Effects of Upper Mantle Phase Transitions on Seismic Motions

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9526905 Wahr This research is to study the possible effects of upper mantle phase transformations, for example at depths of 410 and 660 km, on seismic motions, especially the free oscillations of the earth. Preliminary estimates suggest that during seismic motions, material in the vicinity of a phase boundary could change phase as it oscillates. For example, perturbations to the eigenfrequencies of free oscillations due to the 660 km boundary could be anywhere from a few tenths to several percent, well within the observational uncertainties of the eigenfrequencies. The objectives are to study whether the inversion of free oscillation data for spherical structure could provide useful information on the 410 and 660 km discontinuities; and to judge whether the inclusion of phase transformation effects into the inversion process might improve the inversions for other parameters. For the latter, the work will focus especially, though not exclusively, on implications for the density profile in the transition zone, and on the bulk dissipation models that have been inferred from observations of radial mode attenuation. *** ??