The origin of precursors to core waves

Abstract

The origin of the precursors of the core waves in the range 105°-142° is studied. Between 105° and 125° a long tail is observed after the P wave diffracted by the core. In the range 130° ≦ Δ ≦ 142° we usually observe short-period onsets a few seconds before PKIKP; these are the waves called P(GH). Reflection at a discontinuity in the outer core, near the inner-core boundary, is shown to produce the P(GH) branch. Reflections in the outer core are rejected as a mechanism for the tail of the P diffracted wave. A theoretical study of diffraction of P by the core shows that higher modes of diffracted waves cannot explain the observations of the tail of P diffracted. We conclude, by elimination, that it is due to reflections or multiple paths in the upper mantle.