Just before 9.30am on Sunday 11 November, a series of unusual seismic pulses rippled around the world almost undetected.
The waves rang for over 20 minutes, emanating about 15 miles off the shores of Mayotte – a tiny island in the Indian Ocean between Madagascar and Africa.
From here, they reverberated across Africa, setting off geological sensors in Zambia, Kenya, and Ethiopia.
Despite their huge range, the waves were apparently not felt by anybody. However, one person monitoring the US Geological Survey’s live stream of seismogram displays did notice the unusual waveform and posted it to Twitter, sparking the interest of other geologists and earthquake enthusiasts.
It was not only the power of the seismic waves which puzzled scientists when they began to examine the readings, but also the curiously regular shape of the waveform.
In a typical earthquake, the rapid crash of a tectonic plate movement sends out what is known as a “wave train”, composed of several types of waves moving at different speeds from the epicentre of the quake.
Seismographs measure the fastest waves first (the primary or P waves), which arrive in an abrupt cluster, then the secondary (S waves), and finally, rumbling along later, come slower low-frequency surface waves. In a powerful quake, these can move across the surface of the planet several times.
The wave pattern seen on 11 November resembled these slow-moving waveforms usually seen following large earthquakes – only, in this case, there had not been a perceptible earthquake.
The bizarre waveform is what scientists call “monochromatic”. Earthquakes normally produce waves of so many different frequencies, the wave readings appear more jumbled.
But the mystery waveform from Mayotte was a crisp zigzag, which repeated after steady 17-second intervals.
“They’re too nice. They’re too perfect to be nature,” joked the University of Glasgow’s Helen Robinson, who is study for a PhD in applied volcanology.
Speaking to the National Geographic, she added the location of the island means industrial sources for the unusual wave – such as oil drilling or from wind farms – could be ruled out.
Founder of UK Earthquake Bulletin (UKEQ) Jamie Gurney, who was among those who began looking at the waveform, saying on Twitter he had “no idea if a similar global signal of this nature has ever been observed”.
So what could cause these regular slow waves? Could it have been a meteor strike? An illegal weapons test? Previously undiscovered sea monsters?
Mayotte lies between Tanzania and Madagascar (Google)
Despite the mystery, scientists do have clues as to the cause of the seismic event.
Mayotte, an inhabited French island, was formed as a result of volcanic eruptions around 4,000 years ago, but since then, the area has largely remained volcanically inactive.
However, since May last year, the island nation has been rocked by hundreds of earthquakes, all of which have emanated offshore and the largest of which measured 5.8 on the Richter scale. The island is believed to be moving about two inches southeast each year.
But in recent months, seismic activity has declined, and on the 11 November, no traditional quakes were detected.
According to the National Geographic, analysis by the French Geological Survey suggests the new activity may point to huge movements of magma beneath the earth’s crust, miles offshore and under thousands of feet of water.
Large movements of liquid rock, or reverberations through the magma chamber can cause waves similar to those measured in this instance.
GPS measurements of Mayotte’s slow movements indicate a magma body measuring about a third of a cubic mile is pushing through the subsurface, and this could be the source of the weird waves.
“It doesn’t mean that, in the end, the cause of them is that exotic.”
Another theory is that an underwater volcanic eruption occurred, which would have been capable of producing the waveforms, however no evidence of such an event has yet been seen – satellite images are not believed to have recorded the formation of a “pumice raft” typically seen after a submarine eruption.
Regarding the uniform 17-second waveform, scientists currently estimate the resonance from the magma body could be being sonically filtered by the unique and complicated tectonic geography of the area, only allowing certain frequencies to escape the immediate environment.
While the definitive cause and the conditions in the area remain unknown, the French Geological Survey is planning to undertake ocean bottom surveys to obtain detailed information about this little-explored region, as well as investigating the possibility of a submarine eruption.