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Science 30 October 1998:
Vol. 282. no. 5390, pp. 924 - 927
DOI: 10.1126/science.282.5390.924
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Reports

Seismic Evidence for an Inner Core Transition Zone

Xiaodong Song, Don V. Helmberger

Seismic waves that traverse Earth's inner core along north-south paths produce unusually broad pulse shapes at long periods (compared with waves along east-west paths) and reflections from below the inner core boundary at short periods. The observations provide compelling evidence for a seismic velocity discontinuity along north-south paths about 200 kilometers below the inner core boundary separating an isotropic upper inner core from an anisotropic lower inner core. The triplication associated with such a structure might be responsible for reported waveform complexity of short-period inner core arrivals along north-south paths and, if the depth of the boundary is laterally variable, their large travel-time variation.

X. D. Song, Lamont-Doherty Earth Observatory, Palisades, NY 10964, and Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA. D. V. Helmberger, Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.

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