IN 1983, Poupinet et al.1 observed that compressional seismic waves traversing the inner core along a trajectory parallel to the Earth's rotation axis arrive faster than the same (PKIKP) waves travelling in the equatorial plane. They interpreted this observation as revealing prolate topography of the inner-core boundary. In 1986, Morelli et al.2 and Woodhouse et al.3 suggested that inner-core anisotropy could explain both the travel-time observations and the anomalous splitting of some of the Earth's normal modes. Inner-core anisotropy continues to be the preferred explanation for the travel-time anomalies, although there is disagreement about the magnitude of anisotropy4-6. More recent explanations for the anomalous splitting involve topography of the inner-core and core-mantle boundaries as well as lateral heterogeneity of the core7-11. In particular, Widmer et al.11 dismissed a rather complex recent model of inner-core anisotropy12 because it could not explain the splitting of several previously unidentified modes. Here I show that the anomalous splitting of all currently identified modes can in fact be explained by cylindrical anisotropy of the Earth's inner core that is also compatible with the observed PKIKP travel-time anomalies. The resulting model should be regarded as an upper limit to the amount of anisotropy, as lateral heterogeneity also undoubtedly contributes to the splitting.
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