Mantle Seismic Structure Beneath the MELT Region of the East Pacific Rise from P and S Wave Tomography

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Science  22 May 1998:
Vol. 280, Issue 5367, pp. 1224-1227
DOI: 10.1126/science.280.5367.1224

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Relative travel time delays of teleseismic P andS waves, recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment, have been inverted tomographically for upper-mantle structure beneath the southern East Pacific Rise. A broad zone of low seismic velocities extends beneath the rise to depths of about 200 kilometers and is centered to the west of the spreading center. The magnitudes of the P and S wave anomalies require the presence of retained mantle melt; the melt fraction near the rise exceeds the fraction 300 kilometers off axis by as little as 1%. Seismic anisotropy, induced by mantle flow, is evident in the P wave delays at near-vertical incidence and is consistent with a half-width of mantle upwelling of about 100 km.

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