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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Morelli, A., Dziewonski, A. M. & Woodhouse, J. H. Anisotropy of the inner core inferred from PKIKP travel times. Geophys. Res. Lett. 13, 1545–1548 (1986).
Woodhouse, J. H., Giardini, D. & Li, X. D. Evidence for inner core anisotropy from free oscillations. Geophys. Res. Lett. 13, 1549–1552 (1986).
Creager, K. C. Anisotropy of the inner core from differential travel times of the phases PKP and PKIKP. Nature 356, 309–314 (1992).
Tromp, J. Support for anisotropy of the Earth’s inner core from free oscillations. Nature 366, 678–681 (1993).
Song, X. D. Anisotropy of the Earth’s inner core. Rev. Geophys. 35, 297–313 (1997).
Brown, J. M. & Mcqueen, R. G. Phase transitions, Grüneisen parameter, and elasticity for shocked iron between 77 GPa and 400 GPa. J. Geophys. Res. 91, 7485–7494 (1986).
Shearer, P. M. Constraints on inner core anisotropy from PKP(DF) travel times. J. Geophys. Res. 99, 19647–19659 (1994).
Song, X. D. & Helmberger, D. V. Depth dependence of anisotropy of Earth’s inner core. J. Geophys. Res. 100, 9805–9816 (1995).
Tanaka, S. & Hamaguchi, H. Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP(BC)–PKP(DF) times. J. Geophys. Res. 102, 2925–2938 (1997).
Niu, F. L. & Wen, L. X. Hemispherical variations in seismic velocity at the top of the Earth’s inner core. Nature 410, 1081–1084 (2001).
Sun, X. L. & Song, X. D. Tomographic inversion for three-dimensional anisotropy of Earth’s inner core. Phys. Earth Planet. Inter. 167, 53–70 (2008).
Song, X. D. & Richards, P. G. Seismological evidence for differential rotation of the Earth’s inner core. Nature 382, 221–224 (1996).
Deguen, R. & Cardin, P. Tectonic history of the Earth’s inner core preserved in its seismic structure. Nature Geosci. 2, 418–421 (2009).
Alboussiere, T., Deguen, R. & Melzani, M. Melting-induced stratification above the Earth’s inner core due to convective translation. Nature 466, 744–747 (2010).
Ishii, M. & Dziewonski, A. M. The innermost inner core of the earth: Evidence for a change in anisotropic behavior at the radius of about 300 km. Proc. Natl Acad. Sci. USA 99, 14026–14030 (2002).
Beghein, C. & Trampert, J. Robust normal mode constraints on inner-core anisotropy from model space. Science 299, 552–555 (2003).
Cormier, V. F. & Stroujkova, A. Waveform search for the innermost inner core. Earth Planet. Sci. Lett. 236, 96–105 (2005).
Sun, X. L. & Song, X. D. The inner inner core of the Earth: Texturing of iron crystals from three-dimensional seismic anisotropy. Earth Planet. Sci. Lett. 269, 56–65 (2008).
Lythgoe, K. H., Deuss, A., Rudge, J. F. & Neufeld, J. A. Earth’s inner core: Innermost inner core or hemispherical variations? Earth Planet. Sci. Lett. 385, 181–189 (2014).
Lin, F. C. et al. Extracting seismic core phases with array interferometry. Geophys. Res. Lett. 40, 1049–1053 (2013).
Nishida, K. Global propagation of body waves revealed by cross-correlation analysis of seismic hum. Geophys. Res. Lett. 40, 1691–1696 (2013).
Boue, P. et al. Teleseismic correlations of ambient seismic noise for deep global imaging of the Earth. Geophys. J. Int. 194, 844–848 (2013).
Lin, F. C. & Tsai, V. C. Seismic interferometry with antipodal station pairs. Geophys. Res. Lett. 40, 4609–4613 (2013).
Rial, J. A. & Cormier, V. F. Seismic waves at the epicenter’s antipode. J. Geophys. Res. 85, 2661–2668 (1980).
Niu, F. L. & Chen, Q. F. Seismic evidence for distinct anisotropy in the innermost inner core. Nature Geosci. 1, 692–696 (2008).
Weaver, R. L. & Lobkis, O. I. Diffuse fields in open systems and the emergence of the Green’s function. J. Acoust. Soc. Am. 116, 2731–2734 (2004).
Stixrude, L. & Cohen, R. E. High-pressure elasticity of iron and anisotropy of Earth’s inner core. Science 267, 1972–1975 (1995).
Mattesini, M. et al. Hemispherical anisotropic patterns of the Earth’s inner core. Proc. Natl Acad. Sci. USA 107, 9507–9512 (2010).
Belonoshko, A. B., Skorodumova, N. V., Rosengren, A. & Johansson, B. Elastic anisotropy of Earth’s inner core. Science 319, 797–800 (2008).
Bensen, G. D. et al. Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements. Geophys. J. Int. 169, 1239–1260 (2007).
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).
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) doi:10.1038/ngeo2354
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