Seismic Wave Scattering Near Caustics: Observations of PKKP Precursors

Abstract

ANALYSIS of precursors to PKP^1,2 has provided evidence for the existence of scattering structures in the deep mantle. It has been argued that the observability of the scattered waves is closely related to the presence of a caustic because of the focusing effect and because the scattered waves can enter the shadow zone at the concave side of the caustic, where they may manifest themselves as first arrivals. If that interpretation is correct, then in suitable circumstances the scattering phenomenon should also be identifiable near other caustics. The Earth's core forms a caustic not only for PKP phases, but also for SKP, PKKP, SKKP etc. and thus there is, in principle, the opportunity to test the interpretation. In practice there are a number of limitations: first, the latter phases themselves are one or more orders of magnitude smaller than PKP; second, at the time of arrival of these phases the seismic traces are, in general, still ‘noisy’. This practically excludes identification from single traces. We have used data from the Norwegian Seismic Array (NORSAR) and here observations in the shadow of the PKKP caustic will be reported. The PKKP caustic agrees mostly with the one of PKP; both caustics extend throughout the whole mantle, in contrast to those of, for example, SKP and SKKP.