Shear-Wave Splitting and Implications for Mantle Flow Beneath the MELT Region of the East Pacific Rise-Reference-Cited by-同舟云学术

Shear-Wave Splitting and Implications for Mantle Flow Beneath the MELT Region of the East Pacific Rise

Published:1998-05-22 Issue:5367 Volume:280 Page:1230-1232
ISSN:0036-8075
Container-title:Science
language:en
Short-container-title:Science

Author:

Wolfe Cecily J.¹²,Solomon Sean C.¹²

Affiliation:

1. C. J. Wolfe, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.

2. S. C. Solomon, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.

Abstract

Shear-wave splitting across the fast-spreading East Pacific Rise has been measured from records of SKS and SKKS phases on the ocean-bottom seismometers of the Mantle Electromagnetic and Tomography (MELT) Experiment. The direction of fast shear-wave polarization is aligned parallel to the spreading direction. Delay times between fast and slow shear waves are asymmetric across the rise, and off-axis values on the Pacific Plate are twice those on the Nazca Plate. Splitting on the Pacific Plate may reflect anisotropy associated with spreading-induced flow above a depth of about 100 km, as well as a deeper contribution from warm asthenospheric return flow from the Pacific Superswell region.

Publisher

American Association for the Advancement of Science (AAAS)

Subject

Multidisciplinary

Reference21 articles.

1. Seismic Team MELT, Science280, 1215 (1998).

2. Vinnik L. P., Kind R., Kosarev G. L., Makeyeva L. I., Geophys. J. Int.99, 549 (1989).

3. Silver P. G., Chan W. W., J. Geophys. Res.96, 16429 (1991).

4. SKS is a phase that propagates downward from the source as an S wave, converts to a Pwave at the core-mantle boundary, travels through the fluid outer core, and then converts back to an S wave at the core-mantle boundary nearer to the point beneath the receiver. (An SKKSphase differs from SKS only in that the core path involves one underside reflection at the core-mantle boundary.) The conversion to P in the outer core removes any splitting accrued along the source side of the path, so when SKS orSKKS exits the outer core the converted shear wave is polarized in the direction radial to the source. Several lines of evidence [for example, Meade C., Silver P. G., Kaneshima S., Geophys. Res. Lett.22, 1293 (1995)] suggest that neither Earth's lower mantle nor the transition zone between the upper and lower mantle contributes substantially to SKS orSKKS splitting. The dominant contribution to such splitting is the preferential alignment of the orthorhombic mineral olivine in the upper mantle beneath the receiving seismometer.

5. For a single anisotropic layer, the delay time will be δt = L δβ/β0, where Lis the path length through the anisotropic material and is approximately equal to the layer thickness for near-vertically traveling SKS phases, and β0 is the isotropically averaged shear velocity. δβ is the dimensionless anisotropy (the difference between the fast and slow shear velocities divided by β0) and varies as a function of propagation direction (100 δβ is the percent anisotropy). Although the interpretation of shear-wave splitting measurements is generally made under the assumption of a single layer of anisotropic material, the more complicated case of multiple layers yields apparent splitting parameters that are functions of the anisotropy characteristics of individual layers [Silver P. G., Savage M. K., Geophys. J. Int. 119, 949 (1994)]. Because theSKS and SKKS phases of our study sample only two back azimuths (Table 1), we cannot discount the possibility that there are multiple layers of distinct anisotropy, but the simple spreading-parallel splitting in Fig. 3 argues against such complexity.

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