1932

Abstract

Understanding how Earth's inner core (IC) develops and evolves, including fine details of its structure and energy exchange across the boundary with the liquid outer core, helps us constrain its age, relationship with the planetary differentiation, and other significant global events throughout Earth's history, as well as the changing magnetic field. Since its discovery in 1936 and the solidity hypothesis in 1940, Earth's IC has never ceased to inspire geoscientists. However, while there are many seismological observations of compressional waves and normal modes sensitive to the IC's compressional and shear structure, the shear waves that provide direct evidence for the IC's solidity have remained elusive and have been reported in only a few publications. Further advances in the emerging correlation-wavefield paradigm, which explores waveform similarities, may hold the keys to refined measurements of all IC shear properties, informing dynamical models and strengthening interpretations of the IC's anisotropic structure and viscosity.

  • ▪  What are the shear properties of the IC, such as the shear-wave speed, shear modulus, shear attenuation, and shear-wave anisotropy?
  • ▪  Can the shear properties be measured seismologically and confirmed experimentally?

Loading

Article metrics loading...

/content/journals/10.1146/annurev-earth-071521-063942
2022-05-31
2024-04-14
Loading full text...

Full text loading...

/deliver/fulltext/earth/50/1/annurev-earth-071521-063942.html?itemId=/content/journals/10.1146/annurev-earth-071521-063942&mimeType=html&fmt=ahah

