Mysterious seismic waves in the Indian Ocean that were picked up by monitoring stations from Madagascar to Canada three weeks ago have baffled scientists.
Researchers and earthquake enthusiasts who spotted the signals have narrowed down the origin to a region just off the coast of the island Mayotte.
The slow waves detected on November 11 rumbled for more than 20 minutes, unbeknownst to most people.
They are similar to those typically seen after large earthquakes, which are known to travel great distances – but, no such earthquake took place.
Theories as to what caused the cryptic rumble have ranged from a slow earthquake to an undetected meteor strike.
One researcher told MailOnline that the trembling was ‘almost certainly’ caused by a low-level underwater volcanic eruption off the northeast of Mayotte.
Seismic signals originating off the coast of the small French island of Mayott were detected at seismology station ranging from Chile to New Zealand
Researchers and earthquake enthusiasts who spotted the signals have narrowed down the origin to a region just off the coast of the island Mayotte (shown), in the Indian Ocean. But, no one’s quite sure what caused the strange event
‘I don’t think I’ve seen anything like it,’ Göran Ekström, a seismologist at Columbia University who specialises in unusual earthquakes, told National Geographic.
But, he adds, ‘It doesn’t mean that, in the end, the cause of them is that exotic.’
The monotone ‘ring’ was picked up by seismographs almost 11,000 miles (18,000 km) from Mayotte, and were spotted by chance.
A New Zealand based Earthquake enthusiast who goes by the handle @matarikipax noticed unusual seismology readings from the US Geological Survey.
The agency publishes all of its recordings for free online, allowing anyone across the globe to trawl through its data.
‘This is a most odd and unusual seismic signal. Recorded at Kilima Mbogo, Kenya,’ @matarikipax wrote on Twitter on November 11.
Experts detected strange earthquake-like readings at seismology stations across the globe. Pictured are seismograph readings produced by three detectors at the FOMA seismology station in Madagascar. The strange seismic waves were picked up around 9:30am GMT (4:30am ET). The thick black lines show normal background noise that could be caused by anything from waves hitting a beach to a strong wind. The thinner, larger back lines show something more significant, and would normally indicate that an earthquake has occurred
‘The signal can be seen all around the world.’
Their posts sparked the interest of seismologists on Twitter, starting a global discussion as to what caused the covert shakes.
Many others chimed in to the conversation to point out where else the low-frequency waves were detected: Chile, New Zealand, Canada, and Hawaii.
They are unusual because of their monotone, low-frequency ‘ring’, as well as their global spread.
During a typical earthquake, the build up of tension releases in a flash of mere seconds, releasing a powerful ‘ping’ of pressure.
The strange waves were traced to an origin roughly 15 miles (24 kilometres) off of the French island, Mayotte
The fastest travelling signals, known as primary or P waves, reach seismographs first, and produce a bunched-up reading of repeated tremors.
They are followed by S or secondary waves, which register as longer, side-to-side motions.
Finally, ‘slow waves’ reach seismographs, which are prolonged rumble – much like the waves triggered at Mayotte.
These signals seemed to propagate without a triggering earthquake – and that wasn’t all that baffled scientists.
Earthquake experts on Twitter debated over what could have caused the seismic waves. The Twitter thread pictured first revealed the strange phenomenon on the morning of November 11. Scientists and earthquake enthusiasts alike worked to narrow it down
The waves were monochromatic, meaning it did not send out a bundle of frequencies, like most earthquakes.
Instead, the zigzagging pattern it produced was primarily made up of one type of wave, which took 17 seconds to repeat.
University of Plymouth Geology Graduate and founder of UK Earthquake Bulletin Jamie Gurney said he had ‘no idea if a similar global signal of this nature has ever been observed’.
Scientists are working to understand what spurred the mysterious waves on that day.
So far, many suspect they’re related to an ongoing seismic swarm in the region that began last May.
Hundreds of small quakes have hit the region during that time, with the largest reaching magnitude 5.8 on May 15.
But even then, there was no corresponding earthquake on November 11.
HOW ARE EARTHQUAKES MEASURED?
Earthquakes are detected by tracking the size, or magnitude, and intensity of the shock waves they produce, known as seismic waves.
The magnitude of an earthquake differs from its intensity.
The magnitude of an earthquake refers to the measurement of energy released where the earthquake originated.
Magnitude is calculated based on measurements on seismographs.
The intensity of an earthquake refers to how strong the shaking that is produced by the sensation is.
A 5.3 magnitude earthquake hit the Channel Islands off the coast of southern California on Thursday at 10.30am
According to the United States Geological Survey, ‘intensity is determined from the effects on people, human structures and the natural environment’.
Earthquakes originate below the surface of the earth in a region called the hypocenter.
During an earthquake, one part of a seismograph remains stationary and one part moves with the earth’s surface.
The earthquake is then measured by the difference in the positions of the still and moving parts of the seismograph.
Researchers with the French Geological Survey (BRGM) say it could be a signal that magma beneath the volcanic island is shifting offshore.
Anthony Lomax, an independent seismology consultant, agrees.
He told MailOnline the shakes were ‘almost certainly’ caused by undersea activity, not very deep under the seafloor, to the northeast of Mayotte.
‘There has been ongoing low-level seismic activity there since May,’ Mr Lomax said.
‘Inflation/deflation and collapse of volcano calderas, and movement of magma under a volcano can produce a wide variety of seismic signals, including long period and repetitive waves like those observed November 11.’
Others say there may have been a ‘slow’ earthquake that simply went by unnoticed, or an underwater eruption.
The experts say the complex geology of the region further compounds the issue, potentially filtering some of the waves to make the clean signal, NatGeo reports.
Scientists plan to survey the ocean to find out any additional information that could help to explain the mysterious phenomenon.
But at this stage, the experts agree there’s just too much we don’t know to say what was really to blame.