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.
I thank G. Ekstrom, J. Gaherty and W.W. Webb for discussions, and P. Lognonné and T. Tanimoto for comments and suggestions.
This file contains Supplementary Notes which include a more complete derivation of the equations used in the letter and additional references.