Underneath the noise of bustling cities and the background sound of nature itself; winds rustling, waves rushing and crashing, tree branches creaking… The Earth is “humming.” And scientists were able to measure for the first time this consistent hum coming from the ocean floor. The findings were published in Geophysical Research Letters.

According to the US Geological Survey, the earth undergoes as estimated 500,000 earthquakes per year, but only 100,000 are strong enough to be felt, and only 100 are strong enough to cause damage. However, there’s a lot going on during the rest period between earthquakes.

Scientists have known since the 1990s that the Earth is in constant vibration with microseismic activity, called “free oscillation.” Free oscillation creates a hum that can be detected anywhere on land by seismometers — equipment used to detect and record vibrations.

The source of this hum has baffled researchers for years, with many theories surfacing ranging from the collisions of ocean waves to the rhythmic ebb and flow of ocean waves that reached all the way down to the seafloor. However, scientists ruled that both theories contributed to the Earth’s constant vibration.

Recently, scientists were able to record the humming sound from the seafloor using special spherical ocean seismometers, in an attempt to gather more data than is currently available from readings taken on land, contributing to efforts to map the planet’s interior.

Between 2012 and 2013, the researchers deployed 57 free-fall seismometers around La Réunion Island to the east of Madagascar, over an area measuring about 1242 m2 (772 square miles).

In order to isolate the hum from normal ocean noise, the researchers used filters, noise reduction and calculations. They found “very clear peaks” that appeared consistently over the 11-month study period and were similar to previously recorded humming taken in Algeria. The peaks occurred at several frequencies between 2.9 and 4.5 millihertz — about 10,000 times lower than the human hearing threshold, which is 20 hertz.

Capturing ocean recordings of Earth’s hum will provide scientists with far more data than is currently available from readings taken on land, contributing to efforts to map the planet’s interior, the researchers wrote in the study. ~ Live Science


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