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Equatorial anisotropy in the inner part of Earth’s inner core from autocorrelation of earthquake coda


The Earth’s solid inner core exhibits strong anisotropy1,2,3,4,5, with wave velocity dependent on the direction of propagation due to the preferential alignment of iron crystals6. Variations in the anisotropic structure, laterally and with depth7,8,9,10,11, provide markers for measuring inner-core rotation12 and offer clues into the formation and dynamics of the inner core13,14. Previous anisotropy models of the inner core have assumed a cylindrical anisotropy in which the symmetry axis is parallel to the Earth’s spin axis. An inner part of the inner core with a distinct form of anisotropy has been suggested15, but there is considerable uncertainty regarding its existence and characteristics16,17,18,19. Here we analyse the autocorrelation of earthquake coda measured by global broadband seismic arrays between 1992 and 2012, and find that the differential travel times of two types of core-penetrating waves vary at low latitudes by up to 10 s. Our findings are consistent with seismic anisotropy in the innermost inner core that has a fast axis near the equatorial plane through Central America and Southeast Asia, in contrast to the north–south alignment of anisotropy in the outer inner core. The different orientations and forms of anisotropy may represent a shift in the evolution of the inner core.

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Figure 1: Illustration of data used in this study.
Figure 2: Residuals of II2–I2 differential times (open circles) versus ray angle from the spin axis and best-fit cylindrical anisotropy model (red curve).
Figure 3: Residuals of II2–I2 differential times (open circles) versus ray angle from the best-fit IIC axis.
Figure 4: Residuals of observed II2–I2 differential times (symbols) and model global predictions (colour background).


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We thank L. Zhao for the parallelized version of the direct-solution method. This research was supported by the Natural Science Foundation of China (41330209, 41404037) and the US National Science Foundation (EAR 1215824).

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Authors and Affiliations



X.S. designed the project, performed data analysis, and wrote the paper. T.W. carried out data processing, performed data analysis, and contributed to the paper writing. H.H.X. contributed to data processing.

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Correspondence to Xiaodong Song.

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The authors declare no competing financial interests.

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Wang, T., Song, X. & Xia, H. Equatorial anisotropy in the inner part of Earth’s inner core from autocorrelation of earthquake coda. Nature Geosci 8, 224–227 (2015).

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