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Heterogeneities of the Earth’s Inner Core Boundary from Differential Measurements of PKiKP and PcP Seismic Phases

  • Dmitry KrasnoshchekovEmail author
  • Vladimir Ovtchinnikov
Conference paper
Part of the Springer Proceedings in Earth and Environmental Sciences book series (SPEES)

Abstract

The Earth’s crystalline inner core (IC) solidifies from the liquid Fe alloy of the outer core (OC), which releases latent heat and light elements sustaining the geodynamo. Variability in solidification regime at the inner core boundary (ICB) may result in compositional and thermal multi-scale mosaic of the IC surface and dissimilarity of its hemispheres. Both the mosaic and hemisphericity are poorly constrained, not least due to a lack of available sampling by short-period reflected waves. Measured amplitude ratio of seismic phases of PKiKP and PcP reflected, respectively, off the inner and outer boundary of the liquid core, yields direct estimate of the ICB density jump. This parameter is capable of constraining the inner–outer core compositional difference and latent energy release, but is not well known (0.2–1.2 g/cub. cm), and its distribution is obscure. Travel time measurements of PKiKP and PcP waveforms can be useful in terms of getting an insight into fine structure of ICB and its topography. We analyse a new representative sample of pre-critical PKiKP/PcP differential travel times and amplitude ratios that probe the core’s spots under Southeastern Asia and South America. We observe a statistically significant systematic bias between the Asian and American measurements, and carefully examine its origin. Separating the effects of core-mantle boundary and ICB on the measured differentials is particularly challenging and we note that a whole class of physically valid models involving D″ heterogeneities and lateral variation in lower mantle attenuation can be employed to account for the observed bias. However, we find that variance in PKiKP-PcP differential travel times measured above the epicentral distance of 16° is essentially due to mantle heterogeneities. Analysis of data below this distance indicates the ICB density jump under Southeastern Asia can be about 0.3 g/cub. cm, which is three times as small as under South America where also the thickness of the above liquid core can be by 1–3 km in excess of the one in the East. The findings preclude neither IC hemispherical asymmetry (whereby crystallization dominates in the West and melting in the East) nor patchy IC surface, but provide an improved and robust estimate of the ICB density jump in two probed locations.

Keywords

Inner core boundary Reflected waves Density jump 

Notes

Acknowledgements

This work is supported by the Russian Foundation for Basic Research grant #18-05-00619. The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR-1261681. Seismic data were also obtained from Geophysical Survey of Russia (Federal research center of Russian Academy of Sciences), Data Management Center of the National Research Institute for Earth Science and Disaster Resilience (Japan) and Earthquake Research Institute, the University of Tokyo (Japan). Figure 1 was generated with Generic Mapping Tools [32].

