Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

An initial map of fine-scale heterogeneity in the Earth’s inner core

Abstract

The seismological properties of Earth’s inner core are key to understanding its composition, dynamics and growth history. Within the inner core, fine-scale heterogeneity has previously been identified from backscattering of high-frequency compressional waves. Here we use historical earthquake and explosion data from the Large Aperture Seismic Array, USA, between 1969 and 1975 to build a 3D map of heterogeneity from the inner-core boundary to 500 km depth and determine the geographical distribution of the scatterers across the 40% of the inner core that is visible to the array. Our model has two regions of strong scattering, one beneath eastern Asia and the other beneath South America, both located where past local surveys have identified scattering. We suggest that these loci of strong, fine-scale heterogeneities may be related to random alignments of small, inner-core crystals due to fast freezing. These areas, which have been identified as having high attenuation and lie beneath colder areas of the core–mantle boundary, potentially provide constraints on the dynamics of the inner core and the motions in the outer core, with downwelling in the mantle and outer core possibly associated with strong scattering and inner-core heterogeneity.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Map of events and PKiKP and ICS beam character.
Fig. 2: Beamforming envelopes summed from 20 s to 100 s after PKiKP for eight events.
Fig. 3: ICS strength distribution at various depths below the ICB.
Fig. 4: Illustration of a possible geodynamic model.

Similar content being viewed by others

Data availability

The LASA data are available online (https://github.com/JohnVidale/LASA_data). The events used in this study are listed in Supplementary Table 1.

Code availability

All the code will be available upon request.

References

  1. Vidale, J. E., Dodge, D. A. & Earle, P. S. Fine-scale heterogeneity in the Earth’s inner core. Nature 405, 445–448 (2000).

    Article  Google Scholar 

  2. Cormier, V. F. Texture of the uppermost inner core from forward- and back-scattered seismic waves. Earth Planet. Sci. Lett. 258, 442–453 (2007).

    Article  Google Scholar 

  3. Leyton, F. & Koper, K. D. Using PKiKP coda to determine inner core structure: 1. Synthesis of coda envelopes using single-scattering theories. J. Geophys. Res. Solid Earth 112, B05316 (2007).

    Google Scholar 

  4. Peng, Z., Koper, K. D., Vidale, J. E., Leyton, F. & Shearer, P. M. Inner-core fine-scale structure from scattered waves recorded by LASA. J. Geophys. Res. Solid Earth 113, B09312 (2008).

    Article  Google Scholar 

  5. Monnereau, M., Calvet, M., Margerin, L. & Souriau, A. Lopsided growth of Earth’s inner core. Science 328, 1014–1017 (2010).

    Article  Google Scholar 

  6. Wu, W. & Irving, J. C. E. Using PKiKP coda to study heterogeneity in the top layer of the inner core’s western hemisphere. Geophys. J. Int. 209, 672–687 (2017).

    Article  Google Scholar 

  7. Hedlin, M. A. H., Earle, P. S. & Bolton, H. Old seismic data yield new insights. Eos 81, 469–473 (2000).

    Article  Google Scholar 

  8. Frosch, R. A. & Green, P. E. The concept of the large aperture seismic array. Proc. R. Soc. A 290, 368–384 (1966).

    Google Scholar 

  9. Leyton, F. & Koper, K. D. Using PKiKP coda to determine inner core structure: 2. Determination of QC. J. Geophys. Res. Solid Earth 112, B05317 (2007).

    Google Scholar 

  10. Vidale, J. E. Very slow rotation of Earth’s inner core from 1971 to 1974. Geophys. Res. Lett. 46, 9483–9488 (2019).

    Article  Google Scholar 

  11. Sato, H., Fehler, M. C. & Maeda, T. Seismic Wave Propagation and Scattering in the Heterogeneous Earth (Springer, 2012).

  12. Deuss, A. Heterogeneity and anisotropy of Earth’s inner core. Annu. Rev. Earth Planet. Sci. 42, 103–126 (2014).

    Article  Google Scholar 

  13. Niu, F. & Wen, L. Hemispherical variations in seismic velocity at the top of the Earth’s inner core. Nature 410, 1081–1084 (2001).

