Seismic anisotropy of the Earth's inner core resulting from flow induced by Maxwell stresses

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Abstract

Seismological observations indicate that the inner core of the Earth is elastically anisotropic1. Anisotropic structures are likely to be formed by dynamic processes and therefore such observations have the potential to provide constraints on flow in the inner core and on the geodynamo itself. But in addition to the difficulties in estimating the relevant physical properties of iron under inner-core conditions2,3,4, even the macroscopic processes responsible for generating seismic anisotropy in this region have yet to be determined5,6,7,8,9. As a result, the geodynamic significance of seismic anisotropy in the inner core has remained unknown. Here I propose—based on geodynamic and mineral physics considerations—that flow induced by the stress due to the magnetic field, the Maxwell stress, near the inner-core boundary produces an axisymmetric fabric responsible for the observed seismic anisotropy. The resultant seismic anisotropy reflects the geometry of the magnetic field near the inner-core boundary and therefore seismological observations might provide constraints on the geodynamo. This flow also causes non-uniform release of energy at the inner-core boundary, associated with solidification and melting which may affect the pattern of convection in the outer core.

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Figure 1: A schematic diagram showing the structure and dynamics of the inner core.
Figure 2: Flow field caused by the magnetic field at the boundary between the inner and the outer core.

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Acknowledgements

I thank Y. Abe, S. Banerjee, M. Bergman, J. Bloxham, B. Buffett, F. Busse, R. Coe, K. Creager, D. Gubbins, R. Merrill, F. Stacey and D. Stevenson for discussions. This work was supported by Alexander von Humboldt Stiftung.

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Correspondence to Shun-ichiro Karato.

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Karato, S. Seismic anisotropy of the Earth's inner core resulting from flow induced by Maxwell stresses. Nature 402, 871–873 (1999) doi:10.1038/47235

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