A Thin and Weak Lithosphere-Asthenosphere Boundary (LAB) Beneath the Oceanic Lithosphere and its Effects on Subduction Earthquake Cycle Deformation

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