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.

  • Letter
  • Published:

Hemispherical variations in seismic velocity at the top of the Earth's inner core

Nature volume 410, pages 1081–1084 (2001)Cite this article

Abstract

Knowledge of the seismic velocity structure at the top of the Earth's inner core is important for deciphering the physical processes responsible for inner-core growth1,2,3. Previous global seismic studies4,5,6,7,8,9 have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions10,11,12. Here we present constraints on seismic velocity variations just beneath the inner-core boundary, determined from the difference in travel time between waves reflected at the inner-core boundary and those transmitted through the inner core. We found that these travel-time residuals—observed on both global seismograph stations and several regional seismic networks—are systematically larger, by about 0.8 s, for waves that sample the ‘eastern hemisphere’ of the inner core (40° E to 180° E) compared to those that sample the ‘western hemisphere’ (180° W to 40° E). These residuals show no correlation with the angle at which the waves traverse the inner core; this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.

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

Figure 1: Map view of PKiKP minus PKIKP travel-time residuals displayed as lines along ray segments through the inner core and symbols at the turning points.
Figure 2: PKiKP - PKIKP travel-time residuals as a function of turning depth and angle.
Figure 3: Examples of seismograms.
Figure 4: Distribution of hit points of PKIKP and PKiKP rays at the CMB.

Similar content being viewed by others

References

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

    Article  ADS  CAS  Google Scholar 

  2. Anderson, O. L. Mineral physics of iron and of the core. Rev. Geophys. 33, 429–441 (1995).

    Article  ADS  Google Scholar 

  3. Boehler, R. Temperatures in the Earth's core from melting point measurements of iron at high static pressure. Nature 363, 534–536 (1993).

    Article  ADS  CAS  Google Scholar 

  4. Morelli, A., Dziewonski, A. M. & Woodhouse, J. H. Anisotropy of the core inferred from PKIKP travel times. Geophys. Res. Lett. 13, 1545–1548 (1986).

    Article  ADS  Google Scholar 

  5. Shearer, P. M., Toy, K. M. & Orcutt, J. A. Axi-symmetric Earth models and inner-core anisotropy. Nature 333, 228–232 (1988).

    Article  ADS  Google Scholar 

  6. Creager, K. C. Anisotropy of the inner core from differential travel times of the phases PKP and PKIKP. Nature 356, 309–314 (1992).

    Article  ADS  Google Scholar 

  7. Tromp, J. Support for anisotropy of the Earth's inner core from oscillations. Nature 366, 678–681 (1993).

    Article  ADS  Google Scholar 

  8. 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. 102, 2925–2938 (1997).

    Article  ADS  Google Scholar 

  9. Creager, K. C. Large-scale variations in inner core anisotropy. J. Geophys. Res. 104, 23127–23139 (1999).

    Article  ADS  Google Scholar 

  10. Cormier, V. F. & Choy, G. L. A search for lateral heterogeneity in the inner core from differential travel times near PKP-D and PKP-C. Geophys. Res. Lett. 13, 1553–1556 (1986).

    Article  ADS  Google Scholar 

  11. Song, X. & Helmberger, D. V. Depth dependence of anisotropy of the Earth's inner core. J. Geophys. Res. 100, 9805–9816 (1995).

    Article  ADS  Google Scholar 

  12. Kaneshima, S. Mapping heterogeneity of the uppermost inner core using two pairs of core phases. Geophys. Res. Lett. 23, 3075–3078 (1996).

    Article  ADS  Google Scholar 

  13. Dziewonski, A. M. & Anderson, D. L. Preliminary reference Earth model. Phys. Earth Planet. Int. 25, 297–356 (1981).

    Article  ADS  Google Scholar 

  14. Wen, L. & Niu, F. Seismic velocity and attenuation structures in the top of the Earth's inner core. J. Geophys. Res. (submitted).

  15. Helmberger, D. V. in Earthquakes: Observation, Theory and Interpretation (ed. Kanomori, H.) 173–222 (Soc. Italiana Di Fisica, Bologna, 1983).

    Google Scholar 

  16. 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  CAS  Google Scholar 

  17. J-Array Group. The J-Array program: system and present status. J. Geomag. Geoelectr. 45, 1265–1274 (1993).

    Article  ADS  Google Scholar 

Download references

Acknowledgements

We are grateful to the IRIS, J-array Data Center, W. Shannon and L. Saumure for supplying the data. We thank S. Tanaka for providing us their PKP-PKIKP data. We also thank H. Kawakatsu, S. Sacks and P. Silver for comments on the manuscript. Discussions with J. Aurnou, M. Fouch, W. Holt, S. Solomon, I. Sumita, S. Tanaka and D. Weidner were helpful in preparing the manuscript. This work is supported by the fellowship of the Carnegie Institution of Washington and an NSF grant.

Author information

Authors and Affiliations

  1. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, 20015, DC, USA

    Fenglin Niu

  2. Department of Geosciences, State University of New York at Stony Brook, Stony Brook, 11794, New York, USA

    Lianxing Wen

Authors
  1. Fenglin Niu
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Lianxing Wen
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Fenglin Niu.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Niu, F., Wen, L. Hemispherical variations in seismic velocity at the top of the Earth's inner core. Nature 410, 1081–1084 (2001). https://doi.org/10.1038/35074073

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/35074073

This article is cited by

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