Seismic Anisotropy and Deformation in Earth's Mantle

Seismic anisotropy in the Earth’s mantle arises from the directional dependence of seismic wave speeds, reflecting the alignment of mineral grains and the internal deformation of rocks under high pressure and temperature. As mantle flow transports heat from the core towards the surface, solid-state creep mechanisms shape the texture of dominant minerals such as bridgmanite, ferropericlase and post-perovskite. Crystallographic preferred orientation (CPO) of these phases, together with compositional layering and grain-scale heterogeneity, yields measurable shear wave splitting and variations in attenuation. These seismic observations are essential to constraining convection patterns, the rheological structure of the lower mantle and the dynamics of the D″ layer immediately above the core–mantle boundary. Bridging laboratory experiments, multiscale numerical modelling and full-waveform imaging has transformed our understanding of how individual deformation mechanisms translate into large-scale mantle flow, with implications for plate tectonics, plume dynamics and the thermal evolution of our planet.

Research from Nature Portfolio

Recent multiscale dislocation-dynamics modelling demonstrates that ferropericlase deforms markedly more slowly than bridgmanite under lower mantle conditions. This finding implies that bridgmanite dominates the rheology of the deep mantle, despite the presence of weaker periclase phases, and refines estimates of viscosity profiles at realistic strain rates.

Deformation experiments on post-perovskite under extreme pressures and temperatures reveal that its intrinsic crystal orientation favours (001) slip. This texture can account for the amplitude and polarity of shear wave splitting observed in the D″ layer, suggesting that as little as half of the available post-perovskite deformation is sufficient to generate the anisotropy detected beneath circum-Pacific regions.

An integrated mineral-physics approach has identified diffusion-controlled pure climb creep as the principal mechanism governing the deformation of bridgmanite in the lower mantle. By constraining oxygen diffusion coefficients and vacancy concentrations, researchers have reconciled theoretical creep models with seismically inferred viscosity, providing a unified framework for interpreting lower mantle dynamics.

Seismic Anisotropy and Deformation in Earth's Mantle publication trend

The graph below shows the total number of articles in seismic anisotropy and deformation in earth's mantle across all publications each year (not limited to Nature Index journals).

Technical terms

Seismic anisotropy: Variation in seismic wave speed with direction due to aligned mineral fabrics or compositional layering.

Crystallographic preferred orientation (CPO): Alignment of mineral grains in a rock resulting from deformation, producing directional seismic properties.

Shear wave splitting: Separation of shear waves into fast and slow polarised components when passing through anisotropic media.

Bridgmanite: The dominant high-pressure phase of (Mg,Fe)SiO₃ in the lower mantle, controlling its viscosity.

Ferropericlase: (Mg,Fe)O phase in the lower mantle, typically weaker than bridgmanite and influential in anisotropy.

Post-perovskite: High-pressure polymorph of bridgmanite present near the core–mantle boundary, associated with the D″ seismic discontinuity.

References

1. Periclase deforms more slowly than bridgmanite under mantle conditions. Nature (2023).
2. Seismic anisotropy of the D″ layer induced by (001) deformation of post-perovskite. Nature Communications (2017).
3. Deformation T-Cup: A new multi-anvil apparatus for controlled strain-rate deformation experiments at pressures above 18 GPa. Review of Scientific Instruments (2014).
4. Ferropericlase Control of Lower Mantle Rheology: Impact of Phase Morphology. Geochemistry Geophysics Geosystems (2020).
5. The role of diffusion-driven pure climb creep on the rheology of bridgmanite under lower mantle conditions. Scientific Reports (2019).
6. Inversion of shear wave waveforms reveal deformation in the lowermost mantle. Geophysical Journal International (2022).