Weak asthenosphere of Sumatran backarc revealed by long postseismic geodetic records

Abstract The viscosity of Earth’s asthenosphere regulates plate motion and surface deformation. Assessing asthenosphere rheology in subduction-zone backarcs is as crucial as in forearc and arc regions for understanding subduction dynamics. High heat flow and low seismic velocities in backarcs suggest a low viscosity, yet its geodetic evidence remains elusive. Here, we leverage 12–20 years of geodetic observations from Thailand, Malaysia, and Singapore to provide the evidence. We observe sustained far-field postseismic motion following great earthquakes in the Sumatran subduction zone and Wharton Basin extending over 600 km from the ruptures. Using layered spherical Earth models, we demonstrate this deformation requires a weak asthenosphere with viscosity on the order of ~10 18 Pa s, with or without an even weaker lithosphere-asthenosphere boundary. These findings lend support to the conceptual model of buoyancy-driven small-scale thermal convection in subduction-zone backarcs. The extensive postseismic deformation due to the weak asthenosphere also affects sea-level projections.