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Annual-scale variability in both the rotation rate and near surface of Earth’s inner core

Nature Geoscience volume 18, pages 267–272 (2025)Cite this article

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Abstract

The inner core has been inferred to change its rotation rate or shape over years to decades since the discovery of temporal variability in seismic waves from repeating earthquakes that travelled through the inner core. Recent work confirmed that the inner core rotated faster and then slower than the rest of Earth in the last few decades; this work analysed inner-core-traversing (PKIKP) seismic waves recorded by the Eielson (ILAR) and Yellowknife (YKA) arrays in northern North America from 121 repeating earthquake pairs between 1991 and 2023 in the South Sandwich Islands. Here we extend this set of repeating earthquakes and compare pairs at times when the inner core re-occupied the same position, revealing non-rotational changes at YKA but not ILAR between 2004 and 2008. We propose that these changes originate in the shallow inner core, and so affect the inner-core-grazing YKA ray paths more than the deeper-bottoming ray paths to ILAR. We thus resolve the long-standing debate on whether temporal variability in PKIKP waves results from rotation or more local action near the inner-core boundary: it is tentatively both. The changes near the inner-core boundary most likely result from viscous deformation driven by coupling between boundary topography and mantle density anomalies or traction on the inner core from outer-core convection.

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Fig. 1: Experiment geometry.
Fig. 2: Waveform change versus repeater dates.
Fig. 3: Recordings of earthquake pair 31, spanning 2004 to 2017.
Fig. 4: Non-rotational change evidence.
Fig. 5: Ray paths near inner core.

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Data availability

The seismic waveform data are available online from the Incorporated Research Institutions for Seismology Data Management Center (http://iris.edu) and Canadian National Seismograph Network (http://earthquakescanada.nrcan.gc.ca/stndon/CNSN-RNSC/index-en.php). The events used in this study are listed in Supplementary Table 1.

Code availability

All the code will be available upon request. All figures were generated using python packages, Matplotlib (https://matplotlib.org/), Basemap (https://matplotlib.org/basemap/stable/) and ObsPy (https://docs.obspy.org/).

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Acknowledgements

X. Song (Peking University), Y. Yang (Nanjing University), L. Wen (State University of New York, Stony Brook) and J. Yao (China University of Geosciences) supplied crucial unpublished catalogues and advice. J. Aurnou (University of California, Los Angeles), Y. Ricard (École Normale Supérieure de Lyon) and B. Buffett (University of California, Berkeley) also provided suggestions. This study is supported by National Science Foundation grant EAR-2041892 (J.E.V.) and the National Natural Science Foundation of China (42394114), the National Key R&D Program of China (grant 2022YFF0503203) and the Key Research Program of the Institute of Geology and Geophysics (IGGCAS-201904, IGGCAS-202204) (W.W.).

Author information

Authors and Affiliations

  1. Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA

    John E. Vidale & Ruoyan Wang

  2. Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

    Wei Wang

  3. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China

    Wei Wang

  4. Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA

    Guanning Pang

  5. Department of Geology and Geophysics, University of Utah, Salt Lake City, UT, USA

    Keith Koper

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  1. John E. Vidale
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Contributions

J.E.V. and W.W. engaged throughout. R.W. participated in data analysis. G.P. and K.K. initiated this line of investigation and consulted in interpretation and writing.

Corresponding author

Correspondence to John E. Vidale.

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Nature Geoscience thanks Vernon Cormier, Yanick Ricard and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Stefan Lachowycz, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Plot showing whether YKA and ILAR are similar or different.

Dots are YKA and stars are ILAR. Same axes as Fig. 2. ILAR stars are offset 0.25 years for visibility.

Extended Data Fig. 2 Earthquake repeatability for pair 31.

Waveform comparison at global stations for non-IC phases for repeating pair 31.

Extended Data Fig. 3 YKA repeaters and their difference.

(left) Comparison of the waveforms of seven pairs of repeating earthquakes at YKA. (right) Comparison of their difference after RMS amplitude normalization. The years of the events are noted and the pair index marked. Pair 33 has a slight difference in the very beginning of the time function, also visible in the ILAR PKiKP, which might be a source difference.

Extended Data Fig. 4 Matching YKA longer-offset repeaters.

(left) Comparison of the waveforms of seven pairs of repeating earthquakes at YKA. (right) Comparison of their difference after RMS amplitude normalization. The years of the events are noted and the pair index marked.

Extended Data Fig. 5 Zoom in of differing YKA repeaters shown in Extended Data Fig. 3.

(left) Comparison of the waveforms of seven pairs of repeating earthquakes at YKA. (right) Comparison of their difference after RMS amplitude normalization. The years of the events are noted and the pair index marked.

Extended Data Fig. 6 Zoom in of matching YKA longer-offset repeaters shown in Extended Data Fig. 4.

(left) Comparison of the waveforms of seven pairs of repeating earthquakes at YKA. (right) Comparison of their difference after RMS amplitude normalization. The years of the events are noted and the pair index marked.

Extended Data Fig. 7 YKA very-long-time-offset repeaters and their differences.

(left) Upper frame compares waveforms; lower frame shows their difference for long time window. (right) Same layout for shorter window.

Extended Data Fig. 8 ILAR for pairs in which YKA differed.

Waveforms of repeating events are compared. (left) Comparison of the waveforms of seven pairs of repeating earthquakes at YKA. (right) Comparison of their difference after RMS amplitude normalization. The years of the events are noted and the pair index marked.

Extended Data Fig. 9 ILAR matching, longer-offset repeaters.

Waveforms of repeating earthquakes are compared. (left) Comparison of the waveforms of seven pairs of repeating earthquakes at YKA. (right) Comparison of their difference after RMS amplitude normalization. The years of the events are noted and the pair index marked. Pairs 109 and 140 have high noise levels, but still reveal that the PKIKP phases are in phase as late as when PKP arrive; events that we classify as “different” for ILAR are not in phase at that time, see Extended Data Fig. 10.

Extended Data Fig. 10 ILAR dissimilar repeaters.

Waveforms of repeating earthquakes are compared. (left) Comparison of the waveforms of seven pairs of repeating earthquakes at YKA. (right) Comparison of their difference after RMS amplitude normalization. The years of the events are noted and the pair index marked.

Supplementary information

Supplementary Tables 1–3

Supplementary Table 1: Events and their indices. Index numbering matches Wei et al. (2024), which has further details. Data are augmented by an additional seven more recent events and the 25 additional pairs that they form. Table 2: Event pairs with matching characteristics. Index numbering matches Wei et al. (2024), which has further details. Waveform difference for YKA and ILAR in Table 2: 1—different. 0—similar. −1—noise level too high. −2—no data. Table 3: Multiplet population groupings.

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Vidale, J.E., Wang, W., Wang, R. et al. Annual-scale variability in both the rotation rate and near surface of Earth’s inner core. Nat. Geosci. 18, 267–272 (2025). https://doi.org/10.1038/s41561-025-01642-2

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