By Nanci Bompey 
 
 Scientists have come up with an explanation of why the Earth rings like a bell. 
 
 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. This ever-present background tremor can’t be felt by humans, but can be detected by sensitive seismic instruments. 
 
 In the years since the discovery, scientists have come up with explanations for this continuous vibration. One theory suggests it’s generated by ocean waves moving in opposite directions. When the waves collide, they create very weak, “microseismic” waves that add up to a generalized ringing. 
 
 A group of French researchers have now tested this theory by using models to calculate how ocean waves could generate seismic waves. They found that opposing ocean waves could initiate a particular kind of seismic waves – those that take 13 seconds or less to complete one oscillation. But, when it came to even slower oscillating seismic waves, the theory did not hold up. That left seismic experts in a quandary, because it’s very slow waves, with periods greater than 50 seconds, that are at the heart of the planet’s bell-like resonance. 
 
 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. Long ocean waves can extend all the way down to the seafloor. As they make their way back and forth to the open ocean from the coast, these long waves travel over the bumpy ocean bottom. 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. 
 
 The team published their results in a new study in Geophysical Research Letters, a journal of the American Geophysical Union. 
 
 “We have made a big step in explaining this mysterious signal and where it is coming from and what is the mechanism,” Ardhuin said. 
 
 Understanding the Earth’s ringing could help scientists map the inside of the planet, 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. 
 
 Ardhuin said understanding where these seismic signals are coming from also enables researchers to look for even fainter seismic signals. That could allow scientists to better detect faint earthquakes far away from seismic stations or nuclear explosions, he said. 
 
 “I think it is a relief to the seismologists,” Ardhuin said of figuring out the source of Earth’s ringing. “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. ||||| A view of Earth from above created from images snapped by the Suomi-NPP satellite. 
 
 Even planets can get a bad case of tinnitus, according to a new study that explains Earth's mysterious, never-ending hum. 
 
 Scientists have long known that earthquakes can make the Earth sing like a bell for days or months. However, in the late 1990s, seismologists discovered the world also constantly vibrates at very low frequencies even when there are no quakes. This so-called microseismic activity is too faint for humans to feel. 
 
 Now, researchers say ocean waves are the culprit behind these mysterious tremors. 
 
 Many scientists had previously turned to ocean waves to explain our planet's unusual humming. In one theory, scientists proposed the vibrations were generated by huge ocean waves, which can extend all the way down to the seafloor. The waves can shake the Earth as they tumble over seafloor ridges and underwater continental shelves. Another idea suggested colliding ocean waves triggered the tremors. [In Photos: The 10 Strangest Places on Earth] 
 
 But neither idea could account for the entire range of vibrations seen on earthquake sensors. The new study combines both of these ideas into one model that accounts for these microseismic signals. 
 
 "I think our result is an important step in the transformation of mysterious noise into an understood signal," lead study author Fabrice Ardhuin, an oceanographer at the French Research Institute for Exploitation of the Sea, told Live Science. 
 
 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. Most of the mystery hum comes from these longer waves. The pressure of these longer ocean waves on the seafloor causes most of the Earth's bell-like ringing, the researchers said. 
 
 A better understanding of this hum could help scientists generate better maps of the Earth's interior, Ardhuin said. These seismic waves penetrate deep into the planet's mantle and potentially all the way to Earth's core. This means that analyzing these waves could help yield a more detailed picture of the planet's structure, he said. 
 
 The researchers added there could be still more sources of microseismic activity. For instance, perhaps ocean waves traveling along shorelines or traveling down underwater mountains and mid-ocean ridges can generate seismic waves, Ardhuin said. 
 
 Ardhuin and his colleagues Lucia Gualtieri and Eléonore Stutzmann at the Paris Institute of Earth Physics detailed their findings in the Feb. 16 issue of the journal Geophysical Research Letters. 
 
 Follow Live Science @livescience, Facebook & Google+. Original article on Live Science. ||||| Scientists have long known that Earth produces an eerie low-frequency hum that's inaudible to humans but detectable with seismic instruments. But as for what's causing this "microseismic" activity, scientists have never been sure. 
 
 Until now. 
 
 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. 
 
 "We have made a big step in explaining this," Dr. Fabrice Ardhuin, a senior research scientist at the National Center for Scientific Research in Brest, France and the study's lead author, said in a written statement. "Now we know where this ringing comes from and the next question is: what can we do with it." 
 
 Catching the culprit. With the help of computer models of the ocean, winds, and seafloor, the scientists were able to pinpoint the exact kind of ocean waves that cause the hum and how, Live Science reported. 
 
 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. 
 
 (Story continues below image.) 
 
 
 
 The graphic on the left shows the computed height of giant waves that can reach the seafloor. The picture on the right shows these types of waves during a storm at a beach south of Bordeaux, France. The pressure of these long ocean waves on the seafloor generates seismic waves that cause the Earth to oscillate, scientists say. 
 
 How the hum can help. The microseismic activity caused by the giant ocean waves penetrates deep into Earth’s mantle -- and possibly to its core. That suggests that by recording it, scientists may be able to gain a more detailed picture of our planet's interior, according to the researchers. 
 
 What would the hum sound like if it were at a higher frequency -- within the range of human hearing? 
 
 "If you played it at 10,000 times the speed you could hear 'white noise' like an old TV set between channels," Ardhuin told The Huffington Post in an email. 
 
 Hmm. Maybe it's a good thing we can't hear the hum.
– 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.)