Starting in May 2018, a volcano-tectonic crisis occurred in the vicinity of Mayotte, a volcanic island in the Comoros Archipelago in the Mozambique Channel. The origin of the volcanism but also the ...subsurface architecture and nature of the crust, remain unknown. Here, based on receiver function analyses that provide S-wave velocity profiles, we determine the depth of Mohorovičić discontinuity (Moho) and VP/VS ratios for volcanic islands in the Mozambique Channel. We propose that the crust beneath Mayotte and Juan de Nova islands is of continental nature, while it appears to be of oceanic origin beneath Europa and Grande Glorieuse islands. Our results suggest that Mayotte edifice grew on an isolated continental block abandoned during the Gondwana breakup and the opening of the Mozambique Channel. The continental crust is underlain by a thick (9–10 km) and fast layer, interpreted as magmatic underplating which may result from the 20-Myr-long duration of the volcanism. The new velocity model determined from the seismic station on Mayotte can be used to relocate the seismicity related to the ongoing volcano-tectonic crisis.
•We use receiver functions to image the crustal structure in the Mozambique Channel•Mayotte Island is built on an underplated isolated continental block•Thick underplating developed below the edifices of the MC due to a long activity•Our velocity model help relocate the seismicity of the volcanic crisis of Mayotte
Since May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km\(^{3}\) of basanitic magma. ...The eruption occurred along a WNW–ESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels (\({\le }\)12–15 km) yields a phonolitic residual liquid containing up to 3–4 wt% \(\mathrm{H}_{2}\mathrm{O}\) after \({\ge }\)65 wt% of an assemblage of olivine\(+\)plagioclase\(+\)amphibole\(+\)clinopyroxene\(+\)biotite\(+\)magnetite\(+\)ilmenite\(+\)apatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6–8 km depth, while a shallower differentiation (\({\le }\)4–5 km) results in the production of liquids with trachyte–benmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in \(\mathrm{SiO}_{2}\)–\(\mathrm{Al}_{2}\mathrm{O}_{3}\) but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage at Mayotte is a rather shallow process.
The present study was carried out at 22 stations located on the fringing reefs of the island of Mayotte inside the perimeter of the Marine Natural Park of Mayotte (PNMM). The average assessment of ...the number of samples of marine litter (ML) collected at these stations was 3.9 ± 1.3 ML per 500 m2 in winter and 3.8 ± 1.1 ML per 500 m2 in summer. Plastic ML was dominant, representing approximately 92% of the litter collected at the stations for the two seasons. The majority was plastic ML from fishing gear (lines, nets etc.). Station 18 presented remarkable average densities of 39 ± 4.2 ML per 500 m2 for the two seasons and differed from the other stations in that the ML came solely from the catchment areas, with a majority of aluminium ML (56%). Approximately half the coral colonies coming into contact with ML presented an impact dominated by the category of “Broken or abraded colonies”, with 25% of the colonies being impacted. The colonies most severely impacted by ML were branch or table corals, with the greatest majority for the present study being of the genus Acropora.
•Marine litter was quantified at 22 stations located on the fringing reefs of Mayotte.•Marine litter was collected and sorted at each station by scuba diving along two transects of 500 m2 in winter and summer.•Litter density ranged from 0 to 50 items/500 m2 (average 3.9 items in winter and 3.8 items in summer).•92% of litter was plastic, of this 66% was fishing gear.•Half of the coral colonies containing litter presented an impact.
A detailed morpho-bathymetric study of the Comoros archipelago, based on mostly unpublished bathymetric data, provides a first glimpse into the submarine section of these islands. It offers a ...complete view of the distribution of volcanic structures around the archipelago, allowing to discuss the origin and evolution of this volcanism. Numerous volcanic cones and erosional-depositional features have been recognized throughout the archipelago. The magmatic supply is focused below one or several volcanoes for each island, but is also controlled by lithospheric fractures evidenced by volcanic ridges, oriented along the supposed Lwandle-Somali plate boundary. Massive mass-wasting morphologies also mark the submarine flanks of each island. Finally, the submarine geomorphological analysis made possible to propose a new scheme for the succession of the island's growth, diverging from the east-west evolution previously described in the literature.
