mantle boundary beneath North America,” Thorne added. “It appears these ULVZs are being continuously formed. We’re unsure of the process, but our observations of their presence near subduction zones lead us to believe that mid-ocean ridge basalts may be melting, potentially facilitating their formation.”
The dynamics may spread these layers across the Earth, likely accumulating at the edges of Large Low Velocity Provinces—distinct compositional features beneath the Pacific and Africa, according to Thorne.
“They might additionally cluster beneath the hotspots, though it is uncertain if these ULVZs are created via the same mechanisms,” he noted. The implications of this discovery will need to be explored in future studies.
