Described as explosive booms rumbling in the distance, skyquakes are widely thought to originate from the sky. These mysterious booms are often reported in the eastern coast of the United States where they seem to be coming from offshore.
In North Carolina, some skyquakes were so powerful that they rattled windows and caused buildings to vibrate. Last April, for example, a loud booming noise rang through several neighborhoods in southern Charlotte and caused the ground to shake. Meteorologists detected seismic activity but found no evidence of an actual earthquake or other seismic events that could have caused the celestial boom. Local news outlets reported that officials were not able to locate the cause. (Related: Mysterious booms rattle houses in Florida.)
Such unexplained noises also happen elsewhere in the world and, in fact, are so widespread and common that different countries have different names for them. They are called "barisal guns" in India, "uminari" in Japan, "mistpoeffers" in the Netherlands and Belgium, "lagoni" in Italy and "retumbo" in the Philippines. In the U.S., skyquakes are also called "lake guns" or "Seneca guns" after the Seneca Lake in New York, where residents often complain about loud booming noises.
But though they are widely reported, there is still no satisfactory explanation for skyquakes. To that end, the researchers attempted to narrow down the list of phenomena that could possibly cause skyquakes. They presented their findings last December during the annual meeting of the American Geophysical Union.
For their study, the researchers used the EarthScope Transportable Array, a mobile network of 400 atmospheric sensors and seismographs, to measure the seismic activities that occurred in North Carolina between 2013 and 2015. They then compared the recorded data with news reports featuring mysterious booms or other similar phenomena.
"We wanted to go through local news articles, create a catalog of instances of the Seneca guns, and then try to verify them with actual seismo-acoustic data," said Eli Bird, an undergraduate student at the University of North Carolina at Chapel Hill and one of the study's researchers.
Bird and his colleagues found that two of the sensors detected some unusual signals near Cape Fear in Carolina Beach. The signals varied in length from about one second to nearly 10 seconds. The researchers then examined seismic records but found no earthquake that coincided with the signals. This effectively ruled out ground shaking as the cause of these two noises.
Because of these findings, Bird and his colleagues suspected that skyquakes are really atmospheric phenomena. "We don't think it's coming from seismic activity, we're assuming it's propagating through the atmosphere rather than the ground," Bird said.
Possible atmospheric causes included meteors that exploded above a cover of clouds. The resulting debris could burn up on their way down to the ground, causing such explosions to go unnoticed. Another explanation was that some skyquakes could simply be sounds produced by large ocean waves. These sounds could be heard widely because they were amplified by certain atmospheric conditions near the sea. (Related: Daytime fireball meteor generates sonic boom over UK and France.)
In other cases, skyquakes could be sonic booms generated by fast-moving aircraft that broke the sound barrier. This happened multiple times before during military operations.
For example, the earthshaking noises felt throughout the East Coast in January 2016 turned out to be sonic booms produced by a Navy fighter jet. The Navy owned up to the noises, saying that it was conducting supersonic testing of an F-35C stealth fighter in a "cleared military flight area" off the East Coast.
According to the U.S. Geological Survey, the aircraft generated nine sonic booms over the course of one and a half hours. The booms were heard in southern New Jersey, Long Island and elsewhere on the Eastern Seaboard.
Because of the wide range of possible explanations, Bird said that skyquakes could not be attributed to one cause only. Moving forward, he and his colleagues plan to use an array of at least three stations to collect more data and make a more accurate determination of where a particular skyquake originated.
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