In the early hours of November 11, 2018, an extremely unusual event left geologists baffled. A series of strange seismic waves rippled across the world. They originated 24 kilometers off the coast of an island near Madagascar called Mayotte. The waves passed over oceans, reaching as far as Canada and New Zealand. And experts can’t agree why it happened.
The term “seismic waves” refers to waves of energy that move through the earth. They occur due to the abrupt fracturing of rock within the planet. A variety of occurrences can lead to seismic waves, including man-made explosions, landslides, volcanic eruptions and earthquakes.
Seismology is the examination and analysis of earth’s seismic waves. Experts in the field use a variety of specialist equipment to note and measure the effects of waves and earthquakes. Seismologists record these using a variety of instruments, including accelerometers, seismometers and hydrophones.
There are a variety of seismic waves for seismologists to consider. And each of them demonstrate different properties in relation to the manner with which they move. But there are two primary types: surface and body waves.
Surface waves travel along the earth’s surface, creating a rippling effect on water. Body waves, on the other hand, move through the interior layers of the planet. Meanwhile, earthquakes give off seismic energy as both surface and body waves.
Earthquakes can be so powerless that they go unnoticed on the ground above. However, strong earthquakes are very dangerous and can level entire urban centers. For example, one of the worst such events in recorded history was a magnitude 8 earthquake which took place in the Chinese province of Shaanxi in 1556. It killed over 830,000 people.
In the 21st century though, one of the most devastating earthquakes occurred in Haiti. Here, on the evening of January 12, 2010, a magnitude 7 earthquake shook the country, resulting in relentless destruction. A series of over 50 aftershocks followed, causing additional damage and taking yet more lives.
Estimates vary as to the death toll, but figures range between 100,000 up to as high as 316,000 people. And the Haitian government estimates that around 30,000 commercial buildings and 250,000 homes were damaged or destroyed.
Left homeless after the quake, many Haitians slept out in the open, or in hastily raised shanty towns. And so now, in addition to the direct havoc caused by the earthquake, people were living in unsanitary conditions. Thus in October 2010 cholera – a rare infection since the implementation of contemporary sanitation systems – began spreading throughout the population.
The Haitian cholera epidemic spread to almost 800,000 people in the country, killing over 9,000, according to the United Nations. And if this outbreak illustrates anything, it’s that earthquakes have the ability to devastate areas and populations in ways beyond their initial impact.
Meanwhile, the seismic event in November 2018 was in no way as damaging as such a powerful earthquake. Indeed, despite having occurred for over 20 minutes, nobody within the areas the seismic waves traveled through could feel them. And they covered an incredibly wide expanse.
The seismic waves originated from near the island of Mayotte, around 24 kilometers from its coastline. Mayotte is located between Mozambique and Madagascar. It is made up of a series of islands and islets in the Indian Ocean, and is an overseas department and region of France.
From this point, the waves traveled across the sea along the east of Africa, and were recorded in countries such as Kenya, Ethiopia and Zambia. They also traveled in different directions, and were picked up in Canada, New Zealand, Chile and Hawaii. Yet in spite of all this, if it weren’t for a single person posting to Twitter, the incredible phenomenon might have passed by relatively unnoticed.
The individual in question, known by the Twitter handle @matarikipax, noted the unusual activity on the U.S. Geological Survey’s seismogram displays. These are visual representations of information noted by real-life seismograph stations that are updated frequently throughout the day. When the user saw that something strange was happening, he directed his Twitter followers to the seismogram displays.
According to experts who have now studied the seismogram displays, the features of these particular seismic waves were irregular. Indeed, a seismologist from Columbia University, Göran Ekström, put it rather succinctly. He told National Geographic in 2018, “I don’t think I’ve seen anything like it.”
To understand the confusion, one must look at the specific nature of seismic waves that are typically caused by earthquakes. That’s because the waves originating off Mayotte mirrored the nature of those following a quake. Yet no such event was recorded in the area on November 11, 2018.
After an earthquake, a combination of different sorts of waves called a “wave train” reverberates from the epicenter. First off comes a series primary waves, or P-waves, followed by secondary S-waves. And both of these have quite high frequencies.
Next, lower frequency surface waves travel from the epicenter. In the case of a powerful earthquake, these surface waves can travel across the planet a number of times. Indeed, Stephen Hicks from the U.K.’s University of Southampton told National Geographic that the phenomenon resembles “ringing earth like a bell.”
So the Mayotte slow waves were similar to the surface waves that occur after earthquakes, even though one was not recorded then. And to add to the confusion, earthquakes tend to emit waves of varying frequencies. But the Mayotte waves were monochromatic – meaning that they occurred at a regular frequency.
The nature of these waves has left experts searching for answers. “They’re too nice. They’re too perfect to be nature,” PhD student Helen Robinson of the University of Glasgow explained to National Geographic.
