A Thin and Weak Lithosphere-Asthenosphere Boundary (LAB) Beneath the Oceanic Lithosphere and its Effects on Subduction Earthquake Cycle Deformation 
Bibliographic record
Abstract
Numerous high-resolution seismological and magnetotelluric observations depict a sharp and distinct Lithosphere-Asthenosphere Boundary (LAB) at the base of oceanic lithosphere, in some cases beneath the subducting slab. Many lines of evidence indicate ponding of partial melts at the LAB. A melt-rich oceanic LAB is expected to have a low viscosity to affect plate motion, subduction, and earthquake deformation. Therefore, it is important to seek direct geodetic evidence for the rheological weakness of the LAB and its effects on deformation. Here we summarize our recent progress in finding the evidence. (1) Immediately after several recent large subduction earthquakes (e.g., the 2011 Mw=9 Tohoku-oki and the 2010 Mw=8.8 Maule) in the Japan-Kuril and Chile subduction zones, GNSS observations show enhanced landward motion (ELM) of coastal areas 100s of km outside the rupture area. Using 3-D viscoelastic finite element models, we explained the postseismic ELM in terms of mechanical decoupling of the subducting slab from the underlying asthenosphere due to a low-viscosity LAB (Sun et al., 2024). The ELM observation is thus considered the first geodetic evidence for a weak LAB beneath subducting oceanic lithosphere. Assuming a thickness of no more than 10 km for the LAB, key characteristics of the observed ELM can be explained to first order by an LAB viscosity of no more than 5e16 Pa s, lower than typical mantle viscosities by 2-3 orders of magnitude. (2) In a more detailed investigation of the postseismic deformation following the 2011 Tohoku-oki earthquake, constrained by extraordinarily dense onshore and offshore (seafloor GNSS/Acoustic) geodetic measurements, we find that both near-field deformation and the more distant ELM can be optimally explained by having a thin (~5 km) and low-viscosity (~5e16 Pa s) LAB down to a depth of ~120-150 km. Our geodesy-based research adds a new dimension to the geophysical studies of the LAB and contributes to understanding the origin, spatial distribution, and consequence of the ponded partial melts.Sun, T., Wang, K., & He, J. (2024). Geodetic signature of a weak lithosphere-asthenosphere boundary in postseismic deformation of large subduction earthquakes. Earth and Planetary Science Letters, 630, 118619, https://doi.org/10.1016/j.epsl.2024.118619
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".