    Article  Google Scholar 

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

    Article  Google Scholar 

  15. Yu, W. C. et al. The inner core hemispheric boundary near 180° W. Phys. Earth Planet. Inter. 272, 1–16 (2017).

    Article  Google Scholar 

  16. Dai, Z., Wang, W. & Wen, L. Irregular topography at the Earth’s inner core boundary. Proc. Natl Acad. Sci. USA 109, 7654–7658 (2012).

    Article  Google Scholar 

  17. Cao, A., Masson, Y. & Romanowicz, B. Short wavelength topography on the inner-core boundary. Proc. Natl Acad. Sci. USA 104, 31–35 (2007).

    Article  Google Scholar 

  18. Tanaka, S. & Tkalčić, H. Complex inner core boundary from frequency characteristics of the reflection coefficients of PKiKP waves observed by Hi-net. Prog. Earth Planet. Sci. 2, 34 (2015).

    Article  Google Scholar 

  19. Ibourichene, A. & Romanowicz, B. Detection of small-scale heterogeneities at the inner core boundary. Phys. Earth Planet. Inter. 281, 55–67 (2018).

    Article  Google Scholar 

  20. Wen, L. Localized temporal change of the Earth’s inner core boundary. Science 314, 967–971 (2006).

    Article  Google Scholar 

  21. Yao, J., Tian, D., Sun, L. & Wen, L. Temporal change of seismic Earth’s inner core phases: inner core differential rotation or temporal change of inner core surface? J. Geophys. Res. Solid Earth 124, 6720–6736 (2019).

    Article  Google Scholar 

  22. Tian, D. & Wen, L. Seismological evidence for a localized mushy zone at the Earth’s inner core boundary. Nat. Commun. 8, 165 (2017).

    Article  Google Scholar 

  23. Waszek, L. & Deuss, A. Anomalously strong observations of PKiKP/PcP amplitude ratios on a global scale. J. Geophys. Res. Solid Earth 120, 5175–5190 (2015).

    Article  Google Scholar 

  24. Koper, K. D., Franks, J. M. & Dombrovskaya, M. Evidence for small-scale heterogeneity in Earth’s inner core from a global study of PKiKP coda waves. Earth Planet. Sci. Lett. 228, 227–241 (2004).

    Article  Google Scholar 

  25. Poupinet, G. & Kennett, B. L. N. On the observation of high frequency PKiKP and its coda in Australia. Phys. Earth Planet. Inter. 146, 497–511 (2004).

    Article  Google Scholar 

  26. Pejić, T., Hawkins, R., Sambridge, M. & Tkalčić, H. Transdimensional Bayesian attenuation tomography of the upper inner core. J. Geophys. Res. Solid Earth 124, 1929–1943 (2019).

    Article  Google Scholar 

  27. Bergman, M. I., Agrawal, S. & Carter, M. S. Transverse solidification textures in hexagonal close-packed alloys. J. Cryst. Growth 255, 204–211 (2003).

    Article  Google Scholar 

  28. Aubert, J., Amit, H., Hulot, G. & Olson, P. Thermochemical flows couple the Earth’s inner core growth to mantle heterogeneity. Nature 454, 758–761 (2008).

    Article  Google Scholar 

  29. Vočadlo, L. in Treatise on Geophysics Vol. 2 (eds Price, G. D. & Schubert, G.) Ch. 5 (Elsevier, 2007).

  30. Fearn, D., Loper, D. & Roberts, P. Structure of the Earth’s inner core. Nature 292, 232–233 (1981).

    Article  Google Scholar 

  31. Mao, H. K. et al. Elasticity and rheology of iron above 220 GPa and the nature of the Earth’s inner core. Nature 396, 741–743 (1998).

    Article  Google Scholar 

  32. Gubbins, D., Sreenivasan, B., Mound, J. & Rost, S. Melting of the Earths inner core. Nature 473, 361–364 (2011).

    Article  Google Scholar 

  33. Lasbleis, M., Kervazo, M. & Choblet, G. The fate of liquids trapped during the Earth’s inner core growth. Geophys. Res. Lett. https://doi.org/10.1029/2019GL085654 (2020).