Yet for all the uncertainties surrounding the seismic waves, Robinson is confident that certain theories can be dismissed. Mayotte isn’t located near the site of any mechanical activities such as wind farming or oil drilling. This rules out an industrial source for the phenomenon.
But the question as to what did happen still remains. And a definitive answer has thus far proven elusive within the seismological community. However, it seems to be broadly believed that the unusual event correlates with volcanic activity.
Indeed, since May 2017 a seismic “swarm” of hundreds of mostly small earthquakes have hit Mayotte. Others, though, have been more powerful, and the island actually experiencing the strongest earthquake in its recorded history during the period, with a magnitude of 5.8.
The majority of the earthquakes have had an epicenter located roughly 31 miles offshore, slightly east of where the seismic waves originated. However, the number of earthquakes hitting the area has recently been petering off. And, as mentioned earlier, no conventional earthquake was noted when the curious waves showed up.
The French Geological Survey has suggested that a new hub of volcanic activity is evolving near Mayotte. Indeed, volcanic processes formed the island, though an eruption hasn’t occurred there in more than 4,000 years. This new activity, however, could point to magmatic movement miles from the shore, underneath the deep waters of the ocean.
Indeed, this is similar to what Stephen Hicks, a seismologist from the U.K’s University of Southampton, has concluded. He believes that magma found within a volcanic chamber around ten miles below the seafloor near Mayotte abruptly drained. This, he told The Guardian, would have caused the seismic waves which spread worldwide.
Understandably, the suggestion that the volcanic activity is occurring in the ocean, away from Mayotte itself, is good news for the people who live there. But it’s problematic for geologists, because they haven’t studied the waters around the island in great detail.
And the lack of information regarding the waters surrounding Mayotte isn’t the only issue for geologists. “The location of the swarm is on the edge of the [geological] maps we have,” head of the seismic and volcanic risk unit at the French Geological Survey Nicolas Taillefer told National Geographic. “There are a lot things we don’t know.”
Indeed, the obscurities of this event are illustrated in the scientific community’s inability to agree on its simpler aspects. Columbia University’s Göran Ekström actually believes that the tremor was, in fact, caused by a type of earthquake with an equivalent strength of magnitude 5.
Ekström told National Geographic that scientific instruments didn’t pick up the tremor because of its nature as a “slow earthquake.” Such a quake would be more subtle than a regular one and could even occur between a few minutes up to a matter of days.
Clarity on the question of these waves is also muddied by the deeply complicated geology of Mayotte. The island is actually located in a region defined by ancient faults. These include fracture zones which occurred after the supercontinent Gondwana split, some 180 million years ago. The modern landmasses that were once unified as Gondwana now form South America, India, Africa, Australia, the Arabian Peninsula and Antarctica.
Furthermore, Helen Robinson of the University of Glasgow explained to National Geographic that the underlying crust beneath the area around Mayotte is transitional. This means that it alternates between the more bulky continental and the slimmer oceanic crusts. And it’s intricacies like this which could, potentially, account for the strange nature of the seismic waves.
Meanwhile, seismologist Jean-Paul Ampuero, from the French Université Côte d’Azur, had his own suggestion. He told the publication that focusing on the specific characteristics of the waves will be key to scientists’ understanding. He suggested that they could help to understand the molten matter moving beneath the earth’s surface. And he had an interesting analogy to illustrate this.
“It’s like a music instrument,” Ampuero told National Geographic. “The notes of a music instrument – whether it’s grave or very pitchy – depends on [its size].”
Nevertheless, experts are still struggling to agree on what actually happened on November 11, 2018. As such, the mystery has captured many people’s imaginations. Indeed, some of the more bizarre explanations for the waves include a meteorite crashing to Earth and strange sea creatures.
The University of Glasgow’s Helen Robinson illustrated the lack of an agreed consensus in her interview with National Geographic. “It is very difficult, really, to say what the cause is and whether anyone’s theories are correct – whether even what I’m saying has any relevance to the outcome of what’s going on.”
So a definitive answer has thus far proven elusive. However, as Stephen Hicks pointed out to The Guardian, the fact that so many people have taken an interest and expressed their opinions online is undoubtedly positive. “Overall, [it has been] a fascinating demonstration of open science on Twitter and engagement between scientists and citizen seismologists,” he said.
And while people speculate as to the reasons behind the seismic waves, investigations continue. The French Geological Survey, for instance, is looking into the possibility of conducting investigations at the bottom of the sea. And the group intends to look for evidence of volcanic eruptions underneath the water.
Whether the French Geological Survey – or anyone else for that matter – will uncover evidence of a genuinely novel scientific event is not yet clear. But as seismology consultant Anthony Lomax explained to National Geographic, “… The science — and the fun — is in the chase.”
“Depending on what field and what time in history, 99.9 percent of the time, [a mysterious occurrence is] ordinary, or noise, or a mistake, and 0.1 percent, it’s something,” Lomax continued. “But that’s just the way it goes. That’s the way it should go. That’s scientific advance.”