Geophysics, geography, economy researcher M/W

Our ambition is to provide a quantitative assessment of the impact of tsunami risks in Mayotte in economic terms, in the short and medium term, taking into account the aggravating factors linked to climate change (rise in sea level, destruction of mangroves and coral reefs), based on state-of-the-art numerical models.
For this, we propose to simulate various scenarios of underwater landslides and the wave they would cause or, alternatively and complementary, of an ocean wave caused by a storm. Thanks to the expertise of Hervé Le Treut, we will assess the evolution of the climatic environment of the ar-chipelago in the coming decades: rise in sea level, frequency and severity of extreme events, erosion of biodiversity and in particular of mangroves and the coral reef, playing a major barrier role for the propagation of waves towards the coasts. We will take these effects into account in the simulations.
A key element of our project will be to take into account the latest mathematical developments made on this type of models by including non-hydrostatic effects and vertical movements through the multilayer approach recently developed by Macias et al., 2021 as well as the refinement of the mesh approaching the coast (Arpaia and Richiutto, 2020) for wave propagation in collaboration with BRGM and INRIA with whom we have submitted an ANR project accepted in phase 2. In addi-tion, we will include in these models a complete description of the topography curvature tensor which plays a key role in landslide dynamics (Peruzzetto et al., 2021).
Indeed, precise simulations (spatial resolution less than 10m) will be necessary in order to identify the part of infrastructures, residential and, more broadly, capital which would probably be destroyed by the said waves, which requires refining the simulations of landslides and tsunamis currently available. For this we will benefit from a database produced by our fellow geographers F. Léone and M. Gherardi including dwellings, infrastructures (location, nature, type of manager, etc.) and strategic issues (regional listening center , airport, port, thermal power plant, submarine cables, large industrial and commercial area) located near the coast. Back and forth between economic, geophysi-cal and mathematical studies will allow the former to point out the areas on which geophysicists will have to target high-resolution spatial simulations. Finally, two time horizons can be considered in order to understand the economic consequences of landslides and floods: that of the immediate cost in terms of capital losses (directly impacting the balance sheet of insurers and reinsurers) and that, at horizon 5 -10 years, of the inflection induced on the economic dynamics of the island in terms of GDP, employment, inflation, poverty, etc. To this end, a coherent stock-flow non-linear macro-dynamic economic model, calibrated on the island of Mayotte, could be developed, inspired by the Gemmes model built at the French Development Agency (Bovari et al., 2017, 2019 )). Particular attention will be paid to electricity and communication infrastructure, as well as the supply of drink-ing water and the archipelago's dependence on natural resources which could run out in the coming decades and in the event of a disaster (Vidal et al., 2017, 2019). The preferred macro-economic ap-proach will be built on the interaction of public and private debt dynamics (whose importance Gi-raud and Grasselli (2019) have shown for the macro-economic trajectory of an economy) with the first-line needs necessity and the constrained expenses of an archipelago where most of the popula-tion has a low standard of living.
Beyond the specific case of Mayotte, this study would provide an innovative methodological exam-ple of interaction between geophysical, mathematical, climatic and economic sciences. Finally, it would contribute to our knowledge of the economic impact of climate change based on a concrete example and would lay the foundations for a reflection on the means available to the inhabitants of Mayotte to adapt to this change. How can we guard against the potentially devastating impact of a particularly high wave, the effect of which will be multiplied by the rise in sea level? Different pub-lic policy and land-use planning options in Mayotte can be tested, and their consequences simulated, on the basis of the scenarios that we will have developed.

In a world facing increasing natural and climatic risks, and in a context of increasing population density and infrastructure, this project aims to quantify the economic effects linked to tsunami risks and climate change. through research at the interfaces between geophysics, mathemat-ics, climatology and economics. For this, we propose to quantify the economic impact of tsunamis (generated by potential submarine landslides off the coast of the island of Mayotte) on infrastruc-tures and populations. Indeed, since 2018 the island has been the seat of a major seismo-volcanic crisis linked to the birth of a new underwater volcano which weakens the steep underwater slopes. The context of Mayotte is ideal for conducting this interdisciplinary research with assurance of its impact on society. Indeed, joint studies have been initiated since 2018, involving a tight network of both fundamental and applied research with researchers in geophysics, geography, and social scienc-es as well as State operators (BRGM) and an interministerial committee maintaining frequent ex-changes with local actors (prefect, population). Until now, no study of the economic impacts of this seismo-volcanic crisis has been conducted.

The project will initially consist of collecting high-resolution data to finely represent buildings, infrastructures, strategic issues and coastal bathymetry in order to carry out very precise simulations of the impact of tsunamis on the coasts of Mayotte. . These simulations will be carried out on the basis of submarine landslide scenarios potentially generated by the current seismo-volcanic crisis. On the basis of these simulations, it will be a question of simulating the economic impact in the short and medium term, in particular taking into account the rise in sea level and the potential destruction of mangroves and the coral reef linked to climate change, an aggravating factor for onshore submersion.

Since May 2018, the island of Mayotte has been experiencing significant seismic activity linked to underwater volcanic activity which would have started 50 km from the coast of the island. The eruptive site discov-ered in 2019 revealed the establishment of a new underwater building, created in less than a year, at least 800 m high on the ocean floor and located at 3500 m depth. Since then, new flows have been identified around this building as well as new eruptive plumes. This crisis is monitored by the REVOSIMA (Volcanological and Seismological Surveillance Network of Mayotte) set up by the State in 2019.
Earthquakes affecting the island form swarms with epicenters located between 5 and 25 km east of Petite Terre. The intense seismo-volcanic crisis, the location of earthquakes close to the steep slopes that surround the island and the construction of a new volcano threaten to trigger sub-marine landslides off Mayotte generating tsunamis. Since 2019, significant work has been carried out to develop water level maps estimating the impact of tsunamis linked to different landslide scenarios. These maps were regularly presented to national and local authorities. They were built on the basis of digital models simulating underwater landslides and the waves they generate. These simulations show that the wave heights on the coasts of Mayotte are very heterogeneous and closely dependent on the bathymetry, the coral reef, the mangroves, and the sea level. The waves can reach a few meters af-fecting strategic infrastructures and inhabited areas. Their arrival time at the coast is very short, ranging from a few minutes to around twenty minutes, and these waves are not necessarily preceded by a withdrawal from the sea to the shore. The population therefore cannot count on such a “natural alert”. Taking into account the simulated water speeds made it possible to draw up hazard maps showing that even if the water heights on land and in particular on the airport runway are not very high (1m), the hazard can be strong in the area considered.
In addition, the overall displacements of the GPS stations of Mayotte since the beginning of the crisis indicate a displacement of the island towards the east of approximately 20 cm and a subsid-ence (subsidence) of approximately 15 cm induced by the deflation of a deep magmatic reservoir (Briole et al., 2018,). This subsidence but also climate change have a significant impact on the coast-line, involving an increase in submerged areas and increased sensitivity to potential tsunamis or storm-related waves in this area. Taking this change in the coastline into account is essential to as-sess the risks associated with waves on the island of Mayotte. If it is possible to simulate these pro-cesses numerically to go as far as the evaluation of economic costs, the accuracy of hazard maps could be considerably improved by developing models that include hydrostatic effects and vertical movements during the propagation of the hazard. tsunami as well as the complete tensor of the cur-vatures of the topography during the simulation of landslides. The state-of-the-art models devel-oped in mathematics in this direction are unfortunately only rarely used in a chain going as far as the quantification of economic impacts.