Earth’s solid inner core: Seismic implications of freezing and melting

Abstract

Seismic P velocity structure is determined for the upper 500 km of the inner core and lowermost 200 km of the outer core from differential travel times and amplitude ratios. Results confirm the existence of a globally uniform F region of reduced P velocity gradient in the lowermost outer core, consistent with iron enrichment near the boundary of a solidifying inner core. P velocity of the inner core between the longitudes 45°E and 180°E (quasi-Eastern Hemisphere) is greater than or equal to that of an AK135-F reference model whereas that between 180°W and 45°E (quasi-Western Hemisphere) is less than that of the reference model. Observation of this heterogeneity to a depth of 550 km below the inner core and the existence of transitions rather than sharp boundaries between quasi-hemispheres favor either no or very slow inner core super rotation or oscillations with respect to the mantle. Degree-one seismic heterogeneity may be best explained by active inner core freezing beneath the equatorial Indian Ocean dominating structure in the quasi-Eastern Hemisphere and inner core melting beneath equatorial Pacific dominating structure in the quasi-Western Hemisphere. Variations in waveforms also suggest the existence of smaller-scale (1 to 100 km) heterogeneity.

Keywords

Earth’s inner core / seismic structure / geodynamo

References

[1] Alboussière T, Deguen R, Melzani M. Melting-Induced Stratification above the Earth’s Inner Core due to Convective Tanslation. Nature, 2010, 466(7307): 744-747.

[2] Aster R C, Borcher B, Thurber C H. Parameter Estimation and Inverse Problems, 2004 Amsterdam: Academic Press, 320.

[3] Aubert J, Amit H, Hulot G, . Thermochemical Flows Couple the Earth’s Inner Core Growth to Mantle Heterogeneity. Nature, 2008, 454: 758-761.

[4] Bergman I. Measurements of Elastic Anisotropy due to Solidification Texturing and the Implications for the Earth’s Inner Core. Nature, 1997, 389: 60-63.

[5] Bullen K E. The Density Variation of the Earths Central Core. Bull. Seismol. Soc. Amer., 1942, 32: 19-29.

[6] Calkins M A, Noir J, Eldredge J D, . The Effects of Boundary Topography on Convection in Earth’s Core. Geophys. J. Int., 2012, 189: 799-814.

[7] Calvet M, Margerin L. 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., 2008, 267: 200-212.

[8] Cao A, Masson Y, Romanowicz B. Short Wavelength Topography on the Inner-Core Boundary. Proc. Nat. Acad. Sci., 2006, 104: 31-35.

[9] Cao A, Romanowicz B. Test of the Innermost Inner Core Models Using Broadband PKIKP Travel Time Residuals. Geophys. Res. Lett., 2007, 34 8 L08303

[10] Cormier V F. Gupta H K. Seismic Viscoleastic Attenuation. Encyclopedia of Earth Sciences Series, Encyclopedia of Solid Earth Geophysics, 2011, 1279-1290.

[11] Cormier V F, Attanayake J, He K. Inner Core Freezing and Melting: Constraints from Seismic Body Waves. Phys. Earth Planet. Int., 2011, 188: 163-172.

[12] Cormier V F, Li X. Frequency Dependent Attenuation in the Inner Core: Part II. A Scattering and Fabric Interpretation. J. Geophys. Res., 2002, 107(B12): ESE 14-1-ESE 4-15.

[13] Cormier V F, Li X, Choy G L. Seismic Atenuation of the Inner Core: Viscoelastic or Stratigraphic?. Geophys. Res. Lett., 1998, 25(21): 4019-4022.

[14] Cormier V F, Richards P G. Comments on “The Damping of Core Waves” by Anthony Qamar and Alfredo Eisenberg. J. Geophys. Res., 1976, 81: 3066-3068.

[15] Creager K C. Anisotropy of the Inner Core from Differential Travel Times of the Phases PKP and PKIKP. Nature, 1992, 356: 309-314.

[16] Creager K C. Large-Scale Variations in Inner Core Anisotropy. J. Geophys. Res., 1999, 104(B10): 23127-23139.

[17] Dai Z, Wang W, Wen L. Irregular Topography at the Earth’s Inner Core Boundary. Proc. Nat. Acad. Sci., 2012, 109(20): 7654-7658.

[18] Deguen R, Cardin P. Thermo-Chemical Convection in the Earth’s Inner Core. Geophys. J. Int., 2011, 187(3): 1101-1118.

[19] Deuss A, Irving J C E, Woodhouse J. Regional Variation of Inner Core Anisotropy from Seismic Normal Mode Observations. Science, 2010, 328: 1018-1020.

