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:

Enhanced inner core fine-scale heterogeneity towards Earth’s centre

Nature volume 620, pages 570–575 (2023)Cite this article

Subjects

Abstract

Earth’s inner core acquires texture as it solidifies within the fluid outer core. The size, shape and orientation of the mostly iron grains making up the texture record the growth of the inner core and may evolve over geologic time in response to geodynamical forces and torques1. Seismic waves from earthquakes can be used to image the texture, or fabric, of the inner core and gain insight into the history and evolution of Earth’s core2,3,4,5,6. Here, we observe and model seismic energy backscattered from the fine-scale (less than 10 km) heterogeneities7 that constitute inner core fabric at larger scales. We use a novel dataset created from a global array of small-aperture seismic arrays—designed to detect tiny signals from underground nuclear explosions—to create a three-dimensional model of inner core fine-scale heterogeneity. Our model shows that inner core scattering is ubiquitous, existing across all sampled longitudes and latitudes, and that it substantially increases in strength 500–800 km beneath the inner core boundary. The enhanced scattering in the deeper inner core is compatible with an era of rapid growth following delayed nucleation.

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

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

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

Fig. 1: Example of ICS and map of arrays.
Fig. 2: ICS energy characteristics.
Fig. 3: Inner core 3D scattering structure.
Fig. 4: Variation of scattering strength and porosity in the equatorial plane viewed from Earth’s pole.

Similar content being viewed by others

Data availability

Requests for International Monitoring System array data can be completed online at https://www.ctbto.org/specials/vdec/. Data for ASAR and WRA can be downloaded from Incorporated Research Institutions for Seismology (IRIS; www.iris.edu) with network code AU. Data for ILAR, NVAR, PDAR and TXAR are available from IRIS with the network code IM; requests for YKA data can be submitted online at https://earthquakescanada.nrcan.gc.ca/fdsnws/. The seismic data for XHDR can be downloaded from IRIS with network code XH, and the seismic data of YDWG and YDWS are available from IRIS with the network code YD in 2013–2014. The Global Centroid Moment Tensor earthquake catalogue is available at https://www.globalcmt.org/CMTsearch.html. Source data are provided with this paper.

Code availability

The seismic array processing code used in this study is available online at https://zenodo.org/record/6784342. The numerical code for the inner core compaction model is available online at https://github.com/MarineLasbleis/mushdynamics. The numerical code for the phonon simulation is available online at https://github.com/nmancinelli/psphoton. All figures were generated using pygmt (https://www.pygmt.org/latest/).

References

  1. Deguen, R. Structure and dynamics of Earth’s inner core. Earth Planet. Sci. Lett. 333, 211–225 (2012).

    Article  ADS  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  ADS  CAS  Google Scholar 

  3. Sun, X. & Song, X. Tomographic inversion for three-dimensional anisotropy of Earth’s inner core. Phys. Earth Planet. Inter. 167, 53–70 (2008).

    Article  ADS  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

  5. Souriau, A & Calvet, M. Treatise on Geophysics 2nd edn, Vol. 1 (ed. Schubert, G.) Ch. 1.23 (Elsevier, 2015).

    Google Scholar 

  6. Tkalčić, H. Complex inner core of the Earth: the last frontier of global seismology. Rev. Geophys. 53, 59–94 (2015).

    Article  ADS  Google Scholar 

  7. Vidale, J. E. & Earle, P. S. Fine-scale heterogeneity in the Earth’s inner core. Nature 404, 273–275 (2000).

    Article  ADS  CAS  PubMed  Google Scholar 

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

    Article  ADS  CAS  PubMed  Google Scholar 

  9. Calvet, M. & Margerin, L. Shape preferred orientation of iron grains compatible with Earth’s uppermost inner core hemisphericity. Earth Planet. Sci. Lett. 481, 395–403 (2018).

    Article  ADS  CAS  Google Scholar 

  10. 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  ADS  CAS  Google Scholar 

  11. 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).

    ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  13. Wang, W. & Vidale, J. E. An initial map of fine-scale heterogeneity in the Earth’s inner core. Nat. Geosci. 15, 240–244 (2022).

    Article  ADS  CAS  Google Scholar 

  14. Wu, W. & Irving, J. C. 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  ADS  Google Scholar 

  15. Wu, S. M., Pang, G., Koper, K. D. & Euler, G. A search for large‐scale variations in the fine‐scale structure of Earth’s inner core. J. Geophys. Res. Solid Earth 127, e2022JB024420 (2022).

    Article  ADS  Google Scholar 

  16. Shearer, P. M. & Earle, P. S. The global short-period wavefield modelled with a Monte Carlo seismic phonon method. Geophys. J. Int. 158, 1103–1117 (2004).

