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Science 13 March 1998:
Vol. 279. no. 5357, pp. 1701 - 1703
DOI: 10.1126/science.279.5357.1701
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Reports

Ultra-Low Velocity Zones Near the Core-Mantle Boundary from Broadband PKP Precursors

Lianxing Wen, * Donald V. Helmberger

Short- and long-period precursors of the PKP phase were used to study an ultra-low velocity zone (ULVZ) near the core-mantle boundary beneath the Western Pacific. Synthetic seismograms were computed from a hybrid method, which handles seismic wave propagation through two-dimensional complex structures. Long-period precursors were explained by Gaussian-shaped ULVZs of 60 to 80 kilometers height with P velocity drops of at least 7 percent over 100 to 300 kilometers. Short-period precursors suggest the presence of smaller scale anomalies accompanying these larger Gaussian-shaped structures. These fine structures may be areas of partial melt caused by vigorous small-scale convection or the instability of a thermal boundary layer at the mantle's base, or both.

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

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