Literature Cited

  1. Al-Attar D, Woodhouse JH. 2008. Calculation of seismic displacement fields in self-gravitating earth models—applications of minors vectors and symplectic structure. Geophys. J. Int. 175:31176–208
    [Google Scholar]
  2. Alterman Z, Jarosch H, Pekeris CL, Jeffreys H. 1959. Oscillations of the Earth. Proc. R. Soc. A 252:126880–95
    [Google Scholar]
  3. Anderson DL, Hart RS 1978. Attenuation models of the earth. Phys. Earth Planet. Inter. 16:4289–306
    [Google Scholar]
  4. Andrews J, Deuss A, Woodhouse J 2006. Coupled normal-mode sensitivity to inner-core shear velocity and attenuation. Geophys. J. Int. 167:1204–12
    [Google Scholar]
  5. Antonangeli D, Merkel S, Farber DL 2006. Elastic anisotropy in hcp metals at high pressure and the sound wave anisotropy of the Earth's inner core. Geophys. Res. Lett. 33:24L24303
    [Google Scholar]
  6. Antonangeli D, Occelli F, Requardt H, Badro J, Fiquet G, Krisch M 2004. Elastic anisotropy in textured hcp-iron to 112 GPa from sound wave propagation measurements. Earth Planet. Sci. Lett. 225:1243–51
    [Google Scholar]
  7. Attanayake J, Cormier VF, de Silva SM. 2014. Uppermost inner core seismic structure—new insights from body waveform inversion. Earth Planet. Sci. Lett. 385:49–58
    [Google Scholar]
  8. Aubert J, Amit H, Hulot G, Olson P. 2008. Thermochemical flows couple the Earth's inner core growth to mantle heterogeneity. Nature 454:7205758–61
    [Google Scholar]
  9. Backus GE. 1965. Possible forms of seismic anisotropy of the uppermost mantle under oceans. J. Geophys. Res. 70:143429–39
    [Google Scholar]
  10. Belonoshko AB, Fu J, Bryk T, Simak SI, Mattesini M. 2019. Low viscosity of the Earth's inner core. Nat. Commun. 10:12483
    [Google Scholar]
  11. Belonoshko AB, Skorodumova NV, Davis S, Osiptsov AN, Rosengren A, Johansson B 2007. Origin of the low rigidity of the Earth's inner core. Science 316:58311603–5
    [Google Scholar]
  12. Belonoshko AB, Skorodumova NV, Rosengren A, Johansson B 2008. Elastic anisotropy of Earth's inner core. Science 319:5864797–800
    [Google Scholar]
  13. Benioff H, Press F, Smith S 1961. Excitation of the free oscillations of the Earth by earthquakes. J. Geophys. Res. 66:2605–19
    [Google Scholar]
  14. Bergman MI. 1997. Measurements of electric anisotropy due to solidification texturing and the implications for the Earth's inner core. Nature 389:664660–63
    [Google Scholar]
  15. Biggin AJ, Piispa EJ, Pesonen LJ, Holme R, Paterson GA et al. 2015. Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation. Nature 526:7572245–48
    [Google Scholar]
  16. Birch F. 1940. The alpha-gamma transformation of iron at high pressures, and the problem of the earth's magnetism. Am. J. Sci. 238:3192–211
    [Google Scholar]
  17. Birch F. 1952. Elasticity and constitution of the Earth's interior. J. Geophys. Res. 57:2227–86
    [Google Scholar]
  18. Braginsky SI. 1963. Structure of the F layer and reasons for convection in the Earth's core. Soviet Phys. Dokl. 149:8–10
    [Google Scholar]
  19. Buffett BA, Huppert HE, Lister JR, Woods AW. 1996. On the thermal evolution of the Earth's core. J. Geophys. Res. 101:B47989–8006
    [Google Scholar]
  20. Buffett BA, Wenk H-R. 2001. Texturing of the Earth's inner core by Maxwell stresses. Nature 413:685160–63
    [Google Scholar]
  21. Bullen KE. 1946. A hypothesis on compressibility at pressures of the order of a million atmospheres. Nature 157:3987405
    [Google Scholar]
  22. Bullen KE. 1950. Theoretical travel-times of S waves in the Earth's inner core. Geophys. J. Int. 6:s2125–28
    [Google Scholar]
  23. Bullen KE. 1951. Theoretical amplitudes of the seismic phase PKJKP. Geophys. J. Int. 6:s3163–67
    [Google Scholar]
  24. Calvet M, Margerin L. 2008. Constraints on grain size and stable iron phases in the uppermost inner core from multiple scattering modeling of seismic velocity and attenuation. Earth Planet. Sci. Lett. 267:1200–12
    [Google Scholar]
  25. Cao A, Romanowicz B. 2009. Constraints on shear wave attenuation in the Earth's inner core from an observation of PKJKP. Geophys. Res. Lett. 36:9L09301
    [Google Scholar]
  26. Cao A, Romanowicz B, Takeuchi N. 2005. An observation of PKJKP: inferences on inner core shear properties. Science 308:57271453–55
    [Google Scholar]
