The island of Mayotte is located between Mozambique and Madagascar. Since May 10, the French Geological Survey has been closely monitoring a series of earthquakes that occur just off the east coast of the island, with a peak of magnitude 5.8. This swarm was already quite curious – and then, on November 11, things took a particular turn.
That day, seismographers around the world recorded a tremor of very low frequency, a strange rumor that lasted about 30 minutes. A segment of the signal also contained several high frequency transistors, each separated by about a minute, much like an ordinary clock.
Stephen Hicks, a seismologist at the University of Southampton, highlighted the phenomenon on 12 November. tweet. "Yesterday, something serious, strangely slow, caused seismic rumblings on the surface of a large part of the planet," he wrote. Today, this brief summary is still valid, even if the discussion continues between geoscientists of all types. on Twitter.
To this day, no one is really sure of the cause. It was not a tectonic earthquake, a landslide, or a meteorite impact. Some experts, disconcerted by the seismic puzzle, suggested that perhaps, finally, they are sea monsters.
The strange sound was almost certainly not that Godzilla was not walking in a submarine, but scientists are starting to regroup around what they suspect to be the solution: strange and invisible magma movements , far below the seabed.
To understand why, however, we must go back to the beginning.
Robin Lacassin, a geologist at the Paris Institute of Earth Physics, told Gizmodo that Mayotte was part of the Comoros archipelago, a chain of volcanic islands that witnessed the rainy day. a fireworks 20 million years ago. Eruptions have recently intensified on other islands, including Grande Comore. Conversely, Mayotte, a relatively dormant presence, has not seen fresh lava for about 4,000 years.
In all cases, the volcanism of the region, fed by a small mantle plume or a distorted tectonic plate, is still alive. The earthquake swarms occurring near Mayotte, although new to the region, resemble those produced by magmatic processes, such as the movement of magma and other overheated fluids.
The November 11th signal, however, is a very different kind of fish porridge. No specific natural processes have yet been associated with seismic data, and the lack of monitoring at the event site means that even its precise location is not yet clear.
Let's first see this low-frequency buzz. Representing surface waves traveling across the planet, this has dominated the overall signal. High frequency waves have not been picked up by remote stations.
Hicks told Gizmodo that a good analogy is that of someone living down the street from a nightclub. You may not be the neighbor, but as long as the energy behind the beats is high enough, "what you'll hear is the bass, not the high-pitched sounds," which is what's going on. weakens fairly quickly.
Many slow and powerful events can create powerful bass lines, ranging from tectonic earthquakes to submarine landslides and even glacial winds. In this case, the tide gauges showed no change during the event, which meant that there was no mass movement on the seabed. The few GPS stations in the area recorded local movements of the seabed, but a little, suggesting that the signal source was deep enough.
The only remaining plausible explanation was a kind of volcanism. It is perfectly reasonable; after all, Mayotte is volcanic and the swarm resembles a volcanic activity. Anthony Lomax, a consulting seismologist, told Gizmodo that it was probably "a volcanic activity, causing earthquakes".
Although an underwater eruption can not be ruled out, there is not yet any observable sign, such as fresh lava on the seabed or a pumice raft floating on the surface. Instead, Lacassin stated that all the data indicated the movement of magma in the crust. But where exactly was he going?
A day after the event, researchers from ENS University in Paris suspected that magma was retreating from the site, causing the roof to collapse from a reservoir of magma and tremors. Earth. In a statement, they estimated that 0.3 cubic miles of magma draining from a tank would match GPS data well.
According to Lomax, a good comparison is the one that was recently seen at Kilauea in Hawaii. During this eruption, the magma is continuously drained from the summit crater. The crater was deflated, causing earthquakes, and magma escaped under the east flanks of the volcano.
However, Hicks' initial analysis of the available data seemed to indicate that the magma was moving in a reservoir and inflating it. Jean Paul Ampuero, a seismologist and director of research at the French Institute for Research for Development, accepted. "The events of collapse or inflation have opposite signs," he told Gizmodo. "This one is inflation type."
At the time of writing, Hicks's analysis is leaning back towards drainage. In any case, the magma must be in motion, but nothing is certain yet.
The deficit of instruments near the signal site means that "there are no real data sets before and after". Helen Robinson, a geothermal expert and PhD student at the University of Glasgow, told Gizmodo. Useful information on what physically changed at this place near Mayotte does not really exist.
The key piece of evidence here is the seismic signal, which certainly shows the ghost of something weird. The signal is so strange that it turns out difficult to decipher.
Ampuero said he spoke to one of his colleagues, specialist in seismology of volcanoes. They confirmed that the internal chaos of volcanoes can cause very low frequency snoring, but the incredible 30-minute duration of it was currently inexplicable.
In order to get that kind of prolonged growling and low frequency, you need something big to resonate. "It's like a bell ringing. If you want to get a very low frequency, a very low tone, you need this bell is huge, "said Ampuero. "Maybe the magma reservoir was hit by a wave of small earthquakes, then kept ringing for a little while."
Then there are these high-frequency blips that look like clocks and that came to light after the seismologists started filtering the deep hum of the signal. Robinson explained that such blips resemble those detected when the rock is fractured when the magma makes its way through.
"What bothers me is how much they are spaced evenly," she said. "I do not know how to explain that." The same goes for anyone.
Signals with very regular intervals tend to be generated artificially, but they may have natural origins. Just take pulsars: when, in the 1960s, we discovered these objects in the form of a lighthouse in rapid rotation, we briefly suspected the regularity of lightning to be generated by intelligent life. The original source has even been nicknamed LGM, for "little green men".
Here, high-frequency noises have instinctively recalled Robinson's industrial or military activity, but there is no such sign in the area. In this case, the regularity could be a magma moving at a constant speed in the rock, causing repetitive fractures. As for the signal, this interpretation is far from certain.
Lomax evokes the parable of the blind and the elephant, which tells the story of those men who first met an elephant, each trying to identify what it is like touching that part of it. Similarly, researchers from different layers of geoscience give their own interpretation of the curious signal.
"It's a good start because you want different shots in different directions," said Lomax. Cracking the box, however, is a team effort. Fortunately, judging by the frenetic scientific discussion that always takes place on social media, it is already in full swing.