On the morning of November 11, just before 9:30 UTC, many seismographs around the world began scribbling a strange signal that looked only remotely like a normal earthquake. Someone spotted it, and it didn’t take long until, in the social media, tinhattery boiled over the rim and attention-seekers had a heyday. People were trying to come up with quick explanations for something scientists hadn’t even had time to analyse yet. Which was not surprising… no one felt it; nothing was destroyed; if it wasn’t for the seismic sensors around the world, we might never even have known *something* had happened.
Nor was it the first event that made the news in this regard. Starting in May 2018, a for this area unusual earthquake (EQ) swarm near the French Comoro Island of Mayotte had made the news and caused first speculations. When the swarm didn’t decrease after a few days, the French Geological Survey (BRGM) created a web page with regular updates on it.
WHAT HAPPENED ON 11/11?
On November 11, this very odd signal was observed by near and far away international seismic stations, up to 11000 kilometers from Mayotte. The seismic waves, emitted by an unknown source, were predominantly in a frequency range far below that of normal earthquakes. The classic P and S waves were very weak and remained hidden for most stations. The signal repeated about every 17 seconds (period of 17 s or 0.06 Hz) and was almost monochromatic (of the same wave shape and frequency). This lasted for about 20 minutes in total. People who couldn’t make sense of what they saw on online seismograms started asking the experts on Twitter… and, within a few hours, seismologists from various corners of the world were busily discussing the event.
The source of these waves was soon narrowed down to be the Comores/Mayotte area – the very place where a long-lasting earthquake swarm had already baffled scientists since May of this year.
Normally, this type of long-range waves is generated by earthquakes of great magnitude (greater than M 5 and shallow depths <30 km). But, at that time, there were no strong quakes in the Mayotte area (or anywhere else for that matter). There had been only small EQs (less than M 3.5 during the last months) which could not have generated such type of signal. Moreover, not even the residents on Mayotte became aware of this massive seismic event. They felt no noticeable shaking, even though the epicenter was probably only about 30 km off the island.
While tinhatters were already sure that a UFO had crash-landed on Earth and others had proof of an ancient sea-creature rising from the deep, some more probable causes were also checked up on. There was no deep water drilling going on in the area, and no sign of perhaps a meteor strike could be found.
The seismologists came up with first ideas of what *could* have caused the signal – for example a large submarine phreatic eruption or the slow, prolonged roof collapse of a volcano magma chamber. A. Lomax summarizes his thoughts: “..this suggests that the 11 Nov events 1) are co-located with the swarm E of Mayotte, 2) are a rapid-fire set of small, tectonic-like ruptures that combined (perhaps with additional slow tectonic or volcanic processes) to form the long, M5+ surface waves observed around the world.”
Mayotte belongs geologically, geographically and culturally to the Comoro Islands archipelago. Politically, it is an overseas department of France. It lies in the Mozambique Channel, in the Indian Ocean, between Mozambique on the African continent and the island of Madagascar. Mayotte consists of a main island called Grande-Terre (Big Earth) or Maoré, a small island called Petite-Terre (Small Earth) or Pamanzi, and about 30 little islets. Mayotte is the oldest of the volcanic Comores islands (today, volcanic activity is restricted to Karthala volcano on Grande Comore). Grande-Terre is 39 km long and 22 km wide, its highest point is Mount Benara, at 660 m a.s.l..
Along the 500 km of the Comoros archipelago, EQs do occur regularly and frequently in the entire area, including some of magnitudes close to 5. The BRGM stresses in each of their updates that, therefore, the current episode is part of a known and relatively moderate seismicity in the northern Mozambique Channel. However, considering that the geology around Mayotte has never been studied much in detail – and volcanic activity hasn’t happened on or near Mayotte for the last 4000 years – the nature of this current episode can not be all that well-known. And, whether it is, or stays, moderate remains to be seen.
The continuous seismicity around Mayotte is due to several active zones: the East African Rift, the Davie Ridge, the Mozambique Basin, the Madagascar Block and the Comoros Archipelago itself. All the earthquakes of the Comoros probably have a double origin: their geodynamic position on an active hot spot and their probable position at the front of the deformation activity in the East African rift.