  27. Cormier VF. 2015. Detection of inner core solidification from observations of antipodal PKIIKP. Geophys. Res. Lett. 42:187459–66
    [Google Scholar]
  28. Cormier VF 2020. Seismic viscoelastic attenuation. Encyclopaedia of Solid Earth Geophysics HK Gupta 1–14 Cham, Switz: Springer
    [Google Scholar]
  29. Crampin S. 1977. A review of the effects of anisotropic layering on the propagation of seismic waves. Geophys. J. Int. 49:19–27
    [Google Scholar]
  30. Creager KC. 1999. Large-scale variations in inner core anisotropy. J. Geophys. Res. 104:B1023127–39
    [Google Scholar]
  31. Dahlen FA, Tromp J. 1998. Theoretical Global Seismology Princeton, NJ: Princeton Univ. Press
  32. de Wit RWL, Käufl PJ, Valentine AP, Trampert J. 2014. Bayesian inversion of free oscillations for Earth's radial (an)elastic structure. Phys. Earth Planet. Inter. 237:1–17
    [Google Scholar]
  33. Deguen R. 2012. Structure and dynamics of Earth's inner core. Earth Planet. Sci. Lett. 333–334:211–25
    [Google Scholar]
  34. Derr JS. 1969. Internal structure of the Earth inferred from free oscillations. J. Geophys. Res. 74:225202–20
    [Google Scholar]
  35. Deuss A. 2014. Heterogeneity and anisotropy of Earth's inner core. Annu. Rev. Earth Planet. Sci. 42:103–26
    [Google Scholar]
  36. Deuss A, Woodhouse JH, Paulssen H, Trampert J. 2000. The observation of inner core shear waves. Geophys. J. Int. 142:167–73
    [Google Scholar]
  37. Doornbos DJ. 1974. The anelasticity of the inner core. Geophys. J. Int. 38:2397–415
    [Google Scholar]
  38. Dziewoński AM. 1971. Overtones of free oscillations and the structure of the Earth's interior. Science 172:39901336–38
    [Google Scholar]
  39. Dziewoński AM, Anderson DL. 1981. Preliminary reference Earth model. Phys. Earth Planet. Inter. 25:4297–356
    [Google Scholar]
  40. Dziewoński AM, Gilbert F. 1971. Solidity of the inner core of the Earth inferred from normal mode observations. Nature 234:5330465–66
    [Google Scholar]
  41. Efron B, Tibshirani R. 1991. Statistical data analysis in the computer age. Science 253:5018390–95
    [Google Scholar]
  42. Fischer RA, Campbell AJ. 2015. The axial ratio of hcp Fe and Fe–Ni–Si alloys to the conditions of Earth's inner core. Am. Mineral. 100:11–122718–24
    [Google Scholar]
  43. Frost DA, Lasbleis M, Chandler B, Romanowicz B 2021. Dynamic history of the inner core constrained by seismic anisotropy. Nat. Geosci. 14:531–35
    [Google Scholar]
  44. Fukao Y, Suda N. 1989. Core modes of the Earth's free oscillations and structure of the inner core. Geophys. Res. Lett. 16:5401–4
    [Google Scholar]
  45. Gubbins D, Sreenivasan B, Mound J, Rost S. 2011. Melting of the Earth's inner core. Nature 473:7347361–63
    [Google Scholar]
  46. Hirose K, Labrosse S, Hernlund J 2013. Composition and state of the core. Annu. Rev. Earth Planet. Sci. 41:657–91
    [Google Scholar]
  47. Hollerbach R, Jones CA. 1993. Influence of the Earth's inner core on geomagnetic fluctuations and reversals. Nature 365:6446541–43
    [Google Scholar]
  48. Iritani R, Takeuchi N, Kawakatsu H. 2014. Intricate heterogeneous structures of the top 300 km of the Earth's inner core inferred from global array data: I. Regional 1D attenuation and velocity profiles. Phys. Earth Planet. Inter. 230:15–27
    [Google Scholar]
  49. Jeanloz R, Wenk H-R. 1988. Convection and anisotropy of the inner core. Geophys. Res. Lett. 15:172–75
    [Google Scholar]
  50. Julian BR, Davies D, Sheppard RM. 1972. PKJKP. Nature 235:5337317–18
    [Google Scholar]
  51. Karato S. 1993. Inner core anisotropy due to the magnetic field—induced preferred orientation of iron. Science 262:51401708–11
    [Google Scholar]
  52. Karato S. 1999. Seismic anisotropy of the Earth's inner core resulting from flow induced by Maxwell stresses. Nature 402:6764871–73
    [Google Scholar]
  53. Karato S. 2008. Deformation of Earth Materials: An Introduction to the Rheology of Solid Earth Cambridge, UK: Cambridge Univ. Press
  54. Kennett BLN. 2020. Radial earth models revisited. Geophys. J. Int. 222:32189–204
    [Google Scholar]
  55. Kennett BLN, Engdahl ER, Buland R. 1995. Constraints on seismic velocities in the Earth from traveltimes. Geophys. J. Int. 122:1108–24
    [Google Scholar]
  56. Kennett BLN, Fichtner A. 2021. Exploiting Seismic Waveforms: Correlations, Heterogeneity and Inversion Cambridge, UK: Cambridge Univ. Press
