Evidence for Inhomogeneities near the Core-Mantle Boundary [and Discussion]

Detailed analysis of observed short-period precursors to the seismic phase PKIKP indicates that the precursors are generated by seismic wave scattering from structural inhomogeneities near, or bumps on, the core-mantle boundary. Analysis of PKIKP precursor wavetrains observed at the Norwegian seismic array, Norsar, shows that observed precursors generally arrive from only very limited parts of the full ranges of arrival directions theoretically possible. In some cases the observed arrival directions of precursors migrate systematically with time along the wavetrains. The detailed characteristics of the data can be modelled in particular cases by postulating distributions of structural inhomogeneities that have either strong regional variations or strongly directional character. Theoretical calculations indicate that observed amplitudes of precursors require lateral length scales for the inhomogeneities not exceeding a few tens of kilometres. There is a suggestion that in some cases the observed scattering may occur at lateral boundaries of regions of much larger extent.