Volcanoes on Mayotte
Mayotte Island has been built by two shield volcanoes from the Pliocene to the Holocene. Volcanism continued on Petite-Terre until the Quaternary, with late trachytic ashes dated at approximately 7000 years BP. Morphologically youthful-looking maars are present on Mayotte Island, and Zinke et al. (2003) found several pumice layers of Holocene age (4000 years) in gravity cores on the barrier reef-lagoon complex at Mayotte. Offshore, multibeam bathymetry revealed hundreds of submarine volcanoes, deep canyons and huge debris-flows. The bathymetry also outlines large submarine plateaus that are probably bounded by active faults.
THE EARTHQUAKE SWARM
The appearance of the seismic swarm in May 2018 surprised the scientific community. According to the BRGM, the first quake was an M 4.2 on 10 May at 23:19 UTC, and was felt by the population. Since that day (up to Nov. 27), 263 shallow earthquakes (between 10 and 30 km depth) have been recorded near Mayotte Island, with magnitudes 3.4 to 5.8. The latter is the largest ever recorded in the Comoros Archipelago. Not recorded are probably tens of thousands of smaller quakes that escaped the instruments. As seismic activity continues, an increasing number of scientists are working together to pinpoint the causes of this EQ swarm and its unusual developments.
The geological knowledge of the swarm zone being limited, the grasp of the phenomenon becomes more precise as the observation of the earthquakes generates more data. Another path is to scan the literature: Examples of other swarms, located in a volcano-tectonic context similar to that of Mayotte, ranged only from a few hours to a fortnight. To date, no swarms with such magnitudes over such a long period are known. Earthquake swarms in other contexts (Alpine for example) have long durations but lower magnitudes.
The location and magnitude of EQs is calculated by the BRGM using 9 stations within 1400 km distance, including three stations in Mayotte (distance of 40 to 50 km), and two stations in Grande Comore (about 270 km away). International networks use remote seismic stations, the nearest of which is 480 km from Mayotte. This results in greater uncertainties. Thus, the instruments can only record magnitudes greater than 3.5 and locate them with a precision of about 10 km. The results show that the epicenters are clustered in an area about 20 km in diameter, located about 50 km E of Mayotte, i.e. at distances ranging from 21 to 65 km from the YTMZ station located at Mamoudzou.
Many of the EQs are felt by the 260,000 people living in the archipelago. The countless small rumbles and occasionally larger shocks have already caused nervous and anxious reactions among the population. In June a psychological support facility was opened to help residents to deal with the growing stress. There have been no reports of serious injury or fatality, but some of damage to buildings, including a school in Dembeni. At least 10 families had to be evacuated at that time.
HYPOTHESIS OF A VOLCANIC CAUSE
Mainly from: Pierre Briole – UMR CNRS/ENS 8538 – Note from 12/11/2018
Wandering to the East and Sinking – Ground Deformation
In addition to the seismic measurements, other data were analyzed to picture the events, in particular deformation data of the surface of the island. The French National Geographical Institute (IGN) monitors Mayotte via six GPS stations. The instruments measure the movement of fixed surface points via Satellite, in components of N↕️S, E↔️W and Up🔃Down directions. Since the beginning of the seismic activity in May, these stations are experiencing “abnormal” behaviour of the ground. Small movements were measured at the beginning of the crisis, then larger and regular movements have occurred since mid-July. Since late October there was a slight increase in average speed, especially for the vertical component.
The total movement of GPS stations by mid-Nov. was more than 60 mm to the East➡️ and 30 mm Down⤵️. Given that the measuring points are fairly evenly distributed on the island, the IGN came to the conclusion that the entire archipelago of Mayotte has moved eastwards and sagged a few centimeters.
At this stage, a volcano-tectonic origin for this crisis is one of the working hypotheses envisaged by the scientists. After the deformation details had become known, Pierre Briole of the ENS in Paris came to the conclusion that a sizeable magma body might be squishing its way through the subsurface near Mayotte.
He argumented that models which assume a seismic shift on a fault can not account for these movements. On the other hand, models which involve an emptying magmatic reservoir do reproduce the observations well. In this hypothesis, the magmatic chamber in question would be located 50 km east of Petite-Terre, at a depth of about 30 km. The hypothetical flow rate would be 110 m³/s which is high but quite plausible for a volcanic eruption. A total of 1.3 km³ would have been emplaced up to 12/11/2018, either on the ocean floor or within the crust in shallow depth. This is also a very high amount and was never before observed in the seabed – if it is indeed an eruption.