  57. Kennett BLN, Stipčević J, Gorbatov A. 2015. Spiral-arm seismic arrays. Bull. Seismol. Soc. Am. 105:42109–16
    [Google Scholar]
  58. Krasnoshchekov DN, Ovtchinnikov VM, Usoltseva OA. 2019. Shear wave velocity in the top of the Earth's inner core. Dokl. Earth Sci. 488:21186–89
    [Google Scholar]
  59. Lehmann I. 1936. P′. Publ. Bur. Cent. Seismol. Int. Sér. A Trav. Sci 14:87–115
    [Google Scholar]
  60. Li X, Cormier VF 2002. Frequency-dependent seismic attenuation in the inner core, 1. A viscoelastic interpretation. J. Geophys. Res. 107:B122361
    [Google Scholar]
  61. Lin J-F, Mao Z, Yavaş H, Zhao J, Dubrovinsky L. 2010. Shear wave anisotropy of textured hcp-Fe in the Earth's inner core. Earth Planet. Sci. Lett. 298:3361–66
    [Google Scholar]
  62. Mao H, Shu J, Shen G, Hemley RJ, Li B, Singh AK 1998. Elasticity and rheology of iron above 220 GPa and the nature of the Earth's inner core. Nature 396:6713741–43
    [Google Scholar]
  63. Mao WL, Campbell AJ, Heinz DL, Shen G. 2006. Phase relations of Fe–Ni alloys at high pressure and temperature. Phys. Earth Planet. Inter. 155:1146–51
    [Google Scholar]
  64. Masters G, Gilbert F. 1981. Structure of the inner core inferred from observations of its spheroidal shear modes. Geophys. Res. Lett. 8:6569–71
    [Google Scholar]
  65. Masters TG, Shearer PM. 1990. Summary of seismological constraints on the structure of the Earth's core. J. Geophys. Res. 95:B1321691–95
    [Google Scholar]
  66. Mattesini M, Belonoshko AB, Buforn E, Ramírez M, Simak SI et al. 2010. Hemispherical anisotropic patterns of the Earth's inner core. PNAS 107:219507–12
    [Google Scholar]
  67. Mattesini M, Belonoshko AB, Tkalčić H, Buforn E, Udias A, Ahuja R 2014. Candy wrapper for the Earth's inner core. Sci. Rep. 3:2096
    [Google Scholar]
  68. Montagner J-P, Kennett BLN. 1996. How to reconcile body-wave and normal-mode reference earth models. Geophys. J. Int. 125:1229–48
    [Google Scholar]
  69. Morelli A, Dziewoński AM, Woodhouse JH. 1986. Anisotropy of the inner core inferred from PKIKP travel times. Geophys. Res. Lett. 13:131545–48
    [Google Scholar]
  70. Nissen-Meyer T, van Driel M, Stähler SC, Hosseini K, Hempel S et al. 2014. AxiSEM: broadband 3-D seismic wavefields in axisymmetric media. Solid Earth 5:425–45
    [Google Scholar]
  71. Okal EA, Cansi Y. 1998. Detection of PKJKP at intermediate periods by progressive multi-channel correlation. Earth Planet. Sci. Lett. 164:123–30
    [Google Scholar]
  72. Pejić T, Hawkins R, Sambridge M, Tkalčić H. 2019. Transdimensional Bayesian attenuation tomography of the upper inner core. J. Geophys. Res. Solid Earth 124:21929–43
    [Google Scholar]
  73. Pejić T, Tkalčić H, Sambridge M, Cormier VF, Benavente R. 2017. Attenuation tomography of the upper inner core. J. Geophys. Res. Solid Earth 122:43008–32
    [Google Scholar]
  74. Phạm T-S, Tkalčić H, Sambridge M, Kennett BLN. 2018. Earth's correlation wavefield: late coda correlation. Geophys. Res. Lett. 45:73035–42
    [Google Scholar]
  75. Poupinet G, Pillet R, Souriau A 1983. Possible heterogeneity of the Earth's core deduced from PKIKP travel times. Nature 305:5931204–6
    [Google Scholar]
  76. Resovsky J, Trampert J, Van der Hilst RD. 2005. Error bars for the global seismic Q profile. Earth Planet. Sci. Lett. 230:3413–23
    [Google Scholar]
  77. Roberts AP. 2008. Geomagnetic excursions: knowns and unknowns. Geophys. Res. Lett. 35:17L17307
    [Google Scholar]
  78. Romanowicz BA, Mitchell BJ 2015. Deep Earth structure: Q of the Earth from crust to core. Treatise on Geophysics G Schubert 789–827 Oxford, UK: Elsevier. , 2nd ed..
    [Google Scholar]
  79. Rost S, Thomas C. 2002. Array seismology: methods and applications. Rev. Geophys. 40:31008
    [Google Scholar]
  80. Shearer PM. 2009. Introduction to Seismology Cambridge, UK: Cambridge Univ. Press. , 2nd ed..
  81. Shearer PM, Rychert CA, Liu Q. 2011. On the visibility of the inner-core shear wave phase PKJKP at long periods. Geophys. J. Int. 185:31379–83
    [Google Scholar]
  82. Singh SC, Taylor MAJ, Montagner JP 2000. On the presence of liquid in Earth's inner core. Science 287:54622471–74
    [Google Scholar]
  83. Song X. 1997. Anisotropy of the Earth's inner core. Rev. Geophys. 35:3297–313
    [Google Scholar]
