The Mysterious Origin Of Planet Earth's Hum Signal Explained

NASA/Goddard

Earth as seen by the NASA's Terra satellite. Image in public domain.

The origin of the "hum", as this oscillation is called, is still poorly understood. The hum is especially strong from December to February and June to August, coinciding with winter in the respective hemispheres. These periods are also characterized by strong storms in the Atlantic and the Pacific Ocean. Based on this observation, some researchers suggested, that seasonal atmospheric disturbances or ocean waves cause the planetary oscillations.

Until now, it was possible to measure the hum only with land-based seismometers, covering just one-third of Earth's surface. In recent years, new developments in technology made it possible to put automated seismic stations also on the bottom of the sea. Such stations are used to monitor underwater volcanoes or earthquakes that could trigger a tsunami.  A new study published in the Geophysical Research Letters journal, used the data of such stations to quantify in great detail the extent and magnitude of the hum over a year, comparing the signal with oceanic and atmospheric changes. The scientists used the data from two selected seismometer stations located at the bottom of the Indian Ocean. After eliminating disturbances of single earthquakes, gravity anomalies, and marine currents interferences, only the signal of the hum remains. Earth, so the study, oscillates at a regular frequency of 2.9 to 4.5 millihertz, be it on the bottom of the sea or on land.

The study concludes that atmospheric turbulence can only explain a smaller part of the observed movements, with movements of ocean waves over continental shelves playing the most important role in generating the oscillation. The forces exercised by the sea and the atmosphere on the solid Earth seem to be very weak at a first glimpse. However, assuming Earth's atmosphere or hydrosphere act only as a sort of pacemaker, resonance effects could amplify the signal over time enough to finally make the entire planet oscillate in the observed regular pattern.

Even if we can't feel it, every few minutes the entire Earth contracts, less than a ten-thousandth of an inch, to expand immediately thereafter again. This low-frequency oscillation was discovered in 1998 when seismologist monitored the signals of earthquakes traveling through Earth, but is not associated with such tectonic events.

NASA/Goddard

Earth as seen by the NASA's Terra satellite. Image in public domain.

Until now, it was possible to measure the hum only with land-based seismometers, covering just one-third of Earth's surface. In recent years, new developments in technology made it possible to put automated seismic stations also on the bottom of the sea. Such stations are used to monitor underwater volcanoes or earthquakes that could trigger a tsunami. A new study published in the Geophysical Research Letters journal, used the data of such stations to quantify in great detail the extent and magnitude of the hum over a year, comparing the signal with oceanic and atmospheric changes. The scientists used the data from two selected seismometer stations located at the bottom of the Indian Ocean. After eliminating disturbances of single earthquakes, gravity anomalies, and marine currents interferences, only the signal of the hum remains. Earth, so the study, oscillates at a regular frequency of 2.9 to 4.5 millihertz, be it on the bottom of the sea or on land.

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