Characterisation and numerical simulation of submarine slope instabilities offshore PetiteTerre (Mayotte) in the context of the on-going seismo-volcanic crisis.

The island of Mayotte in the Comoros archipelago (Western Indian Ocean) has experienced an important seismic crisis since May 10 2018 in the context of the voluminous eruption in 2018-2019 of a new submarine volcano about 50 km E of Mayotte (Feuillet et al., submitted). Almost 2000 M>3.5 earthquakes occurred offshore, east of the island (Lemoine et al., 2019). The crisis is still very active after more than one year, with several M 4.5+ earthquakes per month. Since early July 2018, the island is subsiding up to 170 mm/year and moving eastwards at a fast rate of up to 240 mm/year. High-resolution bathymetry collected during the MAYOBS cruise in May 2019 (Feuillet et al., submitted) (RV Marion Dufresne) revealed steep submarine slopes offshore the island of Petite Terre, close to the seismic swarm. Morphological analysis shows that some previous submarine slope instabilities occurred in the past. The fact that the seismic crisis is still very active after more than one year, that the seismic swarm is located very closed to the steep submarines slopes area offshore Petite Terre, and that there is evidence of the migration of large volumes of magmatic fluids within the Mayotte rift zone in the vicinity of Mayotte, increases the hazard of submarine slopes instabilities and associated tsunamis. In this contribution, we present a preliminary study of submarine instabilities. We identify potential instable areas and selected different scenarios with different volumes offshore Petite Terre. We then use the HYSEA Numerical model based on the equations proposed by Fernandez-Nieto et al (2008) in shallow-water conditions, which has been modified in 2019 to take into account the dispersive terms (Escalante et al., 2019). This model is a two layers model with one layer representing the granular avalanche and the other one the fluids. It includes the dispersive terms for the wave generation and propagation which allows to reproduce the fine dynamics of the water and granular flow interaction. We will discuss the maximum amplitude waves in case of different scenarii.