•First morpho-bathymetric study describing the entire submarine section of the Comoros archipelago.•Seafloor geomorphology is controlled by volcanic and mass-wasting processes.•A myriad of volcanic cones, mounds and volcanic ridges dominate the seafloor.•The Comorian volcanism is strongly controlled by regional lithospheric fractures.•A new relative chronology of the Comoros islands is proposed.
Mayotte island experienced a large volcanic eruption 50 km offshore in 2018–2021, creating the submarine volcano “Fani Maoré”. The eruption was accompanied by intense seismicity at mantle depths ...(20–45 km), divided into a “proximal” and a “distal” cluster centered 10 and 30 km east from the island, respectively. Previous studies suggest that two separate magma reservoirs may lie at the top and bottom of the proximal cluster. Here, we assess whether two reservoirs are a mechanically viable explanation for the proximal cluster's truncated conical shape. We developed finite-element models of pressurized magma reservoirs in a 2D axisymmetric domain, modeling the reservoirs as compliant elastic ellipsoids embedded in an elastoplastic host rock. We find that, at these depths, extremely low friction is required to generate failure at realistically low reservoir pressures. This implies in turn that mechanical weakening must occur at these depths. The weakening could be induced by fractures or pore fluid overpressure in the volcanic system. We find that two superimposed reservoirs can generate a plastic domain between them, if they are spatially close enough. Several reservoir geometries (from spherical to sill-like) are plausible. A conical fracture domain is more likely to appear for reservoirs with opposite pressure loads (i.e. one inflating, one deflating). Given the geometrical match with the proximal seismicity cluster at Mayotte, we suggest that the shallower (Moho-depth) reservoir is inflating, creating a potential hazard for Mayotte island.
•Two interacting magma reservoirs can explain the earthquake distribution near Mayotte.•A 20-km deep reservoir near the island may have inflated during the recent eruption.•Shear patterns between modeled magma reservoirs depend on the applied pressure loads.•The occurrence of mantle-deep earthquakes near Mayotte may indicate mantle weakening.
SUMMARY
Airborne systems collecting transient electromagnetic data are able to gather large amounts of data over large areas in a very short time. These data are most often interpreted through 1-D ...inversions, due to the availability of robust, fast and efficient codes. However, in areas where the subsurface contains complex structures or large conductivity contrasts, 1-D inversions may introduce artefacts into the models, which may prevent correct interpretation of the results. In these cases, 2-D or 3-D inversion should be used. Here, we present a 2.5-D inversion code using 3-D forward modelling combined with a 2-D model grid. A 2.5-D inversion is useful where the flight lines are spaced far apart, in which case a 3-D inversion would not add value in relation to the added computational cost and complexity. By exploiting the symmetry of the transmitter and receiver system we are able to perform forward calculations on a reduced 3-D mesh using only half the domain transecting the centre of the transmitter and receiver system. The forward responses and sensitivities from the reduced 3-D mesh are projected onto a structured 2-D model grid following the flight direction. The difference in forward calculations is within 1.4 per cent using the reduced mesh compared to a full 3-D solution. The inversion code is tested on a synthetic example constructed with complex geology and high conductivity contrasts and the results are compared to a 1-D inversion. We find that the 2.5-D inversion recovers both the conductivity values and shape of the true model with a significantly higher accuracy than the 1-D inversion. Finally, the results are supported by a field case using airborne TEM data from the island of Mayotte. The inverted flight line consisted of 418 soundings, and the inversion spent an average of 6750 s per iteration, converging in 16 iterations with a peak memory usage of 97 GB, using 18 logical processors. In general, the total time of the 2-D inversions compared to a full 3-D inversion is reduced by a factor of 2.5 while the memory consumption was reduced by a factor of 2, reflecting the half-mesh approach.