  84. Souriau A, Calvet M 2015. Deep Earth structure: the Earth's cores. Treatise on Geophysics G Schubert 725–57 Oxford, UK: Elsevier. , 2nd ed..
    [Google Scholar]
  85. Steinle-Neumann G, Stixrude L, Cohen RE, Gülseren O. 2001. Elasticity of iron at the temperature of the Earth's inner core. Nature 413:685157–60
    [Google Scholar]
  86. Stipčević J, Kennett BLN, Tkalčić H. 2017. Simultaneous use of multiple seismic arrays. Geophys. J. Int. 209:2770–83
    [Google Scholar]
  87. Stixrude L, Cohen RE. 1995. High-pressure elasticity of iron and anisotropy of Earth's inner core. Science 267:52061972–75
    [Google Scholar]
  88. Sumita I, Bergman M 2015. Inner core dynamics. Treatise on Geophysics G Schubert 297–316 Oxford, UK: Elsevier. , 2nd ed..
    [Google Scholar]
  89. Talavera-Soza S, Deuss A 2020. Constraining 1-D inner core attenuation through measurements of strongly coupled normal mode pairs. Geophys. J. Int. 223:1612–21
    [Google Scholar]
  90. Tkalčić H. 2010. Large variations in travel times of mantle-sensitive seismic waves from the South Sandwich Islands: Is the Earth's inner core a conglomerate of anisotropic domains?. Geophys. Res. Lett. 37:14L14312
    [Google Scholar]
  91. Tkalčić H. 2015. Complex inner core of the Earth: the last frontier of global seismology. Rev. Geophys. 53:159–94
    [Google Scholar]
  92. Tkalčić H. 2017. The Earth's Inner Core: Revealed by Observational Seismology New York: Cambridge Univ. Press
  93. Tkalčić H, Phạm T-S. 2018. Shear properties of Earth's inner core constrained by a detection of J waves in global correlation wavefield. Science 362:6412329–32
    [Google Scholar]
  94. Tkalčić H, Phạm T-S. 2020. Excitation of the global correlation wavefield by large earthquakes. Geophys. J. Int. 223:31769–79
    [Google Scholar]
  95. Tkalčić H, Phạm T-S, Wang S 2020. The Earth's coda correlation wavefield: rise of the new paradigm and recent advances. Earth-Sci. Rev. 208:103285
    [Google Scholar]
  96. Vallée M, Douet V. 2016. A new database of source time functions (STFs) extracted from the SCARDEC method. Phys. Earth Planet. Inter. 257:149–57
    [Google Scholar]
  97. Vočadlo L. 2007. Ab initio calculations of the elasticity of iron and iron alloys at inner core conditions: evidence for a partially molten inner core?. Earth Planet. Sci. Lett. 254:1227–32
    [Google Scholar]
  98. Vočadlo L, Alfè D, Gillan MJ, Wood IG, Brodholt JP, Price GD. 2003. Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core. Nature 424:6948536–39
    [Google Scholar]
  99. Vočadlo L, Dobson DP, Wood IG. 2009. Ab initio calculations of the elasticity of hcp-Fe as a function of temperature at inner-core pressure. Earth Planet. Sci. Lett. 288:3534–38
    [Google Scholar]
  100. Wang S, Tkalčić H 2020. Seismic event coda-correlation's formation: implications for global seismology. Geophys. J. Int. 222:21283–94
    [Google Scholar]
  101. Wang S, Tkalčić H 2021. Shear-wave anisotropy in the Earth's inner core. Geophys. Res. Lett. 48:19e2021GL094784
    [Google Scholar]
  102. Waszek L, Deuss A. 2015. Observations of exotic inner core waves. Geophys. J. Int. 200:31636–50
    [Google Scholar]
  103. Wenk H-R, Baumgardner JR, Lebensohn RA, Tomé CN. 2000b. A convection model to explain anisotropy of the inner core. J. Geophys. Res. 105:B35663–77
    [Google Scholar]
  104. Wenk H-R, Matthies S, Hemley RJ, Mao H-K, Shu J. 2000a. The plastic deformation of iron at pressures of the Earth's inner core. Nature 405:67901044–47
    [Google Scholar]
  105. Widmer R, Masters G, Gilbert F. 1991. Spherically symmetric attenuation within the Earth from normal mode data. Geophys. J. Int. 104:3541–53
    [Google Scholar]
  106. Woodhouse JH, Giardini D, Li X-D. 1986. Evidence for inner core anisotropy from free oscillations. Geophys. Res. Lett. 13:131549–52
    [Google Scholar]
  107. Wookey J, Helffrich G. 2008. Inner-core shear-wave anisotropy and texture from an observation of PKJKP waves. Nature 454:7206873–76
    [Google Scholar]
  108. Yoshida S, Sumita I, Kumazawa M. 1996. Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy. J. Geophys. Res. 101:B1228085–103
    [Google Scholar]
/content/journals/10.1146/annurev-earth-071521-063942
Loading
/content/journals/10.1146/annurev-earth-071521-063942
Loading

Data & Media loading...

  • Article Type: Review Article
This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error