Comparison between observed and predicted eigenfrequencies of normal modes give us constraints on the 3-D structure of the mantle and inner core. We are collaborating with Miaki Ishii at Harvard in this area of research.

Observed (left column) and predicted (right column) splitting functions for mantle-sensitive normal mode ₁S₄ (top row) and inner-core-sensitive mode ₆S₃ (bottom row). The predicted splitting function is based upon shear-velocity model SKS12WM13 (Dziewonski et al 1997). The splitting function of the mantle-mode ₁S₄is well predicted by SKS12WM13, but the splitting of the core-sensitive mode ₆S₃ is not: a large, zonal degree two pattern is missing. Such zonal splitting functions are characteristic of all anomalously split modes. The splitting function represents a local radial average of the Earth structure. This average value is obtain by filtering the Earth's 3-D heterogeneity through kernels such as those shown in the next figure. Because each mode has a unique set of sensitivity kernels, it also has its own unique splitting function that reflects the way in which it 'sees' the Earth. Splitting functions may be determined directly from free oscillation spectra.

Sensitivity kernels (degree zero) of mantle-sensitive mode ₁S₄ (left) and inner-core-sensitive mode ₆S₃ (right). The solid line represents sensitivity to shear velocity perturbations as a function of depth, the short-dashed line sensitivity to compressional velocity, and the long dashed line sensitivity to density. The locations of the 670 km discontinuity and the CMB have been indicated. Mode ₁S₄ is not sensitive to structure in the inner core, whereas the anomalously split mode ₆S₃ 'sees' the inner core.

 

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Harvard's Seismology Normal Modes site

Miaki Ishii's personal page

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