The place where the magma might have issued is not known. Part of the magma could come from a volcanic edifice in formation (there are old ones visible in the bathymetry) or from an eruptive fissure that could be from a few hundred meters to a few kilometers long. Part of the magma might also be captured shallow in the crust, in the form of vertical (dykes) or horizontal (sills) veins. There were no observations of volcanic gas rising, but this was to expect – the ocean floor is at a depth of more than 3.5 km in that area.
Hypothetical Time frames
For the time June-July the scientists pictured a fracturing phase in the crust, preceding the onset of lava effusion. This deformation could mean magma forming a dyke, between a reservoir and the future lava outlet, measuring 15 km long, 3 km high and 3 m thick. The eruption proper may have begun in July/August with effusion of lava on the ocean floor and circulation of magma in the dyke. During the following months fluctuations in seismicity are recorded, and GPS movements suggest a decline, followed by another upsurge, of lava effusions. The modelled lava flow was strongest in September, with an estimated output of 193 m³/s. By mid November the flow had decreased again.
If the volcanic nature of the phenomenon is confirmed by other data and if the throughput is as high as predicted by the model (which, however, does not make it exceptional), the volume moved after one year (at 30 km depth) could be in the ballpark of 3 km³. Particular attention should be paid to ground deformation in the crisis zone and the possibility of caldera formation.
HOW EXCITING, A HUGE ERUPTION! – REALLY?
I have to repeat that all these “exciting” details are just models and estimates, educated guesses if you will. Complementary measures are necessary to confirm – or to refute – this hypothesis of a volcanic eruption and to specify its extent and course. According to BRGM, the French scientific community has joined forces in the last few months, in order to understand the phenomenon and provide answers. Possibilities are being investigated for the deployment of new instruments on land and at sea. Also, an oceanographic campaign, involving numerous French scientific institutes, has been applied for, to explore both the swarm zone and the regional context and to produce more data.
Meanwhile, the people of Mayotte might not find the prospect of a volcano in their backyard very exciting. Yet, there is no danger for the residents. Even in case of an eruption in the swarm area – located 30-50 km from the island and below 3500 m of sea water – there is currently no significant impact expected in Mayotte. A permanent uptick in earthquake activity, though, would be a bigger concern for the people on the islands.
The mechanism of generation of the great unusual wave of 11 November is not yet explained. The hypotheses put forward by the researchers, such as the resonance of a magma chamber, have yet to be confirmed by additional data. Whether it was indeed caused by submarine eruption, a caldera collapse or a massive movement of magma in the earth’s crust, remains yet to be found out. Discussions among scientist are going on – I just saw these tweets by @CoCaudron who wrote:
“Hi all. Together with Philippe Lesage, we have been estimating the Q factor of the potentially volcano-related signal using ARMA analysis. It turns out to be pretty high Q~200+-50.” “After sounding the volcano-seismo community, it appears this kind of monochromatic signal has never been seen elsewhere. Only of much shorter duration, higher frequency and less energetic (e.g., Kilauea).”
I know only dimly about the Q factor, but it’s really nice to know that Nature may still have surprises for us that are not caused by humans…
Merry Christmas to one and all, may all volcanoes around the world take a break and be quiet for this time of peace and happyness!
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
SOURCES & FURTHER READING
– GVP, Mayotte Island
– The Volcanism of the Comoros Archipelago […] (2016, PDF)
– Note sur la crise tellurique en cours à Mayotte (11/2018, PDF)
– Mayotte seismo-volcanic crisis (by ENS)
– BRGM regular updates on the EQ swarm
– BRGM scientific FAQ
– Article by Alex Hutko, PNSN
– Major natural hazards in a tropical volcanic island: A review for Mayotte Island […] (2010, PDF)
– Strange waves rippled around the world (NGeo)
– Earthquake swarm (and the stress) continue in Mayotte (News)
– Das rätselhafte Mayotte-„Erdbeben“ (Blog, in German)