References

  1. 1.
    Alboussiére, T., Deguen, R., Melzani, M.: Melting-induced stratification above the Earth’s inner core due to convective translation. Nature 466, 744–747 (2010)CrossRefGoogle Scholar
  2. 2.
    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)CrossRefGoogle Scholar
  3. 3.
    Bolt, B., Qamar, A.: Upper bound to the density jump at the boundary of the Earth’s inner core. Nature 228, 148–150 (1970)CrossRefGoogle Scholar
  4. 4.
    Buchbinder, G.G.R., Wright, C., Poupinet, G.: Observations of PKiKP at distances less than 110°. Bull. Seismol. Soc. Am. 63, 1699–1707 (1973)Google Scholar
  5. 5.
    de Silva, S., Cormier, V.F., Zheng, Y.: Inner core boundary topography explored with reflected and diffracted P waves. Phys. Earth Planet. Inter. 276, 202–214 (2017)Google Scholar
  6. 6.
    Dziewonski, A.M., Anderson, D.L.: Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, 297–356 (1981)CrossRefGoogle Scholar
  7. 7.
    Dziewonski, A.M., Chou, T.-A., Woodhouse, J.H.: Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852 (1981).  https://doi.org/10.1029/JB086iB04p02825CrossRefGoogle Scholar
  8. 8.
    Ekström, G., Nettles, M., Dziewonski, A.M.: The global CMT project 2004-2010: Centroid-moment tensors for 13,017 earthquakes. Phys. Earth Planet. Inter. 200–201, 1–9 (2012).  https://doi.org/10.1016/j.pepi.2012.04.002CrossRefGoogle Scholar
  9. 9.
    Fearn, D., Loper, D., Roberts, P.: Structure of the earth’s inner core. Nature 292, 232–233 (1981)CrossRefGoogle Scholar
  10. 10.
    Goldstein, P., Dodge, D., Firpo, M., Minner, L.: SAC20.0: Signal processing and analysis tools for seismologists and engineers, in the IASPEI international handbook of earthquake and engineering seismology. In: Lee W.H.K. et al. (eds.), pp. 1613–1614. Academic Press, London (2003)Google Scholar
  11. 11.
    Gubbins, D., Sreenivasan, B., Mound, J., Rost, S.: Melting of the Earth’s inner core. Nature 473, 361–363 (2011)CrossRefGoogle Scholar
  12. 12.
    Jacobs, J.A.: The Earth’s inner core. Nature 172, 297–298 (1953)CrossRefGoogle Scholar
  13. 13.
    Kennett, B., Engdahl, E., Buland, R.: Constraints on seismic velocities in the Earth from travel times. Geophys. J. Int. 122, 108–124 (1995)CrossRefGoogle Scholar
  14. 14.
    Koper, D.K., Pyle, M.L.: Observations of PKiKP/PcP amplitude ratios and implications for Earth structure at the boundaries of the liquid core. J. Geophys. Res. 109, B03301 (2004).  https://doi.org/10.1029/2003JB002750CrossRefGoogle Scholar
  15. 15.
    Krasnoshchekov, D.N., Kaazik, P.B., Ovtchinnikov, V.M.: Seismological evidence for mosaic structure of the surface of the Earth’s inner core. Nature 435, 483–487 (2005)CrossRefGoogle Scholar
  16. 16.
    Krasnoshchekov, D.N., Ovtchinnikov, V.M.: The density jump at the inner core boundary in the eastern and western hemispheres. Doklady Earth Sci. 478, Part 2, 219–223 (2018)CrossRefGoogle Scholar
  17. 17.
    Li, M., McNamara, A.K., Garnero, E.J., Yu, S.: Compositionally-distinct ultra-low velocity zones on Earth’s core-mantle boundary. Nat. Commun. 8, 177 (2017).  https://doi.org/10.1038/s4l467-017-00219-xCrossRefGoogle Scholar
  18. 18.
    Loper, D.E., Roberts, P.H.: On the motion of an iron-alloy core containing a slurry. Geophys. Astrophys. Fluid Dyn. 9(3–4), 289–321 (1978)Google Scholar
  19. 19.
    Monnereau, M., Calvet, M., Margerin, L., Souriau, A.: Lopsided growth of Earth’s inner core. Science 328, 1014–1017 (2010)CrossRefGoogle Scholar
  20. 20.
    Obara, K., Kasahara, K., Hori, S., Okada, Y.: A densely distributed high-sensitivity seismograph network in Japan: Hi-net by National Research Institute for Earth science and disaster prevention. Rev. Sci. Instrum. 76, 021301 (2005)CrossRefGoogle Scholar
  21. 21.
    Okada, Y., Kasahara, K., Hori, S., Obara, K., Sekiguchi, S., Fujiwara, H., Yamamoto, A.: Recent progress of seismic observation networks in Japan -Hi-net, F-net, K-NET and KiK-net. Earth Planets Space 56, xv–xxviii (2004)CrossRefGoogle Scholar
  22. 22.
    Ritsema, J., van Heijst, H.J., Deuss, A., Woodhouse, J.H.: S40RTS: a degree-40 shear-velocity model for the mantle from new Rayleigh wave dispersion, teleseismic traveltimes, and normal-mode splitting function measurements. Geophys. J. Int. 184, 1223–1236 (2011)CrossRefGoogle Scholar
  23. 23.
    Shen, Z., Ai, Y., He, Y., Jiang, M.: Using pre-critical PKiKP–PcP phases to constrain the regional structures of the inner core boundary beneath East Asia. Phys. Earth Planet. Inter. 252, 37–48 (2016)CrossRefGoogle Scholar
  24. 24.
    Simmons, N.A., Myers, S.C., Johannesson, G., Matzel, E.: LLNL-G3Dv3: Global P wave tomography model for improved regional and teleseismic travel time prediction. J. Geophys. Res. 117, B10302 (2012).  https://doi.org/10.1029/2012JB009525CrossRefGoogle Scholar
  25. 25.
    Stevenson, D.J.: Limits of lateral density and velocity variation in the Earth’s outer core. Geoph. J. R. Astron. Soc. 88, 311–319 (1987)CrossRefGoogle Scholar
  26. 26.
    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(B2), 2925–2938 (1997)CrossRefGoogle Scholar
  27. 27.
    Tian, D., Wen, L.: Seismological evidence for a localized mushy zone at the Earth inner core boundary. Nat. Commun. 8, 165 (2017).  https://doi.org/10.1038/s41467-017-00229-9CrossRefGoogle Scholar
  28. 28.
    Tkalčić, H., Kennett, B., Cormier, V.: On the inner-outer core density contrast from PKiKP/PcP amplitude ratios and uncertainties caused by seismic noise. Geophys. J. Int. 179, 425–443 (2009)CrossRefGoogle Scholar
  29. 29.
    Tkalčić, H., Cormier, V., Kennett, B., He, K.: Steep reflections from the Earth’s core reveal small-scale heterogeneity in the upper mantle. Phys. Earth Planet. Inter. 178, 80–91 (2010)CrossRefGoogle Scholar
  30. 30.
    Vidale, J.E., Earle, P.S.: Fine-scale heterogeneity in the Earth’s inner core. Nature 404, 273–275 (2000)CrossRefGoogle Scholar
  31. 31.
    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)CrossRefGoogle Scholar
  32. 32.
    Wessel, P., Smith, W.H.F.: New version of the generic mapping tools. EOS Trans. Am. Geophys. Union 76, 329 (1995)CrossRefGoogle Scholar

Copyright information

© Springer Nature Switzerland AG 2020

Authors and Affiliations

  1. 1.Institute of Dynamics of Geospheres, Russian Academy of SciencesMoscowRussia

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