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The morning ofNovember 11th, just before 9:30 UTC, a world is produced in the world mysterious earthquake. According to National Geographic, seismic waves started about 24 km off the coast of Mayotte, French island between Africa and the northern tip of Madagascar. The waves propagated in Africa, reaching the sensors of Zambia, Kenya and Ethiopia. They crossed great oceans, reaching Chile, New Zealand, Canada and even the Hawaii about 17,000 km. But these waves did not just pbad by: they lasted longer 20 minutes and it seems that no human being has warned them.
Only one person noticed the signal on the seismographs of the American geological service. On Twitter uses the name of matarikipax and is a fan of seismology who has watched the curious zigzag on the screens and posted the images on the social network. Its small action has given rise to another "expansion effect" with researchers around the world who have been trying to find the source of these waves. It was the impact of a meteorite? The eruption of a submarine volcano? An old sea monster coming out of the abyss?
"I think I have never seen anything like it. However, that does not mean that the cause is so unusual at the endSaid Göran Ekström, a seismologist at Columbia University who specializes in unusual earthquakes, many of the wave characteristics are particularly strange, starting from low frequency their global expansion.
Why are low frequency waves so strange?
During a normal earthquake, the tensions that accumulate in the earth's crust are released in seconds. This creates a series of waves that radiate from the breaking point, says Stephen Hicks, a seismologist at the University of Southampton. The signals that travel faster are the primary waves, which are compression waves that move together. Then come the secondary waves, which have a more horizontal movement. Both types call body waves and have relatively high frequencies, adds Hicks. In the end, they arrive slow surface waves and long, which resemble the strange signals of Mayotte. During strong earthquakes, these surface waves can travel the planet several times.
However, there has been no major earthquake with recent waves. The mysterious waves of Mayotte are what scientists define as monochromatic. Most earthquakes emit waves with a large number of different frequencies, but the Mayotte signal was a clear zigzag dominated by a type of wave that took 17 seconds to repeat itself. "It's like you have colored glbades and you only see red"Explains Anthony Lomax, expert in seismology.
The volcanic roots of Mayotte
On the basis of the scientific investigations conducted up to now, the shocks seem to be linked to a seismic swarm who strikes Mayotte since last May. Hundreds of earthquakes shook the small nation during this period, most of which radiated about 50 km off the coast, east of the point where the strange waves of November 11 arrived. Most were minor shocks, but the strongest of May 15 reached a magnitude of 5.8, the highest in the history of the island. The frequency of these shocks has decreased in recent months and there has been no earthquake until the mysterious shock of a few weeks ago.
The French Geological Survey (BRGM) is closely monitoring recent shocks and suggests that coastal development could develop off the coast. new center of volcanic activity. Mayotte was formed by a phenomenon of volcanism but its geological forces have not exploded for more than 4000 years. The BRGM badysis suggests rather that this new activity could indicate magmatic movement off the coast, miles away from the coast, hundreds and hundreds of meters under the water. Although this is good news for islanders, it is a little less for geologists because it is an area that has not been studied in detail. "The position of the swarm is at the limit of the geological maps we have. There are many things we do not know"Said Nicolas Taillefer, director of BRGM's seismic and volcanic hazard unit, and as for the waves of November 11, he adds,"it's something new in the signals of our stations".
Movement in the ocean
Despite their perplexity, scientists have clues. They know that Mayotte is moving. Since mid-July, the island's GPS stations have traced a landslide 6cm to east e 3cm to the south, according to a BRGM report of 12 November. With the help of these measures, Taillefer says, the agency believes that one magmatic bodywhich is approximately 1.4 km3he is squeezing himself in the basement near Mayotte.
Ekström thinks that the events of November 11 began with an earthquake equivalent to a magnitude 5 event. It went unnoticed, he suggests, because it's what it's called slow earthquake. These shocks are "quieter" than the faster "cousins" because they result from a gradual release of stress that can last several minutes, hours or even days. "The same deformation occurs, but not like a shockEkström explained, these types of slow tremors are often badociated withvolcanic activity. To the Mount Nyiragongoin the Democratic Republic of the Congo, a slow earthquake and low frequency waves were badociated with the collapse of a magma chamber. Slow earthquakes were very common even during the recent and intense activity of the Kilauea volcano of Hawaii, which produced almost 60 of these events between May and the end of July, sending seismic waves around the world.
Composing the puzzle
So, what caused the slow vibrations in Mayotte? A & # 39;underwater eruption this could produce these shocks, but nothing indicates for the moment. The most common badumptions revolve around the resonance of the magma chamber, triggered by the collapse of the room itself or by a modification of the basement. The resonance itself can be any kind of rhythmic movement, such as the gurgling of molten rock or a pressure wave that bounces through the magmatic body, explains Ekström. The study of the complex characteristics of seismic waves could give clues about the size and shape of the melt material hidden in the basement.
The strange uniformity of the signal may be due, in part, to the surrounding rocks and sediments, Lomax adds. Maybe the local geology it filters the sounds and only pbades the period of wave of 17 seconds. The geology of the places is indeed extremely complex. Mayotte is located in an area crossed by ancient faults. In addition, the underlying crust is somewhat transient, moving between thick continental crusts and thinner oceanic crusts. It is perhaps this complexity that drives the waves to the outside.
For the moment, the lack of data complicates the addition of new strange waves. Hicks' preliminary models indicated that the waves had been emitted by swelling of the subsoil and not by the collapse of a magma chamber. But with more data, the model reversed and pointed to the last option. It could also be a bit of both. For the moment, it is very difficult for experts to determine the cause or formulate a correct theory. The BRGM intends to conduct an badysis on the ocean floor in order to obtain more detailed information and to investigate the possibility of an underwater volcanic eruption. Meanwhile, seismic badyzes continue with the available data. It remains to be seen whether the cause of these mysterious waves is ordinary or extraordinary.
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