Inner-Core Shear-Wave Anisotropy and Texture from an Observation of PKJKP, Fig 1

Inner-Core Shear-Wave Anisotropy and Texture from an Observation of PKJKP Source, raypath and receiver geometry. We search for evidence of PKJKP in records of the Mw=7.0 shallow (depth approx. 14 km) 22nd February 2006 event in Mozambique at the Japanese Hi-net array (inset). The epicentral distance to the centre of the array is 113.7 degrees. The right panel shows the raypaths for PKKPab, PKiKP and PKJKP at this distance (straight lines are P-wave segments, wiggly are S-wave).<p></p>
James Wookey (University of Bristol, UK), George Helffrich (University of Bristol, UK)
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Since the discovery of the Earth’s core a century ago, and the subsequent discovery of a solid inner core (postulated to have formed by the freezing of iron) seismologists have striven to understand this most remote part of the deep Earth. The most direct evidence for a solid inner core is the observation of shear-mode body waves which traverse it, but these phases — for example, PKJKP — are extremely hard to observe. Two reported observations in short period data have proved controversial. Arguably more successful have been two studies in longer period data but such data somewhat limits the usefulness of the waveform beyond reported sightings. We present two observations of this phase at higher frequencies in stacked data from the Japanese High-Sensitivity Array, Hi-Net. From an analysis of timing, amplitude and waveform of PKJKP we derive constraints on inner core VP and shear attenuation at ~0.3 Hz which differ from standard isotropic core models. 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 inner core shear-wave anisotropy. A simple model of an inner core composed of hcp structured iron with its c-axis aligned perpendicular to the rotation axis yields anisotropy which is compatible with both the shearwave anisotropy that we observe and the well-established 3% P-wave anisotropy.
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References
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Wookey, J., Helffrich, G. (2008) Inner-core shear-wave anisotropy and texture from an observation of PKJKP. Nature, 454, 873-876
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Acknowledgements: Data were provided by Hinet (National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan). JW was supported by a NERC postdoctoral fellowship grant.

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