When Everything Is Silent, the Earth Emanates a Mysterious Hum

If you turned off every video game system, car stereo, and TV on the planet, and if everyone held their breath and didn’t make a peep, there would still be a sound, though you couldn’t exactly hear it. You may not know it, but the Earth is always emitting a low hum, which geophysicists call the free oscillation of the Earth. In a recent study, geophysicists have measured Earth’s background hum on the ocean floor for the first time. That’s right, the Earth makes noises constantly, even when it’s not rumbling or cracking.

Sure, our planet experiences earthquakes as a result of the movement of Earth’s tectonic plates, which float around on the mantle and collide, shear, and pull apart from each other. But this more subtle vibration, whose source scientists have been trying to track down for over 50 years, is not as violent as these more familiar seismic forces.

In a study published November 14 in Geophysical Research Letters, an international team of European geophysicists, led by Martha Deen, a geophysics Ph.D. student at the Paris Institute of Earth Physics in Paris, France, report that they’ve used seismometers at the bottom of the Indian Ocean to make the first direct observations of the Earth’s hum from the bottom of the ocean.

This is important for a couple of reasons. First, it’s a big deal that they measured a signal from the ocean floor. While scientists have previously measured this background vibration on land, the ocean covers 70 percent of the Earth, so if seismologists want to study this phenomenon, they need to figure out how to do it in the ocean.

This leads to the second reason this discovery is significant: It’s really hard to get a clean reading of the hum from the ocean floor. Measuring the background hum there is complicated because the ocean, which is constantly in motion, creates lots of undesirable noise. Therefore, Deen and her colleagues had to use signals from multiple seismometers that they’d placed at the bottom of the Indian Ocean and remove all the noise from waves, earthquakes, and other glitch signals.

After all that correction, they were left with a signal that hums with a period of more than 30 seconds. That means each wave occurs every 30 seconds or more, as compared to a middle A on a piano, which has a period of 0.00227 seconds. To put it simply, the Earth’s free oscillation produces an incredibly low-frequency sound, which is why we don’t have a recording that we can hear.

Even if we can’t hear it, though, it’s significant. Recording the signal is an important step toward figuring out what actually produces it. Hypotheses for what causes the hum include atmospheric disturbances and ocean waves, but so far there’s no definitive answer. This study could help pave the way for one.

“Earth is constantly in movement, and we wanted to observe these movements because the field could benefit from having more data,” said Deen, in a statement from the American Geophysical Union.

Abstract: The Earth’s hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 s. We present the first observations of its fundamental spheroidal eigenmodes on broadband ocean bottom seismometers (OBSs) in the Indian Ocean. At the ocean bottom, the effects of ocean infragravity waves (compliance) and seafloor currents (tilt) overshadow the hum. In our experiment, data are also affected by electronic glitches. We remove these signals from the seismic trace by subtracting average glitch signals; performing a linear regression; and using frequency-dependent response functions between pressure, horizontal, and vertical seismic components. This reduces the long period noise on the OBS to the level of a good land station. Finally, by windowing the autocorrelation to include only the direct arrival, the first and second orbits around the Earth, and by calculating its Fourier transform, we clearly observe the eigenmodes at the ocean bottom.