For the decades since their discovery, seismic signals known as PKP precursors have challenged scientists.
Regions of Earth’s lower mantle scatter incoming seismic waves, which return to the surface as PKP waves at differing speeds.
For nearly a century, geoscientists have used seismic waves to probe Earth’s interior, leading to numerous discoveries that would not be otherwise possible.
When an earthquake rattles Earth’s surface, seismic waves shoot through the mantle—the 2,900-kilometer-thick dynamic layer of hot rock between Earth’s crust and metal core.
Their findings indicate that the PKP precursors likely come from regions that are home to ultra-low velocity zones.
Seismic signals known as PKP precursors have presented scientists with difficulties for decades since they were discovered. Seismic waves that arrive at the surface are scattered by parts of the lower mantle of Earth and return as PKP waves at varying speeds.
The primary seismic waves that travel through Earth’s core arrive before the precursor signals, whose origin has long been a mystery. However, new research led by geophysicists at the University of Utah sheds light on this enigmatic seismic energy.
Based on research published in AGU Advances, PKP precursors appear to originate from deep below North America and the western Pacific, and may be associated with “ultra-low velocity zones,” which are thin layers in the mantle where seismic waves significantly slow down.
“Some of the most extreme features ever found on Earth are some of these. As lead author Michael Thorne, an associate professor of geology and geophysics at the University of Utah, put it, “We legitimately do not know what they are.”. We do, however, know that they eventually seem to gather beneath hotspot volcanoes.”. Hotspot volcanoes appear to have their origins in entire mantle plumes. “.
The volcanism seen in Yellowstone, the Hawaiian Islands, Samoa, Iceland, and the Galapagos Islands is caused by these plumes.
“These enormous volcanoes appear to have persisted in the same location for hundreds of millions of years,” stated Thorne. He also discovered one of the biggest known ultra-low velocity zones in the world in earlier research.
As one of the largest hotspot volcanoes, Samoa is directly beneath it, according to Thorne.
Seismic waves have allowed geologists to explore the interior of the Earth for almost a century, yielding a wealth of previously undiscovered information. For instance, using seismic wave analysis, other U researchers have determined the composition of Earth’s solid inner core and monitored its movement.
Seismic waves travel 2,900 kilometers through the mantle, the dynamic layer of hot rock between the metal core and the crust of Earth, when an earthquake shakes the planet’s surface. The ones that become “scattered” as they travel through asymmetrical features that introduce variations in the mantle’s material composition are of interest to Thorne’s group. A portion of those dispersed waves develop into PKP precursors.
Given that the waves pass through Earth’s mantle twice—that is, before and after they pass through the liquid outer core of the planet—Thorne set out to pinpoint the precise location of this scattering. Determining whether the precursors originated on the source-side or receiver-side of the ray path has proven to be nearly impossible due to that double journey through the mantle.
In order to identify significant effects that had previously gone undetected, Thorne’s team—which included research assistant professor Surya Pachhai—developed a technique to model waveforms.
The researchers examined data from 58 earthquakes that occurred in and around New Guinea and were detected in North America after traversing the planet, using a state-of-the-art seismic array method and new theoretical observations from earthquake simulations.
“Virtual receivers can be placed anywhere on Earth’s surface, and they will tell me what an earthquake’s seismogram should look like there. And that’s something we can contrast with the actual recordings we have,” stated Thorne. “We can now back project the source of this energy.”. “.”.
With the use of their novel technique, they were able to identify the precise location of the scattering along the core-mantle boundary, which is 2,900 kilometers below the surface of the Earth and separates the liquid metal outer core from the mantle.
Their results suggest that the PKP precursors originate most likely from areas with ultra-low velocity zones. Thorne hypothesizes that the formation of these barely 20 to 40 km thick layers occurs at the oceanic crust’s core-mantle boundary, where subducted tectonic plates meet.
It is now known that these extremely low velocity zones are not limited to the area directly below the hotspots. They are dispersed throughout North America’s core-mantle boundary, according to Thorne. The appearance suggests that these ULVZs are being actively generated. We’re not sure how. However, since they are found close to subduction, we believe that melting basalts in mid-ocean ridges is the source of the material. These are then being forced by Earth’s dynamics in all directions, and eventually they will gather beneath the hotspots. “.
According to Thorne, the dynamics are pushing these elements all over Earth, and eventually, they will gather up against the borders of Large Low Velocity Provinces, which are compositionally distinct continent-scale features located beneath the Pacific and Africa.
“They might also build up below the hotspots, but it’s not clear if these ULVZs are produced by the same mechanism,” he added. Future research will need to be conducted to determine the effects of such a process.
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