Evidence for scattering of seismic PKP waves near the mantle-core boundary

The hypothesis of scattering of PKP waves near the mantle-core boundary provides a comprehensive interpretation of the observed precursors to PKIKP, certain features of which have not been adequately accounted for by any alternative interpretation. These features include the variation with distance of the times and slownesses of precursor onsets, and the variations in amplitude, azimuth and slowness observed in precursor wavetrains.The observed times and slownesses of the earliest precursor onsets are in close agreement with the theoretical least time curve for singly scattered waves.Amplitudes and slownesses of scattered waves have been calculated for earth models which are spherically symmetrical except for random variations in density and elastic parameters in a layer 200 km thick at the base of the mantle. The calculations show that observed precursor amplitudes and slownesses can be accounted for by random variations of about one percent having a correlation distance of about 30 km in this layer. In particular, it is shown that scattering structures up to 900 km above the mantle-core boundary inferred by Doornbos and Vlaar (1973) are not required by their data. There is a suggestion that the main scattering may actually occur inside a layer much less than 200 km thick at the base of the mantle.