Since May 2018, a strange seismic
swarm has been rumbling around Mayotte, which includes the island of Chissioua
Mbouzi. Research now shows that the epicenter of the earthquake swarm and a
newly detected volcano both sit northeast of this island.
Strange waves rippled around
Earth. Now we may know why.
An “exceptional phenomenon” near the tiny islands of Mayotte may help
explain a low-frequency rumble that swept around the world last year.
By Maya Wei-Haas, PUBLISHED May
21, 2019
On May 10, 2018, the geologic
beasts of the tiny island of Mayotte began to stir. Thousands of earthquakes
rattled the French island, which is sandwiched between Africa and Madagascar.
Most were minor shakes, but they included a magnitude 5.8 event that struck on
May 15, the largest yet recorded in the region's history.
In the midst of this seismic
swarm, a strange low-frequency rumble rippled around the world, ringing sensors
nearly 11,000 miles away—and baffling scientists.
Now, researchers may have at last
found the source of the unexpected activity: the birth of a submarine volcano
some 31 miles off Mayotte's eastern shore. Sitting about two miles underwater,
the baby volcano stretches nearly half a mile high and extends up to three
miles across.
The observations came after
French scientists launched a multi-pronged mission to get a better grip on the
origin of the ongoing seismic swarm. Coordinated by France's National Center
for Scientific Research (CNRS), the work includes surveys from the ship Marion
Dufresne co-led by Nathalie Feuillet from the Institut de Physique du Globe de
Paris (IPGP) and Stephan Jorry of the French research institute IFREMER. The
latest venture also retrieved six underwater seismometers that have been
listening for earthquakes since February.
Volcanoes
Volcanoes are as dangerous as
they are majestic. Over 50 eruptions rock our planet every year. This video
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The data are still preliminary,
and the scientists are currently working to analyze their findings and publish
the research in a peer-reviewed journal. In the meantime, the team has issued a
joint press release announcing the new volcano and its probable link to the odd
throng of earthquakes.
“In light of this discovery, the
government is fully mobilized to pursue and deepen our understanding of this
exceptional phenomenon and take necessary measures to categorize and prevent
any risks it represents,” the agencies say in the release.
Stephen Hicks, a seismologist at
Imperial College who previously analyzed Mayotte's strange seismic happenings,
adds that the announcement offers some much needed clarity for the island's
inhabitants, who have been thoroughly shaken after months of unexplained tremors.
The geologic mystery
Mayotte is part of the Comoro
archipelago, a string of volcanic islands northwest of Madagascar. While
volcanism isn't unheard of in the region, Mayotte has long remained silent,
with its last eruption bursting free more than 4,000 years ago. But starting in
May of last year, the geologic activity on Mayotte kicked into high gear. Since
the swarm began, more than 1,800 earthquakes greater than magnitude 3.5 have
rattled the tiny island. The land itself also seems to be on the move, drifting
0.6 inches east and sinking roughly 0.4 inches down each month since mid-July.
In November, the curious
low-frequency rumbles began their global spread, sticking around for more than 20
minutes. Too low of a frequency for humans to feel, only one person noticed the
curious waves: An earthquake enthusiast who goes by the handle @matarikipax
spotted the unusual zigzags on the U.S. Geological Survey's real-time
seismogram displays and posted them on Twitter, drawing an international cohort
of scientists to the mystery.
“It was clearly a both concerning
and fascinating event that was happening,” says marine seismologist Wayne
Crawford of IPGP, who was part of the recent expedition. “It was something we’d
never seen before.”
Even back then, the experts'
conclusion was that the quakes and strange seismic signal were likely related
to the movement of molten rock. Perhaps the earthquake swarm was the result of
magma squishing through the subsurface, and the low-frequency rumble was caused
by waves resonating in a collapsing magma chamber.
The link to volcanic activity
gained further support from a preprint study posted to the EarthArxiv server in
February 2019. That research pinned the swarm on a massive magma chamber
starting to drain, in what could be the largest off-shore submarine volcanic
event yet documented.
Surveys from the ship Marion
Dufresne are helping to explain what's happening off Mayotte's eastern shore.
But with limited monitoring of
these earthquakes near their epicenters out at sea, and no direct evidence of
an eruption, nothing more definitive could be said at the time.
Pregnant Earth
Then, on May 16, the French
collaboration issued their press release, and Robin Lacassin of the Institut de
Physique du Globe de Paris, one of the organizations involved in the research,
posted a pair of images on Twitter. One picture shows the newborn volcano as
seen via acoustic imaging, which acts somewhat like a dolphin using sonar to
sense its surroundings.
“It's almost a pregnancy
ultrasound ... only with larger error bars,” said geophysicist Lucile Bruhat, who
was not part of the research team, about the picture on Twitter.
„It's almost a pregnancy
ultrasound ... only with larger error bars.“
Lucile Bruhat, Ecole Normale
Supérieure
In the image, a twisting plume
rises 1.2 miles through the water column from the top of a conical edifice.
