Advances in seismic hazards research.
Telluric hazards can dramatically affect a large number of coastal regions, world-wide and in particular vulnerable insular areas. Europe and its overseas regions are also threatened by marine geohazards, as the Mediterranean-Gibraltar-Azores system, the Caribbean, the area surrounding Mayotte and the Andean subduction zones among others host a number of offshore active faults able to produce major earthquakes and tsunamis. Despite their potential impacts, the processes are still not understood, even though significant conceptual progress has been made over the last fifteen years based on the combination of land-based seismological and geodetic data. For instance, the discovery of slow-slip events (SSE) has radically changed our views of stress release at subduction zones and raised the possibility of predictability of large earthquakes. However, because the active processes occur below the ocean floor, further progress is hampered by the lack of appropriate seafloor seismological and geodetic instrumentation. More generally, while most of the seismic energy is released at plate boundaries below the oceans, only an insignificant fraction of the world’s seismicity is recorded by stations deployed on the ocean bottom.
Monitoring offshore SSEs and seismic activity on the long term requires monitoring of both seismicity and seafloor motion. To address the observational gap in the ocean, a number of developed countries have mobilized important financial resources to develop large-scale, fixed, cabled ocean-bottom observatories, like DONET and SNET in Japan; Neptune/Ocean Networks in Canada; Neptune/OOI in the USA (see review in Favali et al, 2015). Nevertheless, the international community must be ready to collect new data at tectonically active zones with the appropriate offshore instrumentation, at as many sites as possible, including in Europe and in its oversea regions, where the existence of multiple active margins requires a multi-facetted, flexible and mobile approach. We will refer to the example of the USA, that has developed since 1999 a national pool of marine instrumentation, presently hosted (since 2018) at Woods Hole Oceanographic Institution. Our ambition is to start developing such a pool for France, with the idea of extending this initiative at the European level. This seaward extension of RESIF on-land networks and sensors will allow to get closer to the sources of submarine earthquakes which release most of the seismic energy affecting the Earth. We also anticipate that developing seafloor geodesy will revolutionize our understanding of geodynamic processes, as the development of GPS for continental areas did in the nineteen-nineties.
Advances in geohazard assessment and monitoring in Mayotte.
The marine geological, geophysical and geochemical data collected with R/V Marion Dufresne in 2019 and 2020, suggest an ongoing process of magma migration towards Mayotte since the initiation of the crisis in April 2018, and of lithospheric magma reactivation in the most active part of the submarine volcanic zone (e.g. the so-called “Fer à Cheval” area) within 5 to 15 km offshore Mayotte. In this area, very recent undated voluminous lava flows have been mapped, suggesting a potential for caldera reactivation and tsunamigenic collapse due to voluminous magma withdrawal. On land, field studies suggest that renewed magma ascent below Mayotte with explosive activity has occurred in the last 7000 years. Knowledge on Mayotte’s current and past eruptions as well as on similar volcanic systems indicates that magma ascent, eruption, and tsunamigenic caldera collapse can develop in a non-linear fashion within short timescales on the order of days to weeks and sometimes on the order of hours only.
The current geophysical monitoring networks on land and the periodic acquisition of time-lag seismic data from OBS systems that have to be deployed and retrieved every few months, financed in 2018 and 2019 by the French Government for the immediate crisis response, do not constrain with sufficient accuracy and precision the location, nature and magnitude of seismicity and deformation sources. The observatory system we propose for Mayotte will allow the detection and characterization of low magnitude seismicity, the measurement of seafloor deformation and the direct observation of submarine hydrothermal and volcanic degassing plumes. This will provide direct, real-time information to track the transitory short-timescale dynamics governing the processes that might be precursory to new eruptive activity, caldera formation and gravitational instability.
Advances in understanding marine ecosystems evolution in relation to volcanic activity off Mayotte.
The occurrence of this major submarine eruption generates a remarkable diversity of scientific opportunities to understand the dynamics and coupling between lithospheric processes and the hydrosphere and biosphere compartments by providing an open window to the biodiversity and functioning of associated nascent marine ecosystems. The project will contribute to a significant advance in our understanding of the dynamics of marine ecosystems in relation to volcanic activity. The questions addressed by the project are:
- Newly formed surfaces or venting sources are colonized by a succession of fauna, starting with microbial mats, grazers and the establishment of a suite of species whose recruitment is connected to variations in abiotic conditions and biotic interactions : What are the drivers for the appearance and development of life on a new volcanic edifice?
- What are the factors regulating the colonization / recolonization and larval dispersal processes depending on the faunistic and microbial diversity?
- Hydrothermal vents are now considered as crucial areas where Earth will exchange heat and matter with the hydrosphere influencing ocean biogeochemical cycles at a global scale . What are the fluxes of heat, chemicals and biomass to the overlying ocean associated with the eruptive and volcanic activity, and what are the driving processes involved?
- Recent studies  observed that the eruption of submarine volcano eruption off El Hierro Island (Canary Islands) on October 2011 generated important and reversible perturbation of the water column leading to changes in the distribution and dynamics of pelagic fauna. How does this eruption impact the living communities (from micro-organisms to larger fauna) in the benthic and pelagic compartments? What is the impact for the fishery and marine megafauna (such as marine mammals and turtles). This final objective will contribute to the specific management objectives of the Mayotte Marine National park.