[20] Doornbos D J. Observable Effects of the Seismic Absorption Band in the Earth. Geophys. J. R. Astr. Soc., 1983, 75: 693-711.

[21] Dziewonski A M, Anderson D L. Preliminary Reference Earth Model. Phys. Earth Planet. Int., 1981, 25: 297-356.

[22] Garcia R. Constraints on Upper Inner-Core Structure from Waveform Inversion of Core Phases. Geophys. J. Int., 2002, 150: 651-664.

[23] Garcia R, Tkalčić H, Chevrot S. A New Global PKP Data Set to Study Earth’s Core and Deep Mantle. Phys. Earth. Planet. Inter., 2006, 159(1): 15-31.

[24] Gubbins D, Sreenivasan B, Mound J, . Melting of the Earth’s Inner Core. Nature, 2011, 473: 361-363.

[25] J. Geophys. Res., 2011, 116 B4

[26] Ishii M, Dziewonski A M. The Innermost Inner Core of the Earth: Evidence for a Change in Anisotropic Behavior at the Radius of about 300 km. Proc. Nat. Acad. Sci., 2002, 99: 14025-14030.

[27] Kaneshima S, Hirahara K, Ohtaki T, . Seismic Structure near the Inner Core-Outer Core Boundary. Geophys. Res. Lett., 1994, 21(2): 157-160.

[28] Kennett B L N. Seismic Wave Propagation in Stratified Media, 1983 Cambridge: Cambridge University Press, 339.

[29] Kennett B L N, Engdahl E R, Buland R. Constraints on Seismic Velocities in the Earth from Taveltimes. Geophys. J. Int., 1995, 122: 108-124.

[30] Kennett B L N, Gudmundsson O. Ellipticity Corrections for Seismic Phases. Geophys. J. Int., 1996, 127: 40-48.

[31] Koper K D, Franks J M, Dombrovskaya M. Evidence for Small-Scale Heterogeneity Earth’s Inner Core from a Global Study of PKiKP Coda Waves. Earth Planet. Sci. Lett., 2004, 224: 227-241.

[32] Krasnoshchekov D N, Kaazik P B, Ovtchinnikov V M. Seismological Evidence for Mosaic Structure of the Surface of the Earth’s Inner Core. Nature, 2005, 435(7041): 483-487.

[33] Lehmann I. P’. Publications du Bureau Central Seismologique International. SÈrie A, Travaux Scientifique, 1936, 14: 87-115.

[34] Leyton F, Koper K D. Using PKiKP Coda to Determine Inner Core Structure: 2. Determination of QC. J. Geophys. Res., 2007, 112 B05317

[35] Li X, Cormier V F. Frequency Dependent Attenuation in the Inner Core. Part I: A Viscoelastic Interpretation. J. Geophys. Res., 2002, 107 B12 2362

[36] Mattesini M, Belonoshko A B, Ramìrez E B M, . Hemispherical Anisotropic Patterns of the Earth’s Inner Core. Proc. Nat. Acad. Sci., 2010, 107(21): 9507-9512.

[37] Monnereau M, Calvet M, Margerin L, . Lopsided Growth of Earth’s Inner Core. Science, 2010, 328(5981): 1014-1017.

[38] Montagner J P, Kennett B L N. How to Reconcile Body-Wave and Normal-Mode Reference Earth Models?. Geophys. J. Int., 1995, 125: 229-248.

[39] Morelli A, Dziewonski A M, Woodhouse J. Anisotropy of the Inner Core Inferred from PKIKP Travel Times. Geophys. Res. Lett., 1986, 13: 1545-1548.

[40] Niu F, Wen L. Hemispherical Variations in Seismic Velocity at the Top of the Earth’s Inner Core. Nature, 2001, 410: 1081-1084.

[41] Niu F, Wen L. Seismic Anisotropy in the Top 400 km of the Inner Core beneath the “Eastern” Hemisphere. Geophys. Res. Lett., 2002, 29 12

[42] Niu F, Wen L. Difference in the Seismic Velocity between the Eastern and the Western Hemispheres in the Top of the Earth’s Inner Core. Global Tectonics and Metallogeny, 2003, 8(1–4): 109-111.

[43] Ohtaki T, Kaneshima S, Kanjo K. Seismic Structure near the Inner Core Boundary in the South Polar Region. J. Geophys. Res., 2012, 117 B03312

[44] Oreshin S I, Vinnik L P. Heterogeneity and Anisotropy of Seismic Attenuation in the Inner Core. Geophys. Res. Lett., 2004, 31 L02613

[45] Ouzounis A, Creager K C. Isotropy Overlying Anisotropy at the Top of the Inner Core. Geophys. Res. Lett., 2001, 28(22): 4331-4334.