Exactly what this plume is made of remains unknown, but the sound waves might
be bouncing off glassy shards similar to the ash that billows out of erupting
volcanoes on land, notes Helen Robinson, a Ph.D. candidate in applied
volcanology at the University of Glasgow, via email. But even temperature and
density differences of the water would show up in the images, Crawford adds, so
the plume could just be a hot mineral-rich stream, like the roiling waters from
hydrothermal vents.
While the volcano is definitely
young, exactly how young remains up for debate. It was absent in seafloor maps
of the region drawn up in 2015—and the team thinks that it didn't exist prior
to the onset of earthquake activity last May, Crawford says. Its birth could be
as recent as the summer of 2018, when GPS sensors tracked the island sinking
and shifting east as, presumably, magma drained from a chamber below.
“What we know for a fact is that
that thing didn’t exist in 2015, and now it’s here,” he says.
The other image reveals a series
of bumpy structures on the seafloor that seems to form a loose path to the new
volcanic center from where the most recent earthquakes are rattling, between
three and nine miles offshore Mayotte's Petite-Terre island. Even in the area
where the new volcano is forming, abundant ridges and bumps reveal past
eruptions that could have emerged many years ago, Crawford says.
“Maybe that volcanic center has
migrated away from the island itself,” Hicks speculates, but he notes that more
data are needed to date these potential spots of volcanic activity and confirm
this mechanism.
Crawford agrees that the features
seem to be volcanic. And by using the new earthquake data to recalibrate the
old, a curious pattern emerges, he says. The quake swarm seems to split into
three activity centers along the ridge of bumpy structures: one under the new
volcano, the most recent roughly six miles offshore, and a third one halfway in
between. But how these regions are connected is unknown, Crawford says.
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There are a few similarities
between this new structure and Hawaii's Lō'ihi seamount, an underwater volcano
growing south of Kīlauea, adds Ken Rubin, a volcanologist at the University of
Hawaii at Mānoa who specializes in underwater eruptions.
Hawaii is formed from what's
known as hotspot volcanism; each island represents a boil that formed above a
deep plume of molten rock, popping up in a chain as the overlying tectonic
plate inches along. Lō'ihi is the youngest of these volcanoes. In 1996, it
burst into a fit of unrest, generating thousands of quakes similar to what's
been seen around Mayotte, Rubin says. For Lō'ihi, the pulse of activity was the
result of magma draining from a reservoir that caused the empty chamber to
collapse.
In the Comoros, however, the
situation is a bit more complicated. Some geologists believe that the volcanic
chain there comes from similar hotspot activity. But the archipelago also sits
within an ancient rift—the gaping wound where Madagascar tore away from eastern
Africa—and volcanic activity is possible along fissures that formed from this
break. Curiously, the latest activity boiled off the shores of Mayotte, which
is the oldest in the island chain, Rubin notes.
Clues to the source of the latest
volcanic activity may be locked in the minerals of the solidified lava on the
seafloor, Hicks adds. The team nabbed samples of these rocks from the flanks of
the baby volcano, so studies of this material and of the surrounding region can
help researchers start piecing together the picture.
Pure and exciting
So how exactly is the new
volcanic activity linked to the seismicity, including the strange low-frequency
signal?
“That’s the million dollar
question,” Hicks says.
Research presented at a recent
European Geosciences Union meeting revealed that the long, low-frequency signal
noted in November wasn't the only event of its kind at Mayotte, he says.
Instead, it seems to be a common feature of this ongoing swarm of quakes. But
scientists can't yet say what precise situation is causing the low-frequency
signals and the seismic swarm—or even if the new volcano's eruption is ongoing.
“There’s still so much research
to do,” says Mark Tingay, a specialist in geomechanics at the University of
Adelaide via Twitter direct message. “But it’s an opportunity for scientists to
study what is possibly the birth or reawakening of a submarine volcano.”
Still, having a promising lead in
the case has been a boon for the people around the world who have been
following the quake swarm through social media. Researchers have been posting
updates as the work progressed, offering a peek into “science in its most pure
and exciting form,” as Wendy Bohon of the Incorporated Research Institutions
for Seismology says in an email.
And of course figuring out the
mystery has been of great importance for the island's residents. The
uncertainties surrounding the source of the quakes and a lack of adequate communication
early on during the slew of ground-rattling events caused both frustration and
confusion among the locals. That in turn sparked a range of wild theories about
what might be behind the moving earth, says Laure Fallou, a sociologist with
the Euro-Mediterranean Seismic Centre who has studied the role of culture in
effective science communication in the region.
The recent announcement instead
brings a new wave of emotion: “They went from the fear to the fascination,”
Fallou says. “Something incredible was happening there.”
Editor's Note: This story has
been updated with information from the research team, including confirmation
that the volcanic structure is relatively young.
Maya Wei-Haas is a science staff writer for National
Geographic.
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