# Special issue Comptes-Rendus Geoscience: The Mayotte seismo-volcanic crisis of 2018–2021 in the eastern Comoros archipelago (Mozambique channel)

Jérôme van der Woerd, Vincent Famin, Eric Humler

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Jérôme van der Woerd, Vincent Famin, Eric Humler. Special issue Comptes-Rendus Geoscience: The Mayotte seismo-volcanic crisis of 2018–2021 in the eastern Comoros archipelago (Mozambique channel). Comptes Rendus. Géoscience, 2022, 354, pp.1 - 6. 10.5802/crgeos.196. hal-04057656

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# Comptes Rendus
## Géoscience
### Sciences de la Planète

Jérôme van der Woerd, Vincent Famin and Eric Humler

**Special issue Comptes-Rendus Geoscience: The Mayotte seismo-volcanic crisis of 2018–2021 in the eastern Comoros archipelago (Mozambique channel)**

Volume 354, Special Issue S2 (2022), p. 1-6

Published online: 17 January 2023

Issue date: 17 January 2023

https://doi.org/10.5802/crgeos.196

**Part of Special Issue:** The Mayotte seismo-volcanic crisis of 2018-2021 in the Comoros archipelago (Mozambique channel)

**Guest editors:** Jérôme Van der Woerd (Institut Terre Environnement de Strasbourg, UMR 7063 CNRS / Université de Strasbourg, 67084 Strasbourg, France), Vincent Famin (Laboratoire Géosciences Réunion, Université de La Réunion - IPGP, 97744 Saint-Denis, France) and Eric Humler (Professeur Université de Nantes, Laboratoire de Planétologie et Géosciences, UMR 6112, Faculté des Sciences et Techniques, Nantes Université, 44322 Nante, France)

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e-ISSN : 1778-7025
The Mayotte seismo-volcanic crisis of 2018-2021 in the Comoros archipelago (Mozambique channel) / *La crise sismo-volcanique de 2018-2021 de Mayotte dans l'archipel des Comores (Canal du Mozambique)*

Special issue *Comptes-Rendus Geoscience:*
The Mayotte seismo-volcanic crisis of 2018–2021 in
the eastern Comoros archipelago (Mozambique
channel)

Jérôme van der Woerd<sup>a, *, b</sup>, Vincent Famin<sup>b, c</sup> and Eric Humler<sup>d</sup>

Published online: 17 January 2023, Issue date: 17 January 2023

The challenges posed by unexpected and poorly
known natural hazard are manifold: scientific, tech-
nical and societal. Until 2018, Mayotte was classified
as a zone of moderate seismicity and low probabil-
ity of volcanism among French territories. The onset
of the seismic crisis offshore Mayotte in May 2018,
which appeared to be volcanic in the course of its dis-
covery, has triggered an unprecedented mobilization
of the French scientific, administrative and political
communities (Figure 1). Understanding the event,

building a message, communicating to the popula-
tion: all had to be constructed and invented, with the
additional difficulty that nothing was visible given
that the eruption happened offshore on the several
kilometers deep oceanic floor.

Even though it is not possible to cite all the actions
of the French scientific and institutional communi-
ties in response to the seismo-volcanic crisis of May-
otte, some milestones appear in the chronology of
crisis management. After the first earthquake swarms
of 10 May 2018, the local bureau of the BRGM in May-
otte in charge of seismic risk was mobilized and is-
sued an alert. The large chock of magnitude Mw5.9
occurred on 15 May 2018, 5 days after continuous

<footnote><sup>a</sup> Institut Terre et Environnement de Strasbourg, CNRS UMR 7063, Université de Strasbourg, 67000 Strasbourg, France</footnote>

<footnote><sup>b</sup> Université de Paris, Institut de physique du globe de Paris, CNRS, UMR 7154, F-75005 Paris, France</footnote>

<footnote><sup>c</sup> Université de La Réunion, Laboratoire GéoSciences Réunion, F-97744 Saint-Denis, France</footnote>

<footnote><sup>d</sup> Laboratoire de Planétologie et Géodynamique, CNRS UMR 6112, Université de Nantes, 44322 Nantes, France</footnote>

<footnote>E-mails: jerome.vanderwoerd@unistra.fr (J. van der Woerd), vincent.famin@univ-reunion.fr (V. Famin), eric.humler@cnrs-dir.fr (E. Humler)</footnote>