[46] Owens T J, Crotwell H P, Groves C, . SOD: Standing Order for Data. Seismol. Res. Lett., 2004, 75: 515-520.

[47] Reaman D M, Daehn G S, Panero W R. Predictive Mechanism for Anisotropy Development in Earth’s Inner Core. Earth Planet. Sci. Lett., 2011, 312: 437-442.

[48] Shearer P M. Constraints on Inner Core Anisotropy from PKP(DF) Tavel Times. J. Geophys. Res., 1994, 99(B10): 19647-19659.

[49] Shearer P M, Toy K M. PKP(BC) versus PKP(DF) Differential Travel Times and Aspherical Structure in the Earth’s Inner Core. J. Geophys. Res., 1991, 96: 2233-2247.

[50] Song X D, Helmberger D V. Velocity Structure near the Inner Core Boundary from Waveform Modeling. J. Geophys. Res., 1992, 97(B5): 6573-6586.

[51] Song X D, Helmberger D V. A P Wave Velocity Model of Earth’s Core. J. Geophys. Res., 1995, 100(B7): 9817-9830.

[52] Song X D, Helmberger D V. Seismic Evidence for an Inner Core Tansition Zone. Science, 1998, 282: 924-927.

[53] Souriau A., Poupinet G. The Velocity Profile at the Base of the Liquid Core from PKP(BC+Cdiff) Data: An Argument in Favour of Radial Heterogeneity. Geophys. Res. Lett., 1991, 18 10.

[54] Souriau A, Romanowicz B. Anisotropy in Inner Core Attenuation: A New Type of Data to Constrain the Nature of the Solid Core. Geophys. Res. Lett., 1996, 23: 1-4.

[55] Stroujkova A, Cormier V F. Regional Variations in the Upppermost 100 km of the Earth’s Inner Core. J. Geophys. Res., 2004, 109 B10 B10307

[56] Sun X L, Song X D. Tomographic Inversion for Three-dimensional Anisotropy of Earth’s Inner Core. Phys. Earth Planet. Inter., 2008, 167: 53-70.

[57] Tanaka S. Depth Extent of Hemispherical Inner Core from PKP(DF) and PKP(Cdiff) for Equatorial Paths. Phys. Earth Planet. Int., 2012, 210: 50-62.

[58] 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., 1997, 102(B2): 2925-2938.

[59] Tkalčić H, Kennett B L N, Cormier V F. On the Inner-Outer Core Density Cntrast from PKiKP/PcP Amplitude Ratios and Uncertainties Caused by Seismic Noise. Geophys. J. Int., 2009, 179: 425-443.

[60] Tromp J. Support for Anisotropy of the Earth’s Inner Core from Free Oscillations. Nature, 1993, 366: 678-681.

[61] Tseng T L, Huang B S. Depth Dependent Attenuation in the Uppermost Inner Core from the Taiwan Short Period Seismic Array PKP Data. Geophys. Res. Lett., 2001, 28(3): 459-462.

[62] Van Orman J A. On the Viscosity and Creep Mechanism of Earth’s Inner Core. Geophys. Res. Lett., 2004, 31 L20606

[63] Vidale J E, Earle P. Fine Scale Heterogeneity in the Earth’s Inner Core. Nature, 2000, 404: 273-275.

[64] Waszek L, Deuss A. Distinct Layering in the Hemispherical Seismic Velocity Structure of Earth’s Upper Inner Core. J. Geophys. Res., 2011, 116 B12313

[65] Woodhouse J H, Giardini D, Li X. Evidence for Inner Core Anisotropy from Free Ocillations. Geophys. Res. Lett., 1986, 13: 1549-1552.

[66] Wookey J, Helffrich G. Inner-Core Shear-Wave Anisotropy and Texture from an Observation of PKJKP. Nature, 2008, 454: 873-876.

[67] Yu W C, Wen L. Seismic Velocity and Attenuation Structures in the Top 400 km of the Earth’s Inner Core along Equatorial Paths. J. Geophys. Res., 2006, 111 B07308

[68] Yu W, Wen L, Niu F. Seismic Velocity Structure in the Earth’s Outer Core. J. Geophys. Res., 2005, 110 B02302

[69] Zhang J, Richards P G, Schaff D P. Wide-Scale Detection of Earthquake Waveform Doublets and Further Evidence for Inner Core Super-Rotation. Geophys. J. Int., 2008, 174: 993-1006.

[70] Zou Z, Koper K, Cormier V F. The Structure of the Base of the Outer Core Inferred from Seismic Waves Diffracted around the Inner Core. J. Geophys. Res., 2008, 113 B5 B05314