For decades, seismic signals known as PKP precursors have been a mystery to scientists. These signals, which travel faster than the main seismic waves that pass through Earth’s core, have baffled researchers due to their unclear origin. However, a recent study led by University of Utah geophysicists has shed new light on these enigmatic seismic waves. The research indicates that the PKP precursors likely originate from regions deep below North America and the western Pacific, possibly associated with ultra-low velocity zones in the mantle where seismic waves slow down significantly.
The lead author of the study, Michael Thorne, describes the ultra-low velocity zones as some of the most extreme features found on Earth, with their association to hotspot volcanoes raising questions about their origin and impact. Plumes under these zones are believed to be responsible for volcanic activity observed at locations like Yellowstone, the Hawaiian Islands, Samoa, Iceland, and the Galapagos Islands. Thorne’s team has also identified one of the world’s largest ultra-low velocity zones beneath Samoa, a hotspot volcano. These findings point to a potential link between these zones and long-lasting hotspot volcanoes, providing new insights into Earth’s dynamic mantle processes.
Geoscientists have used seismic waves for nearly a century to explore Earth’s interior, leading to groundbreaking discoveries such as characterizing the structure of the solid inner core and tracking its movement through seismic wave analysis. Thorne’s team focuses on waves that are scattered when they pass through irregular features in the mantle, creating PKP precursors. By developing a new method that combines seismic array techniques and theoretical earthquake simulations, the researchers were able to pinpoint where the scattering occurs along the core-mantle boundary, 2,900 kilometers below Earth’s surface.
The study’s findings suggest that PKP precursors likely originate from regions containing ultra-low velocity zones on the core-mantle boundary. Thorne speculates that these thin layers are formed where subducted tectonic plates intersect the core-mantle boundary in oceanic crust. The presence of ultra-low velocity zones beneath North America, not just hotspot regions, indicates an active generation mechanism, possibly related to the melting of mid-ocean ridge basalts near subduction zones. The dynamics of Earth’s mantle push these zones across the planet, accumulating beneath hotspot volcanoes and the boundaries of Large Low Velocity Provinces, which are distinct features beneath the Pacific and Africa.
Thorne emphasizes the need for future research to unravel the complexities of ultra-low velocity zones and their implications for Earth’s geodynamic processes. The active generation and migration of these zones raise questions about the underlying mechanisms driving these phenomena. By studying the impact of these zones on mantle dynamics, researchers can gain further insights into the formation and activity of hotspot volcanoes, as well as Earth’s overall tectonic processes. The study’s innovative approach to modeling seismic waveforms opens new avenues for exploring the origins and effects of PKP precursors and ultra-low velocity zones in Earth’s mantle, providing a deeper understanding of the planet’s deep-seated geophysical phenomena.