<footnote>* Corresponding author.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Summary of major events, decisions and actions triggered by the seismo-volcanic events starting 10 May 2018 in Mayotte.</figcaption></figure>

and increasing seismicity that worried the population in a time of social unrest and tension, with the difficulty for the authorities to transmit an audible message on an unexpected and worrisome event. A macroseismic survey was launched in June 2018. As the seismicity remained important, the seismological community outlined projects for on- and off-shore deployment of seismic stations based on preliminary locations with an inadequate network. In July 2018, the French geodetic survey (IGN) noticed an unexplained change in ground motions recorded by the GNSS stations. The scientific community interpreted these ground motion changes as due to the deflation of magmatic chamber in the crust or in the mantle, with or without seafloor eruption [Lemoine et al., 2020]. Later in November 2018, a strong and long single-frequency, very low frequency seismic signal is observed several thousand kilometers from Mayotte on global seismic networks. Such a “tremor” is usually associated with magmatic circulation or reservoir draining [Cesca et al., 2020].

Governmental and scientific institutions convened and organized their effort towards understanding the seismo-volcanic event in November 2018, leading to the creation of the “Réseau de surveillance Volcanique et Sismologique de Mayotte”—REVOSIMA (Volcanic and Seismological Monitoring Network of Mayotte), with the requirement to develop scientific knowledge, identify and transmit alerts, disseminate regular information. CNRS-INSU opened a call for survey projects at the beginning of 2019. This rapidly mobilized

funding allowed the deployment of ocean bottom seismometers, and the first MAYOBS-1 cruise that led to the discovery of a new submarine volcano [Feuillet, 2019, Feuillet et al., 2021], later called “Fani Maoré”. Since then, recurrent campaigns have been organized to monitor the evolution of the Fani Maoré volcano (up to MAYOBS-27 at the time of writing). In addition, three oceanographic cruises have been conducted to better understand the geologic and geodynamic setting of this volcanic eruption in 2021 (Figure 2), “SISMAORE” and “GEOFLAMME” with the R/V Pourquoi Pas?, and “SCRATCH” using the R/V Marion Dufresnes II. These surveys deployed a huge panel of marine geophysical techniques and resulted in the collection of 16 sediment cores and 18 dredges over the northern Mozambique channel, which will give work to scientists for several decades.

To sustain this unprecedented scientific effort, the French National Research Agency (Agence Nationale de la Recherche—ANR) granted pluri-annual funding to three ambitious projects involving international and multidisciplinary scientific teams: “COYOTES” aiming at understanding the geodynamics of the northern Mozambique channel, “SUBSILAKE” with the objective of monitoring geochemical changes in a crater lake of Mayotte in response to the submarine eruption, and “MARMOR” aiming at building a multi-technique geophysical network of submarine eruption monitoring in the area. The newly discovered geo-hazard in the northern Mozambique channel also motivated the Comorian and French authorities to reinforce their international collabora-
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Summary of research efforts since the 2018 Mayotte seismic and volcanic crisis. Elements shown include seismic/GNSS stations installed in the Comoros archipelago, the number of which has doubled since 2018. Also shown are the oceanographic cruises and their main operations (geophysical surveys, sediment coring and or seafloor dredging): SISMAORE (December 2020–February 2021), GE-OFLAMME (April–May 2021), SCRATCH (July 2021), and MAYOBS-1 to -23 (March 2019–July 2022).</figcaption></figure>

tion through the “HATARI” Interreg project funded by the European Union and coordinated by the Réunion Regional Council.

Four years after the onset of the crisis, while it is apparently coming to a rest, efforts to understand what is happening and to put in place an efficient monitoring network are still ongoing, at the national and international level. For the French community, scientific and risk managers, the implementation of an underwater volcano monitoring network is not only a challenge but may set an example (Marmor project https://www.marmor-project.org/). The recent Mayotte seismo-volcanic crisis has triggered already a large set of scientific studies as can be seen from the many scientific articles and communications over the last 4 years. In this special issue, our aim is to gather a wide spectrum of scientific works around the event, without being exhaustive, in order to give an overall understanding of the crisis. It is composed of 19 articles, including details of the seismic and volcanic sequence, the various risks associated to its occurrence, its societal impact, but also

first local and regional understanding of the geological, volcanic and geodynamic context of Mayotte and the Comoros archipelago.