    Article  ADS  Google Scholar 

  17. Souriau, A. & Souriau, M. Ellipticity and density at the inner core boundary from subcritical PKiKP and PcP data. Geophys. J. Int. 98, 39–54 (1989).

    Article  ADS  Google Scholar 

  18. Tanaka, S. & Hamaguchi, H. Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP (BC)–PKP (DF) times. J. Geophys. Res. Solid Earth 102, 2925–2938 (1997).

    Article  Google Scholar 

  19. Irving, J. C. E. Imaging the inner core under Africa and Europe. Phys. Earth Planet. Inter. 254, 12–24 (2016).

    Article  ADS  Google Scholar 

  20. Przybilla, J., Wegler, U. & Korn, M. Estimation of crustal scattering parameters with elastic radiative transfer theory. Geophys. J. Int. 178, 1105–1111 (2009).

    Article  ADS  Google Scholar 

  21. Sato, H., Fehler, M. C. & Maeda, T. Seismic Wave Propagation and Scattering in the Heterogeneous Earth. Vol. 496, (Springer, 2012).

  22. Kennett, B. L., Engdahl, E. R. & Buland, R. Constraints on seismic velocities in the Earth from traveltimes. Geophys. J. Int. 122, 108–124 (1995).

    Article  ADS  Google Scholar 

  23. 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  ADS  CAS  PubMed  Google Scholar 

  24. Mayeda, K. & Walter, W. R. Moment, energy, stress drop, and source spectra of western United States earthquakes from regional coda envelopes. J. Geophys. Res. Solid Earth 101, 11195–11208 (1996).

    Article  Google Scholar 

  25. Beghein, C. & Trampert, J. Robust normal mode constraints on inner-core anisotropy from model space search. Science 299, 552–555 (2003).

    Article  ADS  CAS  PubMed  Google Scholar 

  26. Ishii, M. & Dziewoński, A. M. The innermost inner core of the earth: evidence for a change in anisotropic behavior at the radius of about 300 km. Proc. Natl Acad. Sci. USA 99, 14026–14030 (2002).

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  27. Stephenson, J., Tkalčić, H. & Sambridge, M. Evidence for the innermost inner core: robust parameter search for radially varying anisotropy using the Neighbourhood Algorithm. J. Geophys. Res. Solid Earth 126, e2020JB020545 (2021).

    Article  ADS  Google Scholar 

  28. Yoshida, S., Sumita, I. & Kumazawa, M. Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy. J. Geophys. Res. Solid Earth 101, 28085–28103 (1996).

    Article  CAS  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

  30. Huguet, L. Van, Orman, J. A., Hauck, S. A. II & Willard, M. A. Earth’s inner core nucleation paradox. Earth Planet. Sci. Lett. 487, 9–20 (2018).

    Article  ADS  CAS  Google Scholar 

  31. Davies, C. J., Pozzo, M. & Alfè, D. Assessing the inner core nucleation paradox with atomic-scale simulations. Earth Planet. Sci. Lett. 507, 1–9 (2019).

    Article  ADS  CAS  Google Scholar 

  32. Sun, Y., Zhang, F., Mendelev, M. I., Wentzcovitch, R. M. & Ho, K. M. Two-step nucleation of the Earth’s inner core. Proc. Natl Acad. Sci. USA 119, e2113059119 (2022).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  33. Lasbleis, M., Kervazo, M. & Choblet, G. The fate of liquids trapped during the Earth’s inner core growth. Geophys. Res. Lett. 47, e2019GL085654 (2020).

    Article  ADS  Google Scholar 

  34. Labrosse, S. Thermal evolution of the core with a high thermal conductivity. Phys. Earth Planet Inter. 247, 36–55 (2015).

    Article  ADS  Google Scholar 

  35. Ekström, G., Nettles, M. & Dziewoński, A. M. The global CMT project 2004–2010: centroid-moment tensors for 13,017 earthquakes. Phys. Earth Planet. Inter. 200–201, 1–9 (2012).

    Article  ADS  Google Scholar 

  36. Margerin, L., Campillo, M. & Van Tiggelen, B. Monte Carlo simulation of multiple scattering of elastic waves. J. Geophys. Res. Solid Earth 105, 7873–7892 (2000).

    Article  Google Scholar 

  37. Mancinelli, N. & Shearer, P. Scattered energy from a rough core‐mantle boundary modeled by a Monte Carlo seismic particle method: application to PKKP precursors. Geophys. Res. Lett. 43, 7963–7972 (2016).

    Article  ADS  Google Scholar 

  38. Takeuchi, N. et al. Determination of intrinsic attenuation in the oceanic lithosphere-asthenosphere system. Science 358, 1593–1596 (2017).

    Article  ADS  MathSciNet  CAS  PubMed  MATH  Google Scholar 

  39. Wang, W. & Shearer, P. M. Using direct and coda wave envelopes to resolve the scattering and intrinsic attenuation structure of Southern California. J. Geophys. Res. Solid Earth 122, 7236–7251 (2017).

    Article  ADS  Google Scholar 

  40. Weaver, R. L. Diffusivity of ultrasound in polycrystals. J. Mech. Phys. Solids 38, 55–86 (1990).

    Article  ADS  MathSciNet  MATH  Google Scholar 

  41. Hedlin, M. A. & Shearer, P. M. An analysis of large‐scale variations in small‐scale mantle heterogeneity using Global Seismographic Network recordings of precursors to PKP. J. Geophys. Res. Solid Earth 105, 13655–13673 (2000).

    Article  Google Scholar 

  42. Kelton, K. & Greer, A. L. Nucleation in Condensed Matter: Applications in Materials and Biology (Elsevier, 2010).

  43. Rosenfeld, D. & Woodley, W. L. Deep convective clouds with sustained supercooled liquid water down to −37.5°C. Nature 405, 440–442 (2000).

    Article  ADS  CAS  PubMed  Google Scholar 

  44. Konôpková, Z., McWilliams, R. S., Gómez-Pérez, N. & Goncharov, A. F. Direct measurement of thermal conductivity in solid iron at planetary core conditions. Nature 534, 99–101 (2016).

    Article  ADS  PubMed  Google Scholar 

  45. Ohta, K., Kuwayama, Y., Hirose, K., Shimizu, K. & Ohishi, Y. Experimental determination of the electrical resistivity of iron at Earth’s core conditions. Nature 534, 95–98 (2016).

    Article  ADS  CAS  PubMed  Google Scholar 

  46. 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  ADS  Google Scholar 

Download references

Acknowledgements

We thank N. Mancinelli for sharing the phonon code and providing helpful modelling suggestions. We thank J. Vidale for helpful comments. This work was funded by the National Science Foundation (EAR-1722542). This work is approved for public release under LA-UR-22-28862.

Author information

Author notes

  1. Guanning Pang

    Present address: Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA

  2. Sin-Mei Wu

    Present address: Swiss Seismological Service, ETH Zurich, Zurich, Switzerland

  3. Wei Wang

    Present address: Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

  4. Marine Lasbleis

    Present address: Bureau Veritas Marine & Offshore, Paris La Défense, France

Authors and Affiliations

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

    Guanning Pang, Keith D. Koper & Sin-Mei Wu

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

    Wei Wang

  3. Laboratoire de Planétologie et Géosciences, UMR 6112, Université de Nantes, CNRS, Nantes, France

    Marine Lasbleis

  4. Los Alamos National Laboratory, Los Alamos, NM, USA

    Garrett Euler

Authors
  1. Guanning Pang
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Keith D. Koper
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Sin-Mei Wu
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Wei Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Marine Lasbleis
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Garrett Euler
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

G.P. developed the array processing package gPar, analysed most array data, performed heterogeneity modelling and analysed inner core geodynamic evolution. K.D.K. supervised and acquired funding for the project and provided advice on data processing. S.-M.W. helped analyse array data. W.W. helped in the phonon scattering modelling. M.L. helped in the inner core compaction modelling. G.E. helped in funding acquisition and provided advice on data processing. All the authors contributed to the interpretation of the observations and in the preparation of the manuscript.

Corresponding author

Correspondence to Guanning Pang.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature thanks Sebastian Rost and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Additional information

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

Extended data figures and tables

Extended Data Fig. 1 Array geometries and station codes.

The triangles (blue for IMS arrays and green for Antarctica networks) are individual array elements and the dashed grey lines link the elements to the array reference points.

Extended Data Fig. 2 Example ICS observations in categories 1–3.

All three earthquakes were recorded in Chiang Mai, Thailand (CMAR) and had similar magnitudes (Mw ~6.3). The blue-dashed lines mark the predicted PKiKP arrival time and the red dashed lines mark the theoretical PKiKP slowness. Rows a through c (back azimuth, slowness, and beam amplitude) show the results from the sliding window slowness analysis, presented as a function of time relative to earthquake origin time. Row d shows envelopes of delay-and-sum beams formed at the theoretical PKiKP slowness. The red line is the best fitting curve to the background energy using the quasi-analytical coda decay model (Eq. (1)). Row e shows the denoised PKiKP envelopes as a function of time relative to PKiKP.

Extended Data Fig. 3 Overlapping PKiKP coda observations.

a, Locations of arrays AKASG (red triangle) and CMAR (blue triangle). The red star is a Category 1 earthquake recorded at AKASG and the blue stars are three Category 2 earthquakes recorded at CMAR. The solid lines are the ray paths from earthquakes to arrays and the dots are the corresponding bounce points on the ICB. These different earthquake-array combinations sample the same inner core region but different crust and mantle structure. Array response functions at 1 Hz with a slowness of 0 s/° are shown for b, AKASG and c, CMAR. d, Single (red) and stacked (blue) denoised PKiKP envelopes from AKASG and CMAR, respectively. e, Example delay-and-sum beam at the YKA seismic array in Canada. The earthquake (Mw 6.1, 2013/04/21 03:22:16 utc, 29.93°N, 138.89.62°E, 422 km depth, USGS-NEIC) occurred near Japan about 71°. Grey windows mark the signals used in f and g. f, Slowness vector analysis of the direct PKiKP phase (first grey window on e). Blue dots mark the theoretical slowness from AK135. White crosses indicate vertical incidence. Pink crosses represent bootstrap solutions determined by repeating the slowness analysis for 50 randomly resampled sets of array elements. g, Similar to f but for a time window deep in the coda (second grey window on e). Note that the coda energy has a lower ray parameter than the direct PKiKP wave but with a similar back azimuth, implying that it is arriving from great circle heterogeneity deep within the inner core beneath the PKiKP bounce point.

Extended Data Fig. 4 Geographic regions sampled in this study.

Names of regions located in the quasi-eastern hemisphere (longitude > 0) start with E. Names of regions in the quasi-western hemisphere (longitude <0) start with W. The colour is the number of earthquakes that sampled the region.

Extended Data Fig. 5 Observations and predictions of inner core scattered (ICS) energy.

The red lines are ICS envelope stacks sampling different inner core geographic regions (shown in Extended Data Fig. 4) observed at various arrays. The blue lines are the corresponding best-fitting simulations from the multiple scattering phonon approach.

Extended Data Fig. 6 Trade-off between a and ε.

a, Backscattered energy (D−1.5) as a function of correlation length (a) and root-mean-square velocity perturbation (ε). Stars mark the (a, ε) pairs used in generating synthetic envelopes shown in b. b, Synthetic envelopes from scattering models with the same diffusivity D but different (a, ε) pairs marked as stars in a.

Extended Data Fig. 7 Phonon models and model fitting algorithm.

a, A demonstration of the model fit using the algorithm in Eq. 4. The grey line is the observed ICS envelope. The red line is the best-fitting model prediction, and the blue line is a model prediction mismatching the observation by a factor of 4. b, All inner core heterogeneity models in this study. Green lines are the Class 1 models with uniform diffusivities. Red lines are the Class 2 models with stronger scattering strength in deep inner core. Blue lines are Class 3 models with weaker scattering with depth. The grey line is the same observation as in a. The brown line is the best-fit model.

Extended Data Fig. 8 Inner core two-stage growth model.

a, Inner core delayed nucleation and two-stage growth model. The inner core growth is in two steps: I. initial rapid growth stage (red segment); and II. Steady growth stage (blue). The right end of the x-axis is the present time. The left end, i.e., 0, is the moment when the centre of the primordial outer core cooled below its melting point. The y-axis is the normalized inner core radius at time t. RICB is the present inner core radius. The radius at the end of step I is a function of the supercooling temperature, which is shown in b. b, The initial inner core radius at the end of the early rapid growth step (step I) as a function of the supercooling temperature at historical Earth centre, modified from Huguet et al.30. The grey zone denotes the historical inner core radius at the end of Step I in the porosity evolution with delayed nucleation at 0.07–0.12τic, where τic is the presumed inner core age in the conventional growth model (dashed line in a).

Extended Data Fig. 9 Scattering strength and porosity along the radius of the inner core in the present day.

a, Global average scattering variation along the inner core radius. Inner core porosity structure based on three growth models: b, conventional inner core growth34, c, inner core nucleation delayed to 0.12τic, and d, inner core nucleation delayed nucleation delayed to 0.18τic. Yellow lines indicate 500 km below ICB, where the scattering strength increases globally.

Extended Data Table 1 Arrays in this study
Full size table

Supplementary information

Source data

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Pang, G., Koper, K.D., Wu, SM. et al. Enhanced inner core fine-scale heterogeneity towards Earth’s centre. Nature 620, 570–575 (2023). https://doi.org/10.1038/s41586-023-06213-2

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41586-023-06213-2

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced 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