The Earth makes a sound, quite literally. Different from things like winds rustling or waves crashing on the beach, the actual planet emits a permanent, low-frequency hum underneath the ceaseless noise the rest of its inhabitants produce.
This droning sound cannot be heard by the human ear, but has been going on since the Earth was born. Now, scientists have managed to measure this humming, Live Science reports.
Most movements that occur deep down under the Earth are not big enough for humans to feel them, with the exception of earthquakes. But the planet actually undergoes far more earthquakes globally than what most people know – as much as 500,000 tremors per year, according to the US Geological Survey. Of those, only around 100,000 are strong enough for those on the ground to feel, and a mere 100 are powerful enough to cause enough damage to structures.
However, even without any earthquakes happening, there’s still a lot going on in the Earth’s outer layer. Since the 1990s, scientists have been studying Earth’s constant vibrations caused by microseismic activity, known as “free oscillation.”
Free oscillation generates a hum that can be detected anywhere on land by seismometers.
This constant vibrating sound has perplexed researchers for years, with some suggesting that the ebb and flow of ocean waves reaching down to the bottom of the sea were responsible for it, while others thought the sound was made by ocean waves colliding.
In 2015, scientists found that both types of ocean movements actually contributed to Earth’s hum. Seismologists have recorded and measured the sound on land, and have now captured the sonic patterns from the depths of the seafloor.
Scientists used spherical ocean seismometers to measure the vibration in the Indian Ocean. Between 2012 and 2013, 57 free-fall seismometers were deployed around La Réunion Island to the east of Madagascar, over an area measuring about 772 square miles. The scientists then isolated the sound created by ocean wave motions and currents, and found “very clear peaks” over 11 months, which had the same amplitude range as measurements taken on land in Algeria. This new development will contribute towards mapping more of the Earth’s interior.
The study was published in Geophysical Research Letters.