Weak upper-mantle base revealed by postseismic deformation of a deep earthquake.

Mantle viscosity plays a key role in the Earth's internal dynamics and thermal history. Geophysical inferences of the viscosity structure, however, have shown large variability depending on the types of observables used or the assumptions imposed 1-3 . Here, we study the mantle viscosity structure by using the postseismic deformation following a deep (approximately 560 km) earthquake located near the bottom of the upper mantle. We apply independent component analysis 4 to geodetic time series to successfully detect and extract the postseismic deformation induced by the moment magnitude 8.2, 2018 Fiji earthquake. To search for the viscosity structure that can explain the detected signal, we perform forward viscoelastic relaxation modelling 5,6 with a range of viscosity structures. We find that our observation requires a relatively thin (approximately 100 km), low-viscosity (10 17 to 10 18  Pa s) layer at the bottom of the mantle transition zone. Such a weak zone could explain the slab flattening 7 and orphaning 8 observed in numerous subduction zones, which are otherwise challenging to explain in the whole mantle convection regime. The low-viscosity layer may result from superplasticity 9 induced by the postspinel transition, weak CaSiO 3 perovskite 10 , high water content 11 or dehydration melting 12 .