2025-04-24 東京大学
スラブが最下部マントルの構造に与える影響を示した模式図
（上図）約3000万年前の様子の想像図。（下図）現在の様子。
＜関連情報＞
- https://www.s.u-tokyo.ac.jp/ja/press/10763/
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024JB030654
D″における太平洋大低剪断速度海域の北縁下の沈み込みスラブすべり Subducted Slab Slipping Underneath the Northern Edge of the Pacific Large Low-Shear-Velocity Province in D″
Keisuke Otsuru, Kenji Kawai, Robert J. Geller
Journal of Geophysical Research: Solid Earth Published: 24 April 2025
DOI:https://doi.org/10.1029/2024JB030654
Abstract
We conduct waveform inversion for the 3-D seismic shear wave (S-wave) velocity structure in the lowermost mantle near the northern edge of the Pacific large low-shear-velocity province (LLSVP). We image a slab-like high-velocity anomaly slipping beneath the Pacific LLSVP in the lowermost 200 km of the mantle, extending toward an ultra-low velocity zone (ULVZ) beneath a point about 2,000 km southwest of Hawaii. Another strong low-velocity anomaly exists along the edge of the LLSVP just above the slab-like sheet, 50–200 km above the core-mantle boundary (CMB). These results suggest in general that (a) slabs can gather ULVZ materials scattered on the CMB and push them into LLSVPs, creating concentrated ULVZs near LLSVP edges, (b) slabs can uplift hot material from the CMB to create strong anomalies along the edges of LLSVPs, and (c) large seismic-wave velocity contrasts between strong low-velocity anomalies and slabs create sharp LLSVP boundaries.
Key Points
- We image a slab-like high-velocity sheet slipping beneath the northern edge of the Pacific large low-shear-velocity province in D″
- Our results suggest that slabs sweep material horizontally at the core-mantle boundary to create concentrated ultra-low velocity zones
- Slabs uplift hot material from the mantle’s base to make strong low-velocity anomalies at the edge of large low-shear-velocity provinces
Plain Language Summary
Previous studies of the lowermost mantle have found two large regions with anomalously low seismic wave velocity, called large low-shear-velocity provinces (LLSVPs), spanning several thousand kilometers horizontally. However, their chemical composition and how they were created is not fully known. In this study, we image the detailed structure of the lowermost mantle around the northern part of one of these LLSVPs, located beneath the Pacific, using seismic waves that sample the region. We find that a slab (an oceanic plate that sank into the lowermost mantle) seems to slip beneath the northern edge of the LLSVP. A strongly anomalous seismic velocity structure is created at the edge of the LLSVP just above this slab, possibly due to the slab uplifting hot (and thus low seismic velocity) material from the base of the mantle. The slab may also have pushed chemically distinct material horizontally across the base of the mantle and thus created small piles that have been detected as low-velocity anomalies in previous studies.
