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

• Published in: Nature (London) Vol. 445; no. 7129; pp. 754 - 756
• Main Author: Webb, Spahr C
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 15-02-2007; Nature Publishing Group
• Subjects: Atmospheric turbulence; Boundary conditions; Continental shelves; Earth; Earth sciences; Earth, ocean, space; Earthquakes; Earthquakes, seismology; Exact sciences and technology; Internal geophysics; Marine geology; Ocean waves; Oceanography; Oceans; Seismic activity; Seismic waves; Seismology; Sound waves; Turbulence; atmosphere; continental shelf; microseisms; earthquakes; turbulence; phase velocity; interplanetary comparison; frequency; coupling; correlation; ocean; magnitude; mechanism; boundary conditions; ocean waves; seismic waves
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature05536

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 earthquakes. This background level of excitation has been called the 'hum' of the Earth, and is equivalent to the maximum excitation from a magnitude 5.75 earthquake. Its origin is debated, with most studies attributing the forcing to atmospheric turbulence, analogous to the forcing of solar oscillations by solar turbulence. Some reports also predicted that turbulence might excite the planetary modes of Mars to detectable levels. Recent observations on Earth, however, suggest that the predominant excitation source lies under the oceans. 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 data have confirmed that ocean waves, rather than atmospheric turbulence, are driving the modes of the Earth.