  34. Sumita, I., Yoshida, S., Kumazawa, M. & Hamano, Y. A model for sedimentary compaction of a viscous medium and its application to inner-core growth. Geophys. J. Int. 124, 502–524 (1996).

    Article  Google Scholar 

  35. Burdick, S., Waszek, L. & Lekić, V. Seismic tomography of the uppermost inner core. Earth Planet. Sci. Lett. 528, 115789 (2019).

    Article  Google Scholar 

  36. Alboussiëre, T., Deguen, R. & Melzani, M. Melting-induced stratification above the Earth’s inner core due to convective translation. Nature 466, 744–747 (2010).

    Article  Google Scholar 

  37. Frost, D. A., Lasbleis, M., Chandler, B. & Romanowicz, B. Dynamic history of the inner core constrained by seismic anisotropy. Nat. Geosci. 14, 531–535 (2021).

    Article  Google Scholar 

  38. Sumita, I. & Olson, P. A laboratory model for convection in Earth’s core driven by a thermally heterogeneous mantle. Science 286, 1547–1549 (1999).

    Article  Google Scholar 

  39. Aubert, J., Finlay, C. C. & Fournier, A. Bottom-up control of geomagnetic secular variation by the Earth’s inner core. Nature 502, 219–223 (2013).

    Article  Google Scholar 

  40. Davies, C. J. & Mound, J. E. Mantle-induced temperature anomalies do not reach the inner core boundary. Geophys. J. Int. 218, 2054–2065 (2019).

    Google Scholar 

  41. French, S. & Romanowicz, B. Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots. Nature 525, 95–99 (2015).

    Article  Google Scholar 

  42. Garnero, E., McNamara, A. & Shim, S. H. Continent-sized anomalous zones with low seismic velocity at the base of Earth’s mantle. Nat. Geosci. 9, 481–489 (2016).

    Article  Google Scholar 

  43. Venet, L., Duffar, T. & Deguen, R. Grain structure of the Earth’s inner core. C. R. Geosci. 341, 513–516 (2009).

    Article  Google Scholar 

  44. Deguen, R. & Cardin, P. Tectonic history of the Earth’s inner core preserved in its seismic structure. Nat. Geosci. 2, 419–422 (2009).

    Article  Google Scholar 

  45. Song, X. & Richards, P. G. Seismological evidence for differential rotation of the Earth’s inner core. Nature 382, 221–224 (1996).

    Article  Google Scholar 

  46. Mound, J. & Buffett, B. Detection of a gravitational oscillation in length-of-day. Earth Planet. Sci. Lett. 243, 383–389 (2006).

    Article  Google Scholar 

  47. Ding, H. & Chao, B. F. A 6-year westward rotary motion in the Earth: detection and possible MICG coupling mechanism. Earth Planet. Sci. Lett. 495, 50–55 (2018).

    Article  Google Scholar 

  48. Souriau, A. in Treatise on Geophysics Vol. 1 (eds Price, G. D. & Schubert, G.) Ch. 19 (Elsevier, 2007).

  49. Roberts, P. H. & Aurnou, J. M. On the theory of core-mantle coupling. Geophys. Astrophys. Fluid Dyn. 106, 157–230 (2012).

    Article  Google Scholar 

  50. Tkalčić, H., Young, M., Bodin, T., Ngo, S. & Sambridge, M. The shuffling rotation of the Earth’s inner core revealed by earthquake doublets. Nat. Geosci. 6, 497–502 (2013).

    Article  Google Scholar 

  51. Wen, L. Intense seismic scattering near the Earth’s core mantle boundary beneath the Comoros hotspot. Geophys. Res. Lett. 27, 3627–3630 (2000).

    Article  Google Scholar 

  52. Cao, A. & Romanowicz, B. Locating scatterers in the mantle using array analysis of PKP precursors from an earthquake doublet. Earth Planet. Sci. Lett. 255, 22–31 (2007).

