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REGIONAL INNER CORE ANISOTROPY FROM SEISMIC NORMAL MODES
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Cross-coupled splitting function 16S5-17S4 showing antisymmetric splitting, which is characteristic of east versus west hemispherical variation in inner core anisotropy. (From Deuss et al., 2010. Regional variation of inner core anisotropy from seismic normal mode observations. Science, doi:10.1126/science.1188596. reprinted with permission from AAAS.)
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Previous seismic body wave studies have suggested hemispherical variation in the isotropic and anisotropic structure of the inner core, but could not constrain their global extent. Theoretical advances to include coupling between normal modes that are close in frequency were motivated partly by the growing number of high-quality records of odd-degree normal modes, including those from the 2004 Sumatra, 2008 Wenchuan, and other recent large earthquakes. The observed odd-degree modes are now seen to suggest more complicated regional variations than a simple east/west hemispherical pattern. Instead, the similarity of the observed seismic pattern with earth’s magnetic field suggests that anisotropy may originate from freezing in of crystal alignment during solidification of the outer core or texturing of the inner core by electromagnetically induced stress.</p>