Inner-core shear-wave anisotropy and texture from an observation of PKJKP waves

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Abstract

Since the discovery of the Earth’s core a century ago1, and the subsequent discovery2 of a solid inner core (postulated to have formed by the freezing of iron3) seismologists have striven to understand this most remote part of the deep Earth. The most direct evidence for a solid inner core would be the observation of shear-mode body waves that traverse it, but these phases are extremely difficult to observe. Two reported observations in short-period data4,5 have proved controversial6. Arguably more successful have been studies of longer-period data6,7, but such averaging limits the usefulness of the observations to reported sightings. We present two observations of an inner-core shear-wave phase at higher frequencies in stacked data from the Japanese High-Sensitivity Array, Hi-Net8. From an analysis of timing, amplitude and waveform of the ‘PKJKP’ phase we derive constraints on inner-core compressional-wave velocity and shear attenuation at about 0.3 Hz which differ from standard isotropic core models9. We can explain waveform features and can partially reconcile the otherwise large differences between core wavespeed and attenuation models that our observations apparently suggest if we invoke shear-wave anisotropy in the inner core. A simple model of an inner core composed of hexagonal close-packed iron with its c axis aligned perpendicular to the rotation axis10 yields anisotropy that is compatible with both the shear-wave anisotropy that we observe and the well-established 3 per cent compressional-wave anisotropy.

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Figure 1: Source, ray path and receiver geometry.
Figure 2: Seismic data.
Figure 3: Real (top) and synthetic waveforms (bottom).
Figure 4: Possible causes of crystal alignment in the inner core.

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Acknowledgements

We thank L. Vočadlo, M. Kendall, M. Bergman and A. Deuss for discussions, C. Beghein for providing her inner-core anisotropy models and L. Stixrude for a review. J.W. was supported by a NERC postdoctoral fellowship grant.

Author Contributions Data analysis and modelling was undertaken by J.W., initiated by, and in consultation with, G.H. Phase-weighted stacking code and inner-core texture models were developed by G.H. Manuscript and figures were produced by J.W.

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Correspondence to James Wookey.

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Wookey, J., Helffrich, G. Inner-core shear-wave anisotropy and texture from an observation of PKJKP waves. Nature 454, 873–876 (2008) doi:10.1038/nature07131

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