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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.

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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.

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Correspondence to Barbara Romanowicz.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Methods and Figure legends

Detailed information on some methods we used for this research and detailed figure captions for supplementary figures. (DOC 39 kb)

Supplementary Figure 1

Illustration of stacking procedure. (PDF 49 kb)

Supplementary Figure 2

Comparison of stacking methods. (PDF 73 kb)

Supplementary Figure 3

Amplitude of array stack as a function of back azimuth and time for a day with an earthquake. (PDF 32 kb)

Supplementary Figure 4

Analysis of a detection during a quiet day (January 31, 2000) on the F-net array. (PDF 388 kb)

Supplementary Figure 5

Analysis of array response. (PDF 12 kb)

Supplementary Figure 6

Forward modeling of source distribution in azimuth for F-net. (PDF 18 kb)

Supplementary Figure 7

Forward modeling of source distribution in azimuth for BDSN. (PDF 18 kb)

Supplementary Figure 8

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)

Supplementary Figure 9

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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Further reading

Figure 1: Analysis of detections for 31 January 2000.
Figure 2: Amplitude of degree one as a function of time and back azimuth for the 64 quiet days in 2000.
Figure 3: Comparison of seasonal variations in the distribution of hum-related noise (degree one only) and significant wave height in the year 2000.
Figure 4: Distribution of sources for a 6 h time window on 31 January 2000 (from 14:00 to 20:00 utc).


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