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

• Published in: Nature geoscience Vol. 8; no. 3; pp. 224 - 227
• Main Authors: Wang, Tao, Song, Xiaodong, Xia, Han H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.03.2015; Nature Publishing Group
• Subjects: 704/2151/123; 704/2151/210; 704/2151/508; Alignment; Anisotropy; Arrays; Autocorrelation; Broadband; Crystals; Earth core; Earth Sciences; Earth System Sciences; Earthquakes; Geochemistry; Geology; Geophysics/Geodesy; letter; Seismic activity; Seismic phenomena; Wave propagation; Wave velocity
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo2354

The speed of seismic waves passing through the Earth’s inner core varies with direction. Analysis of earthquake seismic data suggests that this directional dependence differs between innermost and outer inner core. The Earth’s solid inner core exhibits strong anisotropy 1 , 2 , 3 , 4 , 5 , with wave velocity dependent on the direction of propagation due to the preferential alignment of iron crystals 6 . Variations in the anisotropic structure, laterally and with depth 7 , 8 , 9 , 10 , 11 , provide markers for measuring inner-core rotation 12 and offer clues into the formation and dynamics of the inner core 13 , 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 suggested 15 , but there is considerable uncertainty regarding its existence and characteristics 16 , 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.