Seismological analyses highlighted the stages of formation of the new submarine volcano off the coast of Mayotte and the drainage of a deep magma reservoir.
The birth of a new volcano was announced this year after the oceanographic campaign conducted by the IPGP in May 2019 (MAYOBS1 campaign, carried out within a CNRS-INSU program) off Mayotte. An international team of scientists led by Simone Cesca from the German Geoscience Research Centre (GFZ), in collaboration with the Observatoire-Midi-Pyrénées (OMP) in Toulouse, investigated the seismicity associated with the formation of this new volcano the year before the campaign, and highlighted magma movements under the seabed before and during the underwater eruption that began in June 2018. With new seismological data analysis techniques adapted to compensate for the lack of instrumentation around the volcano prior to the campaign, scientists were able to reconstruct the different stages of the volcano’s formation and the drainage of a very deep reservoir (~30 km). This is the largest underwater eruption (more than 3.4 km3) recorded to date. The study is published on 6 January 2020 in the journal Nature Geoscience.
Since May 2018, many earthquakes have been detected off the island of Mayotte, in the Comoros archipelago, between the African continent and Madagascar, in a region where seismic activity is usually low. Seismic activity began with a “swarm” of thousands of earthquakes, the largest with a magnitude of Mw 5.9. Hundreds of seismological signals of a rarer type were also detected, the first one in January 2018, well before the beginning of the crisis, most of them from June 2018 onwards. These are monochromatic wave arrivals, lasting 20 to 30 minutes, called very long period signals (VLP, very low period), generally associated with the resonance of volcanic structures. The energy generated by the largest of these VLPs is very important (equivalent to the energy released by a magnitude 5 earthquake), generating surface waves that are detected everywhere on Earth. This is a new observation in seismology. Also in early June 2018, a continuous eastward movement and subsidence of Mayotte began to be detected using GPS stations located on the island of Mayotte, with a total displacement of nearly 20 cm to date. From the outset, these observations thus revealed a magmatic episode of very great magnitude.
An international team led by GFZ scientists therefore analysed seismological and geodetic data from the region to study the temporal evolution of the crisis. In the absence of a sparse local network, the seismological analysis relied on the development of innovative methods, combining available local data with regional data (sensors in Madagascar and on the African continent) and even a network of high-quality sensors located in Kazagsthan. The team was thus able to identify the different phases of the volcanic crisis from 2018 to March 2019. First of all, the observation of a rapid upward migration of seismicity in May-June 2018 indicated a propagation of magma from about 30km deep to the sea floor, creating an underwater volcano (The oceanographic campaign then confirmed this observation, the migration of magma estimated by this study is indeed well located at the level of the volcanic edifice). Once the conduit was formed and a passage opened for the magma towards the surface, the eruption started in June 2018 and the magma started to come out without difficulty, leading to a decrease in seismic activity (linked to the opening of the conduit) and a subsidence of the sea floor detected with the help of GPS stations.
From September 2018, another phase began, with a resurgence of seismicity, closer to Mayotte and at depth, due to the drainage and deep collapse of the magma reservoir extending between the island of Mayotte and the volcano. The number of VLPs has also increased. The team hypothesizes that they are the manifestation of the magma reservoir entering into resonance during magma drainage. When the reservoir collapses, the properties of this resonant structure therefore change, which is well observed in the data, with a steady increase in the resonance frequency of the VLPs starting in September 2018.
The team was thus able to reconstruct the various processes involved in the setting up of the volcano off Mayotte and the drainage of a very deep reservoir (~ 30 km). This is the largest underwater eruption ever recorded to date (more than 3.4 km3). The authors identify a potential risk due to the collapse of the reservoir near the island of Mayotte. They also demonstrate that it is possible to detect volcano-tectonic episodes at sea, even without a local monitoring network, and to study them effectively by relying on the analysis of weak signals recorded from global networks.
- Scientific article : « Drainage of a deep magma reservoir near Mayotte inferred from seismicity and deformation » by Simone Cesca, Jean Letort, Hoby N. T. Razafindrakoto, Sebastian Heimann, Eleonora Rivalta, Marius P. Isken, Mehdi Nikkhoo, Luigi Passarelli, Gesa M. Petersen, Fabrice Cotton & Torsten Dahm, Nature Geoscience, 6 janvier 2020
- Jean Letort, firstname.lastname@example.org