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.
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We thank the operators of the following seismic networks for making their data publicly available: BDSN (http://www.seismo.berkeley.edu), F-net (http://www.fnet.bosai.go.jp/freesia/index.html), IRIS (http://www.iris.edu) and TERRAscope. The Monthly Mean Global Surface Ocean Variables were obtained from the Physical Oceanography Distributed Active Archive Center (http://podaac.jpl.nasa.gov/poet). This work was partially supported by the NSF.
The authors declare that they have no competing financial interests.
Detailed information on some methods we used for this research and detailed figure captions for supplementary figures. (DOC 39 kb)
Illustration of stacking procedure. (PDF 49 kb)
Comparison of stacking methods. (PDF 73 kb)
Amplitude of array stack as a function of back azimuth and time for a day with an earthquake. (PDF 32 kb)
Analysis of a detection during a quiet day (January 31, 2000) on the F-net array. (PDF 388 kb)
Analysis of array response. (PDF 12 kb)
Forward modeling of source distribution in azimuth for F-net. (PDF 18 kb)
Forward modeling of source distribution in azimuth for BDSN. (PDF 18 kb)
Results of forward modeling of stack amplitudes as a function of azimuth for F-net and BDSN for a distribution of sources concentrated over different continents. (PDF 23 kb)
Results of forward modeling of stack amplitudes as a function of azimuth, for F-net and BDSN for a distribution of sources concentrated over selected oceanic areas. (PDF 13 kb)
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