Excitation of Earth's continuous free oscillations by atmosphere–ocean–seafloor coupling

The Earth undergoes continuous oscillations, and free oscillation peaks have been consistently identified in seismic records in the frequency range 2–7 mHz (refs 1, 2), on days without significant earthquakes. The level of daily excitation of this ‘hum’ is equivalent to that of magnitude 5.75 to 6.0 earthquakes3,4, which cannot be explained by summing the contributions of small earthquakes1,3. As slow or silent earthquakes have been ruled out as a source for the hum4 (except in a few isolated cases5), turbulent motions in the atmosphere or processes in the oceans have been invoked3,6,7,8 as the excitation mechanism. We have developed an array-based method to detect and locate sources of the excitation of the hum. Our results demonstrate that the Earth's hum originates mainly in the northern Pacific Ocean during Northern Hemisphere winter, and in the Southern oceans during Southern Hemisphere winter. We conclude that the Earth's hum is generated by the interaction between atmosphere, ocean and sea floor, probably through the conversion of storm energy to oceanic infragravity waves that interact with seafloor topography.