Harvard Seismology: Inner Core Anisotropy

The Earth's inner core has this remarkable property: Compressional waves travel faster along its spin axis than in the equatorial plane. Such a directional dependence of wave propogation, together with the anomalous splitting of core-sensitive normal modes, was explained by an anisotropic inner core model first proposed by Harvard Seismology Group in 1986. Since then, we have further investigated its anisotropy extensively using both travel-time anomalies and the normal modes splitting. The symmetry axis of the anisotropy was used as a marker to detect the differential rotation of the inner core relative to the mantle. Our present research focuses on resolving the three-dimensionality and temporal variation of the anisotropy.

Active participants in this project are Miaki Ishii , Adam Dziewonski, and Wei-jia Su.

Previously active participants in these projects are Jeroen Tromp, and Hong Ma.

Recent Developments

• Inner-core Anisotropic Model
  • explained the observed anomalous splitting of normal modes and travel time anomalies by a cylindrical anisotropy model of the inner-core.
  • proposed a titled anisotropy axis(latitude 79.5N, longitude 160S) to fit the travel time data better.
• Three Dimensionality of the Inner-core Anisotropy
  • developed a cylindrical anisotropy stacking procedure to analyze the travel time data and presented the first three dimensional image of inner core anisotropy.
  • derived a compressional velocity perturbation formular in a hexagonally close-packed medium with random spatial orientation to model the three dimensional anisotropic effects on travel time residuals.
  • inverted for an anisotropy pattern which can be used to explore possible mechanisms of inner core anisotropy.
• Temproal Variation of the Inner-core Anisotropy
  • investigated the temporal variations of ISC travel time residual which revealed a differential rotation of the inner-core relative to the mantle at rate about 3 degree faster per year.

Publications

• Three-Dimensional Inner-Core Anisotropy (Ma, H., Tromp, J., Dziewonski,A.M., , IRIS Meeting, June 1997)
• Planet within planet: Rotation of the inner core of the Earth (Su, W.-J., Dziewonski, A.M., Raymond, J., Science, 274, 1883-1887, 1996)
• Investigation of possible mechanisms and three dimensionality of inner core anisotropy (Ma, H., Tromp, J., Dziewonski, A.M., Su, W.-J., Eos Trans. AGU, Fall Meeting Suppl., F51, 1996)
• Inner core anisotropy in three dimension (Su, W.-J., Dziewonski, A.M., J. Geophys. Res., 100, 9831, 1995)
• Normal mode splitting due to inner-core anisotropy (Tromp, J., Geophys. J. Int., 121, 963-968, 1995)
• Support for anisotropy of the earth's inner core from free oscillations (Tromp, J., Nature, 366, 678 - 681, 1993)
• Anisotropy of the inner core inferred from PKIKP travel times (Morelli, A, and Dziewonski, A.M., Geophys. Res. Lett., 13, 1545 - 1548, 1986)