Vibrations off Madagascar baffled experts but now they believe they have the answer

It is the kind of mystery scientists relish. On 11 November, something stirred near the French island of Mayotte off the west coast of Madagascar and sent a rumble around the world. Travelling at 9,000mph, the deep hum hurtled past earthquake detection systems unnoticed. No one appears to have felt a thing.

The event came to light on Twitter when seismology enthusiasts posted weird signals they had spotted in recordings made by seismic stations from Kenya to Hawaii. Having ruled out the violent lurches of an earthquake, educated guesses gave way to more fanciful theories. Was it a landslide? A meteorite exploding in the atmosphere? The awakening of some long-dormant sea monster?

Now researchers believe they have an answer. Stephen Hicks, a seismologist at the University of Southampton, was on the case fast. He downloaded data from a global network of seismic stations and set about analysing them. “What’s unusual is you see this very long signal travelling most of the way around the world which hasn’t been detected by operational earthquake detection systems,” he said.

By looking at when different seismic stations dotted around the planet detected the rumble, the 30-minute or so signal was traced back to an event that took place at about 9.30am GMT in the sea near Mayotte. Geologists knew that a number of tremors had already rocked the region since a magnitude-5.8 earthquake in May. But earthquakes unleash high frequency seismic waves that vibrate back and forth and from side to side. “This source was completely deficient in those waves,” said Hicks. “It wasn’t picked up because the signal had a very low frequency. It was a low, gentle rumbling.”

The Mayotte vibrations took about 40 minutes to reach Britain, and an hour and 15 minutes to reach Hawaii, more than 11,000 miles from their point of origin.

Such low frequency rumbles are rare but not unheard of. Scientists have detected them before after glacier calving, landslides and sudden shifts of magma beneath volcanoes. There are no glaciers near Mayotte and an underwater landslide would have been picked up by hydrophones in the surrounding ocean, said Hicks. That leaves a magma shift somewhere under the seabed as the prime culprit.

Hicks believes magma may suddenly have drained from a volcanic chamber about 10 miles under the seafloor near Mayotte, setting off the deep rumble that spread around the world. While strong enough to be picked up by sensitive seismometers, the vibrations would have been minuscule: far smaller than a millimetre. “It’s something that you wouldn’t perceive,” he said.

Pierre Briole, a geoscientist at École Normale Supérieure in Paris, has reached a similar conclusion. He believes that a third of a cubic mile of magma may have drained from a volcanic chamber under the seafloor, unleashing deep vibrations when its roof collapsed.

Much of the seismic sleuthing played out on social media with professional and amateur scientists working together. “Overall, [it has been] a fascinating demonstration of open science on Twitter and engagement between scientists and citizen seismologists,” said Hicks.