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Science 20 August 1999:
Vol. 285. no. 5431, pp. 1231 - 1236
DOI: 10.1126/science.285.5431.1231
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Research Articles

Normal-Mode and Free-Air Gravity Constraints on Lateral Variations in Velocity and Density of Earth's Mantle

Miaki Ishii, * Jeroen Tromp

With the use of a large collection of free-oscillation data and additional constraints imposed by the free-air gravity anomaly, lateral variations in shear velocity, compressional velocity, and density within the mantle; dynamic topography on the free surface; and topography on the 660-km discontinuity and the core-mantle boundary were determined. The velocity models are consistent with existing models based on travel-time and waveform inversions. In the lowermost mantle, near the core-mantle boundary, denser than average material is found beneath regions of upwellings centered on the Pacific Ocean and Africa that are characterized by slow shear velocities. These anomalies suggest the existence of compositional heterogeneity near the core-mantle boundary.

Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
*   To whom correspondence should be addressed. E-mail: ishii{at}seismology.harvard.edu

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