Letter | Published:

The Earth’s ‘hum’ is driven by ocean waves over the continental shelves

Nature volume 445, pages 754756 (15 February 2007) | Download Citation


Observations show that the seismic normal modes of the Earth at frequencies near 10 mHz are excited at a nearly constant level in the absence of large earthquakes1. This background level of excitation has been called the ‘hum’ of the Earth2, and is equivalent to the maximum excitation from a magnitude 5.75 earthquake3. Its origin is debated, with most studies attributing the forcing to atmospheric turbulence, analogous to the forcing of solar oscillations by solar turbulence2,4,5,6,7. Some reports also predicted that turbulence might excite the planetary modes of Mars to detectable levels4. Recent observations on Earth, however, suggest that the predominant excitation source lies under the oceans8,9,10. Here I show that turbulence is a very weak source, and instead it is interacting ocean waves over the shallow continental shelves that drive the hum of the Earth. Ocean waves couple into seismic waves through the quadratic nonlinearity of the surface boundary condition, which couples pairs of slowly propagating ocean waves of similar frequency to a high phase velocity component at approximately double the frequency. This is the process by which ocean waves generate the well known ‘microseism peak’ that dominates the seismic spectrum near 140 mHz (refs 11, 12), but at hum frequencies, the mechanism differs significantly in frequency and depth dependence. A calculation of the coupling between ocean waves and seismic modes reproduces the seismic spectrum observed. Measurements of the temporal correlation between ocean wave data and seismic data9,10 have confirmed that ocean waves, rather than atmospheric turbulence, are driving the modes of the Earth.

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  1. 1.

    , & Earth’s background free oscillations. Science 279, 2089–2091 (1998)

  2. 2.

    , & Resonant oscillations between the solid Earth and atmosphere. Science 287, 2244–2246 (2000)

  3. 3.

    Time domain analysis of the Earth’s background seismic radiation. J. Geophys. Res. 106, 26483–26494 (2001)

  4. 4.

    & Continuous excitation of planetary free oscillations by atmospheric disturbances. Nature 395, 357–360 (1998)

  5. 5.

    et al. Origin of Earth’s ground noise from 2 to 20 mHz. Geophys. Res. Lett. 29 1413 doi: 10.1029/2001GL013862 (2002)

  6. 6.

    Continuous free oscillations: Atmosphere-solid Earth coupling. Annu. Rev. Earth Planet. Sci. 29, 563–584 (2001)

  7. 7.

    et al. A theory of the Earth’s background free oscillations. J. Geophys. Res. 107 (B9)2206 doi: 10.1029/2001JB000153 (2002)

  8. 8.

    & Excitation of the Earth’s continuous free oscillations by atmosphere-ocean-seafloor coupling. Nature 431, 552–556 (2004)

  9. 9.

    & Correlation of Earth's long-period background seismic radiation with the height of ocean waves. Eos 86(52), Fall Meet. Suppl. abstr. S34B–02. (2005)

  10. 10.

    , & Insights into the origin of the Earth’s hum and microseisms. Eos 86(52), Fall Meet. Suppl. abstr. S31A–0271. (2005)

  11. 11.

    A statistical analysis of the generation of microseisms. Rev. Geophys. 1, 177–209 (1963)

  12. 12.

    Broad seismology and noise under the ocean. Rev. Geophys. 36, 105–142 (1998)

  13. 13.

    & Theoretical Global Seismology (Princeton Univ. Press, Princeton, New Jersey, 1998)

  14. 14.

    et al. Infragravity-frequency (0.005–0.05 Hz) motions on the shelf. Part II: Free waves. J. Phys. Oceanogr. 25, 1063–1079 (1995)

  15. 15.

    et al. Generation and propagation of infragravity waves. J. Geophys. Res. 100, (C12)24863–24872 (1995)

  16. 16.

    et al. Computation of seismograms and atmospheric oscillations by normal mode summation for a spherical earth model with a realistic atmosphere. Geophys. J. Int. 135, 388–406 (1998)

  17. 17.

    in Seismological Algorithms, Computational Methods and Computer Programs (ed. Doornbos, D. J.) 321–370 (Academic, London, 1988)

  18. 18.

    & Seismic structure of the upper mantle in a central Pacific corridor. J. Geophys. Res. 101, (B10)22291–22309 (1996)

  19. 19.

    The oceanic excitation hypothesis for the continuous oscillations of the Earth. Geophys. J. Int. 160, 276–298 (2005)

  20. 20.

    On aerodynamic infrasound. J. Appl. Atmos. Terr. Phys. 33, 149–155 (1971)

  21. 21.

    On sound generated aerodynamically, 1. General theory. Proc. R. Soc. Lond. A 211, 564–587 (1952)

  22. 22.

    Surface pressures and sound produced by turbulent flow over smooth and rough walls. J. Acoust. Soc. Am. 90, 1041–1047 (1991)

  23. 23.

    & The transfer of energy from a tornado to the ground. Seismol. Res. Lett. 72, 12–21 (2001)

  24. 24.

    et al. Infrasound generation by turbulent convection. Preprint at 〈〉 (v1, 15 Sept., 2004)

  25. 25.

    SAX04 experiment data set 〈〉 (2006)

  26. 26.

    et al. Ambient Earth noise: A survey of the Global Seismographic Network. J. Geophys. Res. 109 B11307 doi: 10.1029/2004JB003408 (2004)

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I thank G. Ekstrom, J. Gaherty and W.W. Webb for discussions, and P. Lognonné and T. Tanimoto for comments and suggestions.

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  1. Lamont Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA

    • Spahr C. Webb


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Reprints and permissions information is available at www.nature.com/reprints. The author declares no competing financial interests.

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Correspondence to Spahr C. Webb.

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    Supplementary Notes

    This file contains Supplementary Notes which include a more complete derivation of the equations used in the letter and additional references.

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