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Published March 13, 1998 | public
Journal Article

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


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 withP 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.

Additional Information

© 1998 American Association for the Advancement of Science. Received for publication 27 October 1997; accepted for publication 2 February 1998. We thank T. Ahrens, D. Anderson, R. Clayton, M. Gurnis, H. Kanamori, T. Melbourne, and D. Stevenson for reviews. Funded by NSF grants EAR-9316441 and EAR-9629279. This paper is Contribution No. 8501, Division of Geological and Planetary Sciences, California Institute of Technology.

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