Scientists are unable to explain a strange seismic event that shook the planet on November 11 and that was detected by seismic sensors installed around the world.
Although the cause of this mysterious disturbance remains unknown, it is in some way linked to an ongoing seismic swarm that has rumbled over the archipelago of Mayotte in the Indian Ocean for several months – but the exact meaning of these unusual tremors remains unclear .
"I do not think I have seen anything like it," said Göran Ekström, seismologist, from Columbia University. National Geographic about the anomaly of November 11th.
Almost six months before this strange signal was detected, seismologists were surprised by another type of abnormal seismic activity in the same neighborhood: a swarm of hundreds of small and frequent earthquakes from about 50 kilometers of the east coast of Mayotte. .
The network of islands and islets, located roughly halfway between Africa and Madagascar, is governed by France, but is also claimed by the island nation of Comoros.
On the morning of May 10, this region was shaken by an earthquake that occurred without warning and that did not occur alone, followed by a series of hundreds of tremors that still have to go away. .
The most dramatic of them – an event of magnitude 5.8 on May 15 – was the largest earthquake ever recorded in the Comoros basin and, although the intensity of the swarm has generally decreased since then, a resurgence of magnitude 5.1 this week did not seem so enjoyable. Subtle reminder that this earth turbulence is not over.
Although earthquake swarms seem alarming, they are not necessarily dangerous.
In this case, a preliminary analysis of the seismic swarm carried out by researchers from the École Normale Supérieure de Paris suggests that tremors can only be explained by the tectonic movement alone, which means that the lor Volcanic activity in the area must also be involved.
Which brings us to November 11th.
– ******* Pax (@matarikipax) November 11, 2018
Less than three weeks ago – during the swarm, but one day when no swarming was detected – scientists recorded something else: a strange, long, flat vibration which buzzed steadily without the spiky fluctuations characteristic of earthquake activity.
Instead, this "atypical very low frequency signal" – to quote the Bureau of Geological Research of France (BRGM) – repeats itself in a wave every 17 seconds, with a total duration of about 20 minutes.
"There are many things we do not know," said Nicolas Taillefer, head of BRGM's seismic and volcanic risk unit, National Geographic.
"It's something quite new in the signals of our stations."
Which does not mean that the team does not have a hypothesis. With what we already suspect of the seismic swarm, the researchers' best guess is that the abnormal vibrations are also related to volcanic activity, perhaps due to a huge movement of magma under the Indian Ocean.
If this is the case, it could also explain something else: Mayotte is not immobile.
GPS readings indicate that since July – after the start of the swarm – the island has moved about 60 mm (2.4 inches) to the east. and 30 mm to the south.
According to one analysis, this move could be due to the emptying of a nearby magma reservoir, although additional research is needed to verify it.
The SBV, like the other stations, displays a long monochromatic signal with a period of ~ 17 s (single-frequency Rayleigh waves). But filtered above 1Hz SBV (lower plot) also shows seismic signals (?) From repeated sources, about 50 seconds apart. Maybe a large, shallow, oscillating volcanic source? pic.twitter.com/bPqdQFwAgm
– Anthony Lomax (@ALomaxNet) November 11, 2018
If the hypothesis proves correct, no one can say for sure what could happen, but the modeling indicates that Mayotte could continue to move as long as the swarm persists.
As for knowing if we will find the mysterious signal, nobody knows it.
"These observations therefore support the hypothesis of a combination of tectonic and volcanic effects explaining a geological phenomenon involving a seismic sequence and a volcanic phenomenon," explains the BRGM.
"This hypothesis will have to be confirmed by future scientific studies."