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The shuffling rotation of the Earth’s inner core revealed by earthquake doublets

Nature Geoscience volume 6, pages 497–502 (2013)Cite this article

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Abstract

Geodynamical models and seismic observations suggest that the Earth’s solid inner core rotates at a different rate than the mantle. However, discrepancies exist in rotation rate estimates based on seismic waves produced by earthquakes. Here we investigate the inherent assumption of a constant rotation rate using earthquake doublets—repeating earthquakes that produce similar waveforms. We detect that the rotation rate of the Earth’s inner core with respect to the mantle varies with time. We perform an inverse analysis of 7 doublets observed at the College station, Alaska, as well as 17 previously reported doublets, and reconstruct a history of differential inner-core rotation between 1961 and 2007. We find that the observed doublets are consistent with a model of an inner core with an average differential rotation rate of 0.25–0.48° yr−1 and decadal fluctuations of the order of 1° yr−1 around the mean. The decadal fluctuations explain discrepancies between previous core rotation models and are in concordance with recent geodynamical simulations.

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Figure 1: Newly observed doublet and PKP waves.
Figure 2: Waveform doublets and measurement methods.
Figure 3: Solution model and posterior distribution of the number of nodes.
Figure 4: Inner-core differential rotation rate as a function of time.

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Acknowledgements

We would like to acknowledge X. D. Song and P. G. Richards for making available previously observed and published waveform doublets. The data consist of digitized analogue data (from the WWSSN archive in Lamont) as well as the data available through the IRIS DMC archive. H.T. is grateful to R. Deguen, M. Fang and the colleagues from RSES, ANU for useful discussions.

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Authors and Affiliations

  1. Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory 0200, Australia

    Hrvoje Tkalčić, Mallory Young, Silvie Ngo & Malcolm Sambridge

  2. Berkeley Seismological Laboratory, University of California at Berkeley, Berkeley, California 94720, USA

    Thomas Bodin

Authors
  1. Hrvoje Tkalčić
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  2. Mallory Young
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  3. Thomas Bodin
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  4. Silvie Ngo
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Contributions

S.N. and H.T. contributed algorithms and methods for doublet search. M.Y., S.N. and H.T. worked together on improving methods for identification of doublets and their measurements. M.Y. performed time measurements and made the waveform figures under the supervision and verification of measurements by H.T. The inversion approach was designed by T.B. based on his PhD thesis supervised by M.S. and discussions with H.T. Synthetic tests were designed by H.T. and T.B., and T.B. contributed figures and supplementary text related to the Bayesian method. H.T. designed the study, wrote the manuscript and produced the figures, which all authors commented on and edited.

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Correspondence to Hrvoje Tkalčić.

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Tkalčić, H., Young, M., Bodin, T. et al. The shuffling rotation of the Earth’s inner core revealed by earthquake doublets. Nature Geosci 6, 497–502 (2013). https://doi.org/10.1038/ngeo1813

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