Seismic Secrets: PKP Precursors and Earth's Mantle Mysteries

For decades, scientists have been baffled by seismic signals known as PKP precursors. These signals, which arrive before the main seismic waves that travel through Earth's core, have puzzled researchers since their discovery. Now, a new study led by geophysicists at the University of Utah has shed light on the origins of these mysterious signals, linking them to intriguing anomalies deep within Earth's mantle.

PKP precursors are scattered seismic waves that originate from regions in the lower mantle, returning to the surface at varying speeds. The research, published in *AGU Advances*, suggests these precursors may be connected to "ultra-low velocity zones" (ULVZs), thin layers within the mantle where seismic waves significantly slow down.

"These are some of the most extreme features discovered on the planet," explained lead author Michael Thorne, a University of Utah associate professor of geology and geophysics. "We don't fully understand what they are, but one thing is clear: they seem to accumulate beneath hotspot volcanoes, potentially acting as the roots of whole mantle plumes that fuel these volcanic hotspots."

Hotspot volcanoes, such as those found at Yellowstone, Hawaii, Samoa, Iceland, and the Galapagos Islands, are known for their persistent activity over millions of years. These plumes rise from deep within the mantle, bringing molten rock to the surface.

Thorne's previous research identified one of the largest known ULVZs, situated directly beneath Samoa, a prominent hotspot volcano. This new study aimed to pinpoint the precise location of PKP precursor scattering, especially considering the waves' double journey through the mantle, making it difficult to distinguish their origin.

To achieve this, Thorne and his team, including research assistant professor Surya Pachhai, employed a novel approach using cutting-edge seismic array methods and earthquake simulations. They analysed data from 58 earthquakes near New Guinea, which were recorded in North America after traversing the planet.

"We can place virtual receivers anywhere on Earth's surface, allowing us to model what the seismogram should look like from an earthquake at that location," explained Thorne. "By comparing these models to real recordings, we can trace the energy back to its source."

This advanced technique enabled the researchers to identify the scattering locations along the core-mantle boundary, approximately 2,900 kilometres below Earth's surface. Their findings indicate that the PKP precursors likely originate from regions containing ULVZs.

Thorne believes these thin layers, measuring just 20 to 40 kilometres thick, are formed when subducted tectonic plates interact with the core-mantle boundary in oceanic crust.

"We've found that ULVZs aren't just confined to areas beneath hotspots," said Thorne. "They're spread across the core-mantle boundary beneath North America, suggesting they're actively generated. While we don't know the exact process, we believe that mid-ocean ridge basalts are melted and then transported across Earth by dynamic processes, ultimately accumulating beneath hotspots."

These findings suggest that ULVZs play a crucial role in the Earth's dynamic processes, influencing the movement of heat and material within the mantle. However, the precise consequences of this interplay remain to be explored in future research.

This study highlights the ongoing discoveries being made through the analysis of seismic waves, offering invaluable insights into the complex and dynamic processes occurring within our planet. As scientists continue to explore Earth's interior, we can expect even more intriguing revelations about the hidden world beneath our feet.