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Mid-mantle deformation inferred from seismic anisotropy
James Wookey1, J-Michael Kendall, Guilhem Barruol
1School of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.
Nature
|February 15, 2002
Summary
Seismic wave splitting reveals mid-mantle anisotropy near the 660-km discontinuity. This suggests a potential boundary layer impeding mantle flow between the upper and lower mantle.
Area of Science:
- Geophysics
- Seismology
- Earth Science
Background:
- Convective mantle processes align crystals, creating seismic anisotropy detectable through directional seismic-wave velocity.
- Anisotropy is typically observed in the uppermost mantle (lithosphere) and lowermost mantle (D" region) due to flow changes.
- The 660-km discontinuity marks the boundary between the Earth's upper and lower mantle.
Purpose of the Study:
- To investigate seismic anisotropy in the mid-mantle, specifically around the 660-km discontinuity.
- To present evidence for a potential mid-mantle boundary layer influencing mantle flow.
Main Methods:
- Utilizing shear-wave splitting measurements from seismograph stations in Australia.
- Analyzing seismic waves from deep-focus earthquakes in the Tonga-Kermadec and New Hebrides subduction zones.
Main Results:
- Observed some of the largest shear-wave splitting values reported to date.
- Detected significant mid-mantle anisotropy in the vicinity of the 660-km discontinuity.
Conclusions:
- Evidence suggests the existence of a mid-mantle boundary layer, at least locally.
- This boundary layer may impede the flow of material between the upper and lower mantle in this region.
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