Seismic sensors first picked up the event originating near an island between Madagascar and Africa. Then, alarm bells started ringing as far away as Chile, New Zealand and Canada.
Hawaii, almost exactly on the other side of the planet, also picked up the 'event'.
Nobody knows what it was.
Meteorite? Submarine volcano? Nuclear test?
"I don't think I've seen anything like it,"National Geographic reports Colombia University seismologist Göran Ekström as saying. "It doesn't mean that, in the end, the cause of them is that exotic."
At the centre of the mystery is the tiny island of Mayotte, positioned about half way between Africa and Madigascar. It's been subjected to a swarm of earthquakes since May. Most have been minor, but the biggest — on May 8 — was the largest in the island's recorded history, topping magnitude 5.8.
But the earthquake swarm had been in decline before the mysterious ringing was detected earlier this month.
Ekström, who specialises in unusual earthquakes, points out much about the November 11 event was weird. It was as though the planet rang like a bell, maintaining a low-frequency monotone as it spread.
Earthquakes, by their very nature, usually register as short-sharp 'cracks'. As tensions in the Earth's crust suddenly release, pulses of clearly identifiable seismic waves radiate outwards from where the slippage occurs.
The first signal is called a primary wave: high-frequency compression waves that radiate in bunches.
Then comes a secondary wave: these high-frequency waves tends to 'wiggle' somewhat more.
Only then comes the surface waves: these slow, deep rumbles tend to linger, and can circle the Earth several times.
The November 11 event is notable in that no primary or secondary waves were detected.
All that registered was the deep, resonating surface wave. And it didn't 'rumble' as an earthquake's surface wave tends to. Instead, it maintained a much cleaner — almost musical — frequency.
National Geographic reports the French Geological Survey suspects a new volcano may be developing off the coast of Mayotte. While the island was created by volcanic activity, it's been dormant for more than 4000 years.
The French believe the weird ringing may have been generated by a movement of magma some 50km off the coast and under deep water. This is supported by GPS sensors detecting Mayotte has moved some 5cm to the southeast in less than five months.
But it's a poorly mapped region. Exactly what's beneath the ocean can only be guessed at.
Ekström believes the unusually pure signal could have been caused by magma sloshing about inside a chamber, or being forced through a gap in the subsurface rocks.
But he's not certain.
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