Ground deformation monitoring of the eruption offshore Mayotte

In May 2018, the Mayotte island, located in the Indian Ocean, was affected by an unprecedented seismic crisis, followed by anomalous on-land surface displacements in July 2018. Cumulatively from July 1, 2018 to December 31, 2021, the horizontal displacements were approximately 21 to 25 cm eastward, and subsidence was approximately 10 to 19 cm. The study of data recorded by the on-land GNSS network, and their modeling coupled with data from ocean bottom pressure gauges, allowed us to propose a magmatic origin of the seismic crisis with the deflation of a deep source east of Mayotte, that was confirmed in May 2019 by the discovery of a submarine eruption, 50 km offshore of Mayotte ([Feuillet et al., 2021]). Despite a non-optimal network geometry and receivers located far from the source, the GNSS data allowed following the deep dynamics of magma transfer, via the volume flow monitoring, throughout the eruption

Installation de l'antenne GPS à la station PMZI à Mayotte, dans le cadre du projet Tellus

This photograph was taken at the PMZI seismological station, located in the municipality of Pamandzi, Mayotte. The site was instrumented as part of the CNRS-INSU Tellus project, following the seismo-volcanic crisis that began in May 2018. The site hosts three types of instruments: an accelerometer (RAP), a wideband seismometer (SISMOB) and a GPS antenna, currently being installed by BRGM staff in this photo. 
This instrument belongs to the GPSMOB mobile instrument park of RESIF, the French Seismological and Geodetic Network. 
RESIF is a national research infrastructure dedicated to the observation and understanding of the structure and dynamics of the Inner Earth. RESIF is based on high-tech observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout France. The data collected make it possible to study with high spatial and temporal resolution the deformation of the ground, surface and deep structures, seismicity on a local and global scale and natural hazards, and more particularly seismic events, on French territory. RESIF is part of the European (EPOS - European platform observing system) and global systems of instruments used to image the Earth's interior as a whole and to study many natural phenomena.Cette image est prise à la station sismologique PMZI, installée sur la commune de Pamandzi, à Mayotte. Le site a été instrumenté dans le cadre du projet CNRS-INSU Tellus, suite à la crise sismo-volcanique débutée en mai 2018. Le site héberge trois types d’instruments : un accéléromètre (RAP), un sismomètre large-bande (SISMOB) et une antenne GPS, en cours d'installation par les personnels du BRGM sur cette photo. 
Cet instrument appartient au parc d'instruments mobiles GPSMOB de RESIF, le Réseau sismologique et géodésique français. 
RESIF est une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. RESIF se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. RESIF s’intègre aux dispositifs européens (EPOS - European plate observing system) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels

Mayotte seismic crisis: building knowledge in near real-time by combining land and ocean-bottom seismometers, first results

International audienceSummary The brutal onset of seismicity offshore Mayotte island North of the Mozambique Channel, Indian Ocean, that occurred in May 2018 caught the population, authorities, and scientific community off guard. Around 20 potentially felt earthquakes were recorded in the first 5 days, up to magnitude Mw 5.9. The scientific community had little pre-existing knowledge of the seismic activity in the region due to poor seismic network coverage. During 2018 and 2019, the MAYOBS/REVOSIMA seismology group was progressively built between four French research institutions to improve instrumentation and data sets to monitor what we know now as an on-going exceptional sub-marine basaltic eruption. After the addition of 3 medium-band stations on Mayotte island and 1 on Grande Glorieuse island in early 2019, the data recovered from the Ocean Bottom Seismometers were regularly processed by the group to improve the location of the earthquakes detected daily by the land network. We first built a new local 1D velocity model and established specific data processing procedures. The local 1.66 low VP/VS ratio we estimated is compatible with a volcanic island context. We manually picked about 125,000 P and S phases on land and sea bottom stations to locate more than 5,000 events between February 2019 and May 2020. The earthquakes outline two separate seismic clusters offshore that we named Proximal and Distal. The Proximal cluster, located 10km offshore Mayotte eastern coastlines, is 20 to 50 km deep and has a cylindrical shape. The Distal cluster start 5 km to the east of the Proximal cluster and extends below Mayotte's new volcanic edifice, from 50 km up to 25 km depth. The two clusters appear seismically separated, however our dataset is insufficient to firmly demonstrate this