Earth is in a constant state of vibration, even when there aren’t earthquakes rocking the surface, and now scientists have locked onto the planet’s frequency. NASA/JPL

Scientists are getting to the bottom of Earth’s mysterious and constant vibration, tuning their instruments to the planet’s natural frequency to learn more.

It’s one of our planet’s little quirks that it is always vibrating even when there isn’t an actual earthquake shaking the land beneath our feet. According to a new study, published in the journal Geophysical Research Letters, scientists have now identified the frequency of that vibration, what they refer to as “the Earth’s hum.” The team of researchers used instruments that pick up seismic activity at the bottom of the ocean to decode the Earth’s frequency, separating it from other noise generated by things like waves.

Previous work to lock onto the planet’s signal has focused on land-based seismometers, so this research opens up the ocean floor as a laboratory setting in which to take these kinds of readings.

The study says the new measurement is important for understanding the vibrations’ source and what they sound and feel like at their point of origin. And according to the American Geophysical Union, which is also the journal publisher, “the new findings could be used to map the interior of Earth with more detail and accuracy.”

Scientists already use vibrations detected with land-based instruments to learn more about the planet’s interior, but collecting data from the seafloor can improve that process and give researchers more spots from which to take measurements than land ever could, since most of the Earth is covered in ocean and the new method would not require an earthquake to be shaking before observations can be made.

The group says the seafloor data will help scientists get a view of Earth as deep as about 300 miles.

“Earth is constantly in movement, and we wanted to observe these movements because the field could benefit from having more data,” geophysicist and lead study author Martha Deen said in the AGU report.

They detected the planet’s natural vibration at 2.9 millihertz and 4.5 millihertz, which is a measurement of how many vibrations there are per second. You don’t hear this frequency while you are just walking around during the day because the human hearing range is between about 20 Hertz and 20 kilohertz. The lowest end of our hearing range has a frequency that is thousands of times higher than that of Earth’s hum.

Experts are at odds over the source of Earth’s constant vibration. The AGU notes that ideas have ranged from ocean currents to atmospheric turbulence.

“Combining data from both land and ocean bottom seismometers gives seismologists a more complete picture of the entire hum signal compared to using land stations alone,” according to the AGU.