It’s long been known that earthquakes can cause the Earth to oscillate, or ring, for days or months, but in the late 1990s, seismologists discovered that the planet constantly vibrates at very low frequencies, even when there are no earthquakes. Using computer models of the ocean, winds and seafloor, the scientists found that colliding ocean waves could generate seismic waves that take 13 seconds or less to complete one ripple. When it came to slower waves, they found that ocean waves moving over the seafloor could generate seismic waves with a frequency of 13 to 300 seconds. The researchers then turned to another theory that suggests the movement of ocean waves over the bottom of the ocean generates these slower oscillating, very long waves. The researchers found that collisions between ocean waves generate some seismic activity, but it was mostly the movement and pressure of giant, slow-moving ocean waves that extend all the way down to the seafloor that cause the Earth to hum. The pressure of the ocean waves on the seafloor generates seismic waves that cause the Earth to oscillate, said Fabrice Ardhuin, a senior research scientist at Centre National de la Recherche Scientifique in Brest, France, and lead author of the new research. When the team plugged this theory into their models, they found that it could explain the longer microseismic signals, ranging from 13 seconds to 300 seconds. The researchers concluded that instead of one theory explaining all of the microseismic activity, both theories are needed – one to explain the shorter seismic waves and another to explain the longer seismic waves responsible for the Earth’s hum. They suggest that most of the hum comes from the movement of the long ocean waves over the steep continental shelves. A new study published online Feb. 10, 2015 in the journal Geophysical Research Letters indicates that the hum is largely due to ocean waves that cause our planet to vibrate subtly -- or "ring," as the researchers put it. A better understanding of this hum could help scientists generate better maps of the Earth's interior, Ardhuin said. The long, microseismic waves causing the resonance penetrate deep into Earth’s mantle, possibly to the planet’s core, so recording and understanding these waves could give scientists a much more detailed picture of the Earth’s structure, he said. “Now we know where this ringing comes from and the next question is: what can we do with it.” 
 
 — Nanci Bompey is a public information specialist/writer in AGU’s public information office.
– Even though we can't feel it, the Earth is humming all the time. "If you played it at 10,000 times the speed, you could hear 'white noise' like an old TV set between channels," Fabrice Ardhuin, an oceanographer in France, tells the Huffington Post. Experts discovered the planet's very low vibrations in the late 1990s, and now researchers may be able to explain them, Live Science reports. According to Ardhuin's team, the hum is a result of ocean waves. Waves don't just occur at the ocean's surface, Live Science explains: They can reach to the bottom of the ocean. There, they move along a "bumpy" sea floor, creating pressure that leads to the Earth oscillating, the American Geophysical Union reports. That could explain the so-called "microseismic" waves behind the hum—at least those lasting from 13 to 300 seconds, according to researchers' models. Shorter microseismic waves could be the result of waves crashing into each other. The wave theory isn't entirely new, Live Science notes, but the findings offer a fuller explanation of what could be happening. "I think (the results are) a relief to the seismologists," says the oceanographer. "Now we know where this ringing comes from, and the next question is: What can we do with it?" One possibility: The research could help experts spot earthquakes occurring a long distance away, AGU notes. (In other recent geological finds, it seems the Earth's core has a core of its own.)