Three-dimensional structure of the Earth from splitting in free-oscillation spectra

Observations of splitting in free oscillations spectra can be used to constrain the three-dimensional distribution of heterogeneity in the Earth. Modes sampling mainly the Earth's mantle are compatible with the distributions of mantle heterogeneity inferred from P-wave travel times. The anomalously large splitting of modes penetrating deeply into the core requires large departures from sphericity which are difficult to accommodate within the framework of an isotropic model', it is argued that anisotropy of the inner core offers the most natural explanation of these data. © 1987 Nature Publishing Group