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). The team was thus able to identify the different phases of the volcanic crisis from 2018 to March 2019. 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.
From the outset, these observations thus revealed a magmatic episode of very great magnitude.Īn international team led by GFZ scientists therefore analysed seismological and geodetic data from the region to study the temporal evolution of the crisis. 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. 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. 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.
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. Seismic activity began with a “swarm” of thousands of earthquakes, the largest with a magnitude of Mw 5.9. 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. The study is published on 6 January 2020 in the journal Nature Geoscience. This is the largest underwater eruption (more than 3.4 km3) recorded to date. 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). 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.
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. © équipe MAYOBS (CNRS/IPGP-Université de Paris/Ifremer/BRGM)
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