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Science 12 November 1999:
Vol. 286. no. 5443, pp. 1326 - 1331
DOI: 10.1126/science.286.5443.1326
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Reports

Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle

Igor Sidorin, Michael Gurnis, * Don V. Helmberger

A sharp discontinuity at the base of Earth's mantle has been suggested from seismic waveform studies; the observed travel time and amplitude variations have been interpreted as changes in the depth of a spatially intermittent discontinuity. Most of the observed variations in travel times and the spatial intermittance of the seismic triplication can be reproduced by a ubiquitous first-order discontinuity superimposed on global seismic velocity structure derived from tomography. The observations can be modeled by a solid-solid phase transition that has a 200-kilometer elevation above the core-mantle boundary under adiabatic temperatures and a Clapeyron slope of about 6 megapascal per kelvin.

Seismological Laboratory 252-21, California Institute of Technology, Pasadena, CA 91125, USA.
*   To whom correspondence should be addressed. E-mail: gurnis{at}caltech.edu

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