In 2019, following the swarm of earthquakes that affected Mayotte for several months, BRGM was involved in the discovery of a new submarine volcano. This was an opportunity to model potential tsunamis in a context of active volcanism, and to map the impacts of coastal flooding.
15 September 2021
Surveillance du poste de suivi d’acquisition acoustique lors de la mission à bord du Marion Dufresne qui a permi la découverte du nouveau volcan en mai 2019.

Map of maximum water level rise for all the tsunami simulations considered, caused by gravity-related instability. 

© BRGM - I. Thinon

This is the largest nascent volcano observed since 1783. On 10 May 2018, the island of Mayotte in the Indian Ocean suddenly experienced a spectacular telluric phenomenon. For months the island was struck by intense seismic activity. In the summer, shifting and sinking were recorded on the island itself. In order to better understand the phenomenon both on land and at sea, BRGM, CNRS, ENS, IFREMER, IGN and IPGP started working together on an observation campaign to monitor the seabed and water column. This led to the discovery of a new volcano in May 2019. The eruption, still ongoing in 2021, was found to be of the effusive type, and it is causing lava flows and seismic activity that are being continuously mapped. REVOSIMA, the volcanological and seismological monitoring network of Mayotte, is overseeing the monitoring of the phenomenon. 

The volcano is located 50 kilometres off the coast of Mayotte at a depth of approximately 3,500 metres, and the huge 800 metre-tall underwater edifice has a diameter of four kilometres. The island itself, which covers only 376 square kilometres, has shifted eastwards by 25 centimetres and sunk by 19 centimetres because of the remarkable draining of a deep magma chamber (about 6.5 cubic kilometres of lava have been discharged). The volcano, which has not yet been officially named, but has been dubbed The Imaginary Monster (Bagug) and The King of the Sea (Mfaloumle Wa Bahari) by Mayotte residents, could pose a threat to the island.

The General Directorate for Risk Prevention of the Ministry for Ecological Transition commissioned BRGM to undertake a study to estimate the potential impacts of tsunamis on the island.

Modélisation d’un scénario de tsunami à Mayotte en cours de propagation.

Modelling of tsunami propagation in Mayotte: the map colours show the extent of the water level rise and the black arrows represent water speed at certain shallow points. 

© BRGM

Improving the prediction of tsunami hazards

The aim was to use numerical modelling to carry out predictive mapping of coastal flooding in the event of a tsunami occurring, to better estimate the associated hazard. The project has been completed; it was part of the steps taken in June 2019 by four ministries in collaboration with the scientific community to monitor volcanic activity and to better characterise and refine its sources, and improve the prediction and mapping of impacts.

The research teams worked together to determine the types of events that can cause tsunamis near Mayotte, identifying three, namely earthquakes, underwater landslides and collapse of the magma chamber and sides of the submarine volcano.

BRGM modelled 60 simulations from the generation and propagation of tsunamis to coastal flooding. The scientists assessed the protection afforded by the reef and mangrove, along with wave magnitudes and arrival times. The simulations with the greatest impacts on the coast of Mayotte were identified, some of which had high uncertainties, due to a lack of knowledge of the geological structures and the magma system. The models produced to estimate the hazard will need to be refined over time.

The final objective of this project was to develop a tsunami decision support tool and to identify research needs for improving the modelling of tsunami impacts, in collaboration with IPGP. The results were also useful for mapping evacuation routes and documenting future spatial planning projects such as the extension of the airport.

Surveillance du poste de suivi d’acquisition acoustique lors de la mission à bord du Marion Dufresne qui a permi la découverte du nouveau volcan en mai 2019.

Monitoring of the acoustic acquisition monitoring station during the observation campaign on board the Marion Dufresne, which led to the discovery of the new volcano in May 2019. 

© BRGM - I. Thinon

A flagship model of collaborative research

As far as basic research is concerned, this work has also improved interactions between different scientific teams and the status of knowledge in their respective disciplines, bearing in mind the need to forecast events more efficiently in future. This field is very complex indeed, and modelling software and available geological data have been found to be inadequate. There is therefore a need to develop simulation tools that can take into consideration different types of tsunami sources and highly complex field conditions, which influence wave propagation and related flooding.

Furthermore, in the context of civil protection and spatial planning, modelling should not be limited to tsunamis and the flooding they can cause, but should instead include their impacts on buildings and infrastructure. This means that modelling should encompass risks as well.

Teamwork played a major role in this project. The issue requires a truly multidisciplinary approach, involving numerical modelling, computing infrastructure, coastal flooding, hydrodynamic models, landslides, oceanography, volcanology, seismology, communication, geomatics, etc., and can be addressed from different angles, such as from an exploratory, operational or research perspective, and it involves several organisations and ministries.

It is essentially a vast experimental endeavour which the whole scientific community works together on a complex issue, across disciplinary and organisational divides. It serves as a flagship model for open, collaborative research. The Mayotte event also proved to be a useful case study since the work carried out by BRGM and partners can be applied to other geographical areas.

Predictive mapping of tsunami-related coastal flooding in Mayotte

BRGM’s scientific advances in 2020: predictive mapping of tsunami-related coastal flooding in Mayotte.

Advances presented in the framework of the scientific programme “Natural hazards and regional resilience”.

© BRGM