A Humming Planet
Like a permanent undertone to the dissonance of busy urban centers or nature's own harmonious melodies, the Earth itself hums a low-frequency tune. The permanent drone, which has baffled scientists for a while now, comes from continuous vibrations too faint to be detected without a specialized instrument.
Now, for the first time ever, scientists have captured Earth's hum — its "song," if you will. They measured the constant humming from from the Indian Ocean seafloor using special spherical Ocean-Bottom Seismometers (OCBs).
"It's like taking a piano and slamming all the keys at the same time," Spahr Webb, a professor at Columbia's Lamont-Doherty Earth Observatory not associated with the study, told National Geographic. "Except they're not nice harmonics. They're oddball frequencies."
From September 2012 to November 2013, the researchers deployed 57 of these OCBs, and measured several frequencies between 2.9 and 4.5 millihertz. That's 10,000 times lower than the average human hearing threshold, the researchers noted in their study, published in the journal Geophysical Research Letters.
Measuring Movement
Scientists have known that the Earth produces this "humming" since at least 1959, but haven't yet identified what exactly is causing it. The hum doesn't appear to be caused by earthquakes, which happen sporadically as the earth releases stress; in contrast, Earth's hum goes on continually.
Some theories suggest atmospheric disturbances, or the movement of ocean waves over the seafloor, trigger the continuous vibrations. Webb, who has also been searching for the hum's cause, told the Washington Post he thought ocean waves were the most likely source, “banging on the sea floor pretty much all the way around the Earth.”
Attempts to measure the low-frequency drone had previously attempted using seismometers based on land. Now, the new measurement taken from the ocean suggests that the humming occurs globally, which the researchers found by stripping data from other vibrations — like ocean currents, waves, seismic activity and glitches — from their recordings, and comparing the recorded hum with measurements taken by a land-based station in Algeria.
Combining the measurements taken from land seismometers and the new numbers from the OCBs, the researchers said that their findings can provide new insight into the mechanisms behind the Earth's hum. Aside from this, their study could also help map the inside of the Earth with more detailed accuracy, the researchers pointed out.
"Earth is constantly in movement, and we wanted to observe these movements because the field could benefit from having more data," lead researcher Martha Deen from the Institut de Physique du Globe de Paris said in a press statement.
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