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

Sunyoung Park (), Jean-Philippe Avouac, Zhongwen Zhan and Adriano Gualandi
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Sunyoung Park: The University of Chicago
Jean-Philippe Avouac: California Institute of Technology
Zhongwen Zhan: California Institute of Technology
Adriano Gualandi: Istituto Nazionale di Geofisica e Vulcanologia

Nature, 2023, vol. 615, issue 7952, 455-460

Abstract: Abstract 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 imposed1–3. 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 analysis4 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 modelling5,6 with a range of viscosity structures. We find that our observation requires a relatively thin (approximately 100 km), low-viscosity (1017 to 1018 Pa s) layer at the bottom of the mantle transition zone. Such a weak zone could explain the slab flattening7 and orphaning8 observed in numerous subduction zones, which are otherwise challenging to explain in the whole mantle convection regime. The low-viscosity layer may result from superplasticity9 induced by the postspinel transition, weak CaSiO3 perovskite10, high water content11 or dehydration melting12.

Date: 2023
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DOI: 10.1038/s41586-022-05689-8

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