The volcano-seismic crisis afflicting Mayotte since May 2018 has motivated France-based seismologists to consider the installment of a permanent seafloor observatory with one or more seismometers for ...monitoring surfacing magma and the associated seismicity. In general, deploying a seismometer offshore is known to improve earthquake location – particular in depth – and lower magnitude detection. However, how true are these claims for Mayotte when a land-based seismic network already exists? To address this, we investigate location and detection performance when deploying permanent seismometers offshore Mayotte. We modeled location and detection performance using both real and synthetic data in different network configurations. We found that, in the case of Mayotte, only longitude error is significantly reduced by adding seismometers offshore, perhaps due to the North-South configuration of the land network. Moreover, the size of the Mayotte volcano monitoring area, which spans depths and distances up to 50 km for both, prevents accurate location and detection performance with less than 2 permanent seismometers offshore. Therefore, we would need at least 2 cabled seismometers to monitor this volcanic system, i.e. locate and detect events in real-time. Overall, our modeling suggests that a one-side land network can perform relatively well by itself in location (errors <5 km) and detection (magnitude >1.3) so long as the seismicity occurs at epicentral distances and depths <20 km. However, beyond this distance, one or more seafloor seismometers would be needed to improve location and detection performance.
•We model network location and detection capabilities using real and synthetic data.•The one-side land network can perform relatively well by itself for events nearby.•Seafloor sites mainly reduce longitude error and affect latitude and depth less.•Improved event detection requires at least 2 offshore, real-time seismometers.
Introduction
A high incidence of human leptospirosis is recorded on Mayotte, an oceanic island located in southwestern Indian Ocean, but the severity of the disease appears relatively mild in terms ...of mortality rate and admission to the intensive care unit. It has been proposed that mild leptospirosis may result from a limited virulence of some of the occurring
Leptospira
species to which the population is exposed.
Methods
Clinical and biological data of patients admitted to the Centre Hospitalier de Mayotte were collected and the infecting
Leptospira
species were determined through molecular typing.
Results
Leptospira
interrogans was detected in the minority of admitted patients but most of these patients suffered from severe forms, with 50% admitted to intensive care unit and suffering from organ failures. Nineteen percent of patients infected with
Leptospira
borgpetersenii were admitted to the intensive care, with 13% displaying organ failures, and one patient died.
Leptospira
mayottensis was found in 28% of the patients and not a single severe case was observed.
Discussion
The distribution of
Leptospira
species in patients was not different from that reported 10-15 years ago and bacterial genotypes were very closely related to those previously reported. These results highlight the importance of the diversity of pathogenic
Leptospira
circulating on Mayotte island and are in keeping with distinct outcome of the disease depending on the infecting
Leptospira
. Altogether, presented data support that the infecting
Leptospira
species is an important driver of disease severity in humans.
L’épidémie de Covid-19 a atteint l’île de Mayotte, département français d’outremer en mars 2020. Depuis cette date, et jusqu’à début septembre, 3 374 cas ont été diagnostiqués, dont 75 cas en service ...de réanimation, ayant évolué vers un décès dans 40 cas. Le dépistage et les mesures barrière sont difficiles à contrôler dans ce pays
The Covid-19 epidemic has reached the island of Mayotte, a french overseas department on march 2020. Since that date, and until the beginning of September, 3 374 cases have been diagnosed, including 75 cases in intensive care unit which progressed to death in 40 cases. Screening and barrier measures are difficult to control in this country.
From 10 May 2018 to 1 November 2022 (time of writing), an unprecedented seismic activity is observed east of Mayotte Island (France), related to the largest submarine eruption ever recorded with ...offshore geophysical studies. Using signals from regional and local seismic stations, we build a comprehensive catalog of the local seismicity for the first ten months of the sequence. This catalog includes a total of 2874 events of magnitude (Mlv) ranging from 2.4 to 6.0, with 77% of them relocated using a double difference location procedure. The hypocentral locations over this period are highly dependent on the small seismic network available. Therefore we compare the locations of later events using a similar network and those estimated from a local ocean bottom seismometer (OBS) network installed since March 2019. Based on the time space evolution and characteristics of the seismicity, five distinct phases can be identified, corresponding to the successive activation of two deep seismic swarms, related to the lithospheric-scale magma ascent up to the seafloor, along with progressive deepening of the seismicity interpreted as decompression of a 40 km deep reservoir.
PDF