The first part of this thematic issue is a compilation of the latest advances about the geological and geodynamic context of the Mozambique channel. Most of these advances are direct outcomes from the ANR COYOTES project (https://anr.fr/Projet-ANR-19-CE31-0018), from oceanic cruises such as SISMAORE (doi:10.17600/18001331), and from the now regular MAYOBS surveys (doi:10.18142/291). In Thion et al. [2022] for instance, extension of the submarine volcanism in the northern Mozambique channel is evidenced, and the regional geodynamics is investigated by means of marine geophysics. This first part also questions the nature of the lithosphere beneath the Comoros archipelago, by probing its thermal state from literature and newly published heat flow measurements [Rolandone et al., 2022] and its structure from teleseismic receiver functions [Dofal et al., 2022]. Seismic reflection across the new volcano East of Mayotte provides unprecedented in-
sights about the chronology and architecture of vol-
canic seamount construction in this zone [Masquelet
et al., 2022]. Ocean bottom volcanic geomorphology
is also investigated from high-resolution bathymetry
and towed camera on the eastern submarine slope of
Mayotte [Puzenat et al., 2022].

In a second part of the thematic issue, a set of
studies on seismicity, geodesy, petrology, and gas
emissions focuses on the development of the May-
otte seismo-volcanic sequence and its evolution. De-
spite an inadequate network before 2019, the first
year of seismicity of the Mayotte sequence has been
carefully analyzed and is now presented in a new cat-
alogue of seismicity [Mercury et al., 2022]. A tele-
seismic source study of the largest events at the
onset of the sequence reveals their link to mag-
matic fracturing and transport [Morales-Yáñez et al.,
2022]. With the seafloor and onland seismic mon-
itoring in place since 2019, the large amount of
data is investigated using automatic detection tech-
niques [Retailleau et al., 2022]. Although dissemi-
nated on few oceanic islands, large ground deforma-
tions (20–25 cm) are monitored via geodesy, which
are explained by magma transfer at depth in the
volcanic edifice [Peltier et al., 2022]. Eruptive ma-
terial collected on the seafloor thanks to oceano-
graphic campaigns helps constraining eruptive sce-
narios [Berthod et al., 2022]. The conditions required
for the genesis and eruption of phonolitic lavas such
as those found in this new volcano are explored
[Andújar et al., 2022]. Monitoring lavas flows with an
hydroacoustic network seems a promising develop-
ment for the study of the eruptive sequence [Bazin
et al., 2022]. Onland gases emissions on Petite Terre
offer a window on magmatic processes occurring at
depth during volcanic unrest [Liuzzo et al., 2022].
The co-eruptive evolution of these emissions also
impacts the water chemistry of maars as shown by
geochemical monitoring and modeling of the Dziani
Dzaha Lake on Petite Terre in Mayotte [Cadeau et al.,
2022].

In a third part, the possible consequences and
impacts of the eruption and its possible evolution
are explored in term of hazard and risk assessment.
Knowledge of the geological nature of the surface
rocks and deposits constrain possible site effects
during seismic shaking [Roullé et al., 2022]. Being
prepared and preventing damages in case of seis-
mic shaking can be explored for crisis management

[Taillefer et al., 2022]. The unstable submarine slopes
of Mayotte could be prone to landslides themselves
triggering tsunamis [Poulain et al., 2022].

The last part addresses how one communicates
in the midst of a crisis with so many unknowns. A
study based on news accounts reveals the various
facets of the narrative from the various communities
involved [Devès et al., 2022]. Another study explores
the perception of the crisis and how it is revealed in
the languages [Mori, 2022].

This issue is a step toward the better understanding of the multiple sides of a volcanic eruption. It gathers together a wide spectrum of scientific approaches and studies, which testify of the strong involvement of the scientific community for assessing natural hazard and risk to the populations. In addition, the organization of an operational multi-partner monitoring system in emergency and in a little known geological area may serve as a model for other regions subject to equivalent hazards and risks.

Conflicts of interest

Authors have no conflict of interest to declare.

Acknowledgements

The authors are grateful towards François Chabaux
for his suggestion to publish a special issue on the
Mayotte seismo-volcanic crisis and for his constant
support during the preparation of the issue. They also
thank Adenise Lopes and the Comptes Rendus Geo-
science team for their help in the editorial process-
ing.

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