DATE2025.04.24 #Press Releases

Do Subducted Plates Act Like a “Broom” in the Lowermost Mantle?

-- Seismic Waveforms Reveal the Dynamics Beneath the Pacific and the Memory of Ancient Plates--

Summary

A research group led by Associate Professor Kenji Kawai and graduate student Keisuke Otsuru of the School of Science at The University of Tokyo, together with Professor Emeritus Robert Geller of The University of Tokyo, has revealed how subducted slabs (tectonic plates) behave in the lowermost mantle beneath the Pacific Ocean using three-dimensional imaging based on seismic waveforms. By analyzing approximately 80,000 seismic waveforms, the team discovered that the slabs slide into the edge of the Large Low Shear Velocity Province (LLSVP) at a shallow angle, deforming the internal structure while entraining materials to form a Ultra-Low Velocity Zone (ULVZ) (Figure). Furthermore, the imaged slabs in the lowermost mantle correspond to subduction zones that are inferred to have existed at the Earth’s surface about 200 million years ago—specifically, the "Mendocino" and "Telkhinia" subduction zones—which have left no traces on the present Earth's surface. This suggests that deep Earth structure preserves records of past tectonic activity. These findings, representing a form of “subsurface geohistory” through analysis of seismic waveforms, mark a significant advancement in our understanding of the Earth’s evolutionary history (Figure).

Figure: Cross-section of the estimated structure
A cross-section of the three-dimensional S-wave velocity structure estimated by this study along the black line A–A′ shown on the map. The slab (enclosed by the blue solid lines in the cross-section) extending southwestward from the location where the Mendocino subduction zone is thought to have existed 200 million years ago (green line on the map) can be seen subducting into the edge of the LLSVP (region to the left of the red dotted line) just above the core–mantle boundary (CMB), forming a strong low-velocity anomaly and a ULVZ.

Journal

Journal Name	Journal of Geophysical Research: Solid Earth
Title of paper	Subducted Slab Slipping Underneath the Northern Edge of the Pacific Large Low-Shear-Velocity Province in Dʺ