Scientists cannot explain the strange seismic events that rocked the planet on November 11 and were picked up by earthquake sensors placed around the world.
While the cause of this mysterious disorder is still unknown, it is somehow related to the ongoing seismic herd that has floundered the Mayotte islands in the Indian Ocean for several months – but only anything indicated by this unusual tremor is still unclear.
"I don't think I've seen anything like this," said seismologist Göran Ekström from Columbia University National geography about the November 11 anomaly.
Nearly half a year before this strange signal emerged, seismologists were shocked by other abnormal seismic activity around the same: a swarm of hundreds of small and frequent earthquakes originating around 50 kilometers (31 miles) off the east coast of Mayotte. .
The network of islands and small islands, which lie roughly between Africa and Madagascar, is governed by France, but also claimed by the Comoros island nation.
On the morning of May 10, the region was rocked by an earthquake that appeared without warning, and which did not come alone – followed by a series of hundreds of earthquakes that had not disappeared.
The most dramatic of these – the magnitude 5.8 on May 15 – is the largest earthquake ever recorded in the Comoros basin, and while herds generally have decreased in intensity since then, a 5.1-magnitude revival this week serves as a not-so-reminder subtle that this earthly turbulence is not over yet.
While earthquake herd sounds alarming, they are not dangerous events.
In this regard, the initial analysis of seismic swarm by researchers at the École normale supérieure in Paris showed that tremor cannot be explained by tectonic movements alone, which means volcanic activity in the region must also be involved.
Which brings us to November 11.
– ******* Pax (@matarikipax) November 11, 2018
Less than three weeks ago – during the herd, but on the day when no tremor was detected – scientists registered something else: strange, long and flat vibrations that hummed consistently, without sharp fluctuations that were a sign of an earthquake ordinary activity.
On the contrary, this "very low atypical frequency signal" – to quote the French Bureau of Recherches Géologiques (BRGM) – is repeated in a wave around every 17 seconds, which lasts for about 20 minutes in total.
"There are many things that we don't know," said research engineer Nicolas Taillefer, head of the unit for earthquake risk and BRGM volcanic risk, to National geography.
"That's something new in the signal at our station."
Which does not mean the team does not have a hypothesis. With what we have already suspected about seismic swarms, the researchers' best guess is that anomalous vibrations are also associated with volcanic activity, probably because of the movement of very large magma beneath the Indian Ocean.
If so, this can also explain something else: Mayotte is not stationary.
GPS readings show that since July – after a swarm began – the island has shifted around 60 mm (2.4 in) to the east and 30 mm (1.2 in) to the south.
According to one analysis, this movement could be caused by emptying the nearby magma reservoir, although additional research would be needed to verify this.
SBV, like other stations, shows a long monochromatic signal with a period of ~ 17 (Mono-freq Rayleigh wave?). But filtered above 1Hz SBV (lower plot) also shows a seismic signal (?) From a recurrent source, with several ~ 50s apart. Maybe some large, shallow, volcanic sources oscillate? pic.twitter.com/bPqdQFwAgm
– Anthony Lomax ??? (@ALomaxNet) November 11, 2018
If the hypothesis turns out to be true, no one can say exactly what might happen, but modeling shows Mayotte can continue to move as long as the herd stays.
As if we will find a mysterious signal again, nobody knows.
"Therefore, this observation supports the hypothesis of a combination of tectonic and volcanic effects that explain geological phenomena involving seismic sequences and volcanic phenomena," explained BRGM.
"This hypothesis needs to be confirmed by scientific research in the future."