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Seismic evidence for small-scale dynamics in the lowermost mantle at the root of the Hawaiian hotspot

Nature volume 396, pages 255–258 (1998)Cite this article

Abstract

The hot thermal boundary layer produced by heat transport from the Earth's core to the base of the mantle is thought to contain strong horizontal shear flows and to nucleate instabilities in which hot material rises into the convecting mantle as thermal plumes1,2,3. A recent study4,5 proposes that the Hawaiian plume is deflected by mantle convection and, in the lowermost mantle, is located to the southeast of its surface manifestation. Here we present seismic data that densely sample, with core-reflected shear waves, a region beneath the central Pacific Ocean which includes the predicted location of the deflected root of the Hawaiian hotspot. Our mapping of the structure in this region of the lowermost mantle reveals strong lateral gradients in shear-wave velocity and anisotropic shear-wave polarization direction over distances of only several hundred kilometres. We interpret these gradients as being indicative of small-scale dynamical structure in the thermal boundary layer, where vertical flow into the Hawaiian plume at its root is accompanied by horizontal flow towards the plume.

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Figure 1: Geometry of ray-paths.
Figure 2: Map views of the travel-time residuals and anisotropy measurements plotted at the ScS CMB reflection points.
Figure 3: Two seismograms from different events and stations which illustrate the presence of variable anisotropy across our study region.
Figure 4: A cartoon of the boundary layer at the CMB that incorporates the differential travel time and anisotropy observations.

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Acknowledgements

We thank B. Steinberger for providing a preprint and details of his calculations for the Hawaiian hotspot, and R. Hartog for providing several codes for measuring anisotropy. Data were obtained through the IRIS, BDSN and TERRAscope data centres. This work was supported by the US NSF.

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Authors and Affiliations

  1. Earth Sciences Department, University of California, Santa Cruz, 95064, California, USA

    Sara A. Russell & Thorne Lay

  2. Seismological Laboratory, University of California, Berkeley, 94720, California, USA

    Edward J. Garnero

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  2. Thorne Lay
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  3. Edward J. Garnero
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Russell, S., Lay, T. & Garnero, E. Seismic evidence for small-scale dynamics in the lowermost mantle at the root of the Hawaiian hotspot. Nature 396, 255–258 (1998). https://doi.org/10.1038/24364

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