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Reconciling the hemispherical structure of Earth’s inner core with its super-rotation


Earth’s solid inner core grows through solidification of material from the fluid outer core onto its surface at rates of about 1 mm per year1, freezing in core properties over time and generating an age–depth relation for the inner core. A hemispherical structure of the inner core is well-documented: an isotropic eastern hemisphere with fast seismic velocities contrasts with a slower, anisotropic western hemisphere2,3,4. Independently, the inner core is reported to super-rotate at rates of up to 1° per year5,6,7. Considering the slow growth, steady rotation rates of this magnitude would erase ’frozen-in’ regional variation and cannot coexist with hemispherical structure. Here, we exploit the age–depth relation, using the largest available PKIKP–PKiKP seismic travel time data set, to confirm hemispherical structure in the uppermost inner core, and to constrain the locations of the hemisphere boundaries. We find consistent eastward displacement of these boundaries with depth, from which we infer extremely slow steady inner core super-rotation of 0.1°–1° per million years. Our estimate of long-term super-rotation reconciles inner core rotation with hemispherical structure, two properties previously thought incompatible. It is in excellent agreement with geodynamo simulations8,9, while not excluding the possibility that the much larger rotation rates inferred earlier5,6,7 correspond to fluctuations in inner core rotation on shorter timescales10.

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Figure 1: Ray paths, travel time curves and an example of the seismic phases PKIKP and PKiKP.
Figure 2: Map showing all PKIKP–PKiKP differential travel time residual data collected.
Figure 3: PKIKP–PKiKP differential travel time residuals as a function of PKIKP turning point longitude, separated according to PKIKP turning depth.


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The research was funded by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 204995. We thank M. Dumberry and V. Cormier for their constructive and helpful comments.

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L.W. compiled and analysed the data and produced the manuscript and figures. J.I. wrote the cross-correlation code. J.I. and A.D. supervised the analysis. All authors discussed the results and implications at all stages.

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Correspondence to Lauren Waszek.

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The authors declare no competing financial interests.

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Waszek, L., Irving, J. & Deuss, A. Reconciling the hemispherical structure of Earth’s inner core with its super-rotation. Nature Geosci 4, 264–267 (2011).

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