    Article  Google Scholar 

  53. Frost, D. A., Rost, S., Selby, N. D. & Stuart, G. W. Detection of a tall ridge at the core–mantle boundary from scattered PKP energy. Geophys. J. Int. 195, 558–574 (2013).

    Article  Google Scholar 

Download references

Acknowledgements

P. Earle recovered the LASA data from deteriorating nine‐track tapes in the 1990s, and S. Gibbons provided digital copies from his archives in 2019. Discussions with J. Aurnou, S. Ni, L. Wen and J. Yao were very helpful. This study is supported by National Science Foundation grant EAR-2041892.

Author information

Authors and Affiliations

Authors

Contributions

W.W. and J.E.V. each contributed equally to project design, methodology, data processing and manuscript preparation.

Corresponding author

Correspondence to Wei Wang.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Geoscience thanks Jessica Irving and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely, in collaboration with the Nature Geoscience team.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Observation quality as a function of magnitude and depth vs distance of the 73 events that we analyse.

Observation quality as a function of magnitude and depth vs distance of the 73 events that we analyse. PKiKP (a) & (c) and ICS (b) & (d) arrivals rated as “good”, “noisy” and “no signal” are shown as green, blue, and magenta dots, respectively.

Extended Data Fig. 2 Comparison of observed vs predicted PKiKP and PcP slownesses for all “good” events and the empirical correction of the slowness.

Comparison of observed vs predicted PKiKP and PcP slownesses for all “good” events and the empirical correction of the slowness. (a) Predicted (purple) slownesses of the well-constrained observed PKiKP (blue dots) and PcP (green dots) arrivals. Radial axis is total slowness from LASA in s/km, transverse axis is back-azimuth to the event at LASA. The blue and green dashed circles outline the slownesses of the rims of the inner and outer core, respectively. (b) Empirical correction of the slowness. The black and grey dots are PKiKP and PcP arrivals, respectively. The purple dotted line indicates no correction. The green dashed line (\(a = 0.83,\;s_0 = 0.007\)) shows the best fitting empirical correction using all PcP and PKiKP arrivals, and blue dotted line (\(a = 0.41,\;s_0 = 0.011\)) shows the best fitting correction using only PKiKP arrivals.

Extended Data Fig. 3 Lapse time after PKiKP, total slowness, and back azimuth of the scattered waves located across isodepth slices of the IC.

Lapse time after PKiKP, total slowness, and back azimuth of the waves scattered across slices at various depths of the IC. The top panel shows results for the scatterers located at the ICB, and the bottom panel shows results for the scatterers located at the depth of 300 km below the ICB. The black triangle shows the location of LASA and black dashed line indicates the outline of IC scatterers that would be visible at LASA from events everywhere. The magenta star indicates the source location, and the magenta dashed line shows the outline of the potential IC scatterers illuminated by this source that would be visible given arrays everywhere.

Extended Data Fig. 4 Illustration of application of the new technique to data.

Illustration of application of the new technique to data. This example uses the 11/27/73 event. (a) shows the seismogram (black) and envelope (cyan) from beamforming at N-S slowness 0.012 km/s and E-W slowness −0.012 km/s. The red dashed line marks energy corresponding to a particular scattering volume in the IC and the blue dashed lines delimit the time window used for the energy estimate for that lapse time and slowness vector. (b) and (c) correspond to Extended Data Figs. 3a and 3b, and the black diamond indicates the location of the resolved scatterer. (d) shows the 2D beamforming summation as a function of the slowness vector for the time window 20–100 seconds after PKiKP. The black diamond is the slowness chosen for the illustration in (a), (b), and (c). The two dashed rings indicate the slowness expected for arrivals from the rims of the inner core and outer core.

Supplementary information

Supplementary Information

Supplementary Figs. 1–10, Discussion and Table 2.

Supplementary Table 1

Information of events.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Wang, W., Vidale, J.E. An initial map of fine-scale heterogeneity in the Earth’s inner core. Nat. Geosci. 15, 240–244 (2022). https://doi.org/10.1038/s41561-022-00903-8

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41561-022-00903-8

This article is cited by

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing