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
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 12 print issues and online access
$259.00 per year
only $21.58 per issue
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
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
Vidale, J. E., Dodge, D. A. & Earle, P. S. Fine-scale heterogeneity in the Earth’s inner core. Nature 405, 445–448 (2000).
Cormier, V. F. Texture of the uppermost inner core from forward- and back-scattered seismic waves. Earth Planet. Sci. Lett. 258, 442–453 (2007).
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).
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).
Monnereau, M., Calvet, M., Margerin, L. & Souriau, A. Lopsided growth of Earth’s inner core. Science 328, 1014–1017 (2010).
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).
Hedlin, M. A. H., Earle, P. S. & Bolton, H. Old seismic data yield new insights. Eos 81, 469–473 (2000).
Frosch, R. A. & Green, P. E. The concept of the large aperture seismic array. Proc. R. Soc. A 290, 368–384 (1966).
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).
Vidale, J. E. Very slow rotation of Earth’s inner core from 1971 to 1974. Geophys. Res. Lett. 46, 9483–9488 (2019).
Sato, H., Fehler, M. C. & Maeda, T. Seismic Wave Propagation and Scattering in the Heterogeneous Earth (Springer, 2012).
Deuss, A. Heterogeneity and anisotropy of Earth’s inner core. Annu. Rev. Earth Planet. Sci. 42, 103–126 (2014).
Niu, F. & Wen, L. Hemispherical variations in seismic velocity at the top of the Earth’s inner core. Nature 410, 1081–1084 (2001).
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).
Yu, W. C. et al. The inner core hemispheric boundary near 180° W. Phys. Earth Planet. Inter. 272, 1–16 (2017).
Dai, Z., Wang, W. & Wen, L. Irregular topography at the Earth’s inner core boundary. Proc. Natl Acad. Sci. USA 109, 7654–7658 (2012).
Cao, A., Masson, Y. & Romanowicz, B. Short wavelength topography on the inner-core boundary. Proc. Natl Acad. Sci. USA 104, 31–35 (2007).
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).
Ibourichene, A. & Romanowicz, B. Detection of small-scale heterogeneities at the inner core boundary. Phys. Earth Planet. Inter. 281, 55–67 (2018).
Wen, L. Localized temporal change of the Earth’s inner core boundary. Science 314, 967–971 (2006).
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).
Tian, D. & Wen, L. Seismological evidence for a localized mushy zone at the Earth’s inner core boundary. Nat. Commun. 8, 165 (2017).
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).
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).
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).
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).
Bergman, M. I., Agrawal, S. & Carter, M. S. Transverse solidification textures in hexagonal close-packed alloys. J. Cryst. Growth 255, 204–211 (2003).
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).
Vočadlo, L. in Treatise on Geophysics Vol. 2 (eds Price, G. D. & Schubert, G.) Ch. 5 (Elsevier, 2007).
Fearn, D., Loper, D. & Roberts, P. Structure of the Earth’s inner core. Nature 292, 232–233 (1981).
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).
Gubbins, D., Sreenivasan, B., Mound, J. & Rost, S. Melting of the Earths inner core. Nature 473, 361–364 (2011).
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).
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).
Burdick, S., Waszek, L. & Lekić, V. Seismic tomography of the uppermost inner core. Earth Planet. Sci. Lett. 528, 115789 (2019).
Alboussiëre, T., Deguen, R. & Melzani, M. Melting-induced stratification above the Earth’s inner core due to convective translation. Nature 466, 744–747 (2010).
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).
Sumita, I. & Olson, P. A laboratory model for convection in Earth’s core driven by a thermally heterogeneous mantle. Science 286, 1547–1549 (1999).
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).
Davies, C. J. & Mound, J. E. Mantle-induced temperature anomalies do not reach the inner core boundary. Geophys. J. Int. 218, 2054–2065 (2019).
French, S. & Romanowicz, B. Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots. Nature 525, 95–99 (2015).
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).
Venet, L., Duffar, T. & Deguen, R. Grain structure of the Earth’s inner core. C. R. Geosci. 341, 513–516 (2009).
Deguen, R. & Cardin, P. Tectonic history of the Earth’s inner core preserved in its seismic structure. Nat. Geosci. 2, 419–422 (2009).
Song, X. & Richards, P. G. Seismological evidence for differential rotation of the Earth’s inner core. Nature 382, 221–224 (1996).
Mound, J. & Buffett, B. Detection of a gravitational oscillation in length-of-day. Earth Planet. Sci. Lett. 243, 383–389 (2006).
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).
Souriau, A. in Treatise on Geophysics Vol. 1 (eds Price, G. D. & Schubert, G.) Ch. 19 (Elsevier, 2007).
Roberts, P. H. & Aurnou, J. M. On the theory of core-mantle coupling. Geophys. Astrophys. Fluid Dyn. 106, 157–230 (2012).
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).
Wen, L. Intense seismic scattering near the Earth’s core mantle boundary beneath the Comoros hotspot. Geophys. Res. Lett. 27, 3627–3630 (2000).
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).
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).
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
Contributions
W.W. and J.E.V. each contributed equally to project design, methodology, data processing and manuscript preparation.
Corresponding author
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
About this article
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
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/s41561-022-00903-8