Seismic array evidence for a two-layer core transition zone

Details Seismic array evidence for a two-layer core transition zone Seismic array evidence for a two-layer core transition zone

May 1974

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974pepi....8..251b&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 8, Issue 3, p. 251-268.

Physics

Scientific paper

Measurements of traveltimes and traveltime gradients for PKP phases recorded at the Warramunga seismic array from distances between 113° and 176° provide evidence for a two-layer transition zone in the earth's core. The basic data consist of paper recordings played out at 40 mm/sec from analog magnetic tape with amplitude gain control and narrow bandpass filters. Measured values of dT/dΔ are perturbed by structure beneath the array and it is necessary to correct for this effect by an empirical approach. On the basis of the dT/dΔ values and traveltimes, two precursor branches to PKPDF for Δ < 143° are identified consistently. These continue as branches intermediate to PKPDF and PKPAB for Δ > 145°. The corrected dT/dΔ values of all phases are smoothed and used to derive a velocity model for the core. The preferred model includes two velocity discontinuities of less than 0.1 km/sec each in the core transition zone to generate the two additional PKP branches. Together with the velocity jump at the outer-inner core boundary, these discontinuities define two layers each a few hundred kilometers thick. It is argued that recent proposals concerning inhomogeneities at the base of the mantle or within the transition zone as the source of PKP precursors do not agree with our observations nor those of other researchers. Inhomogeneities are not excluded but are considered a secondary effect for PKP phases, the primary effect being due to two discontinuous velocity increases in the transition zone.

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