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Since May 2018, Mayotte island has experienced an important seismic activity linked to the on-going sismo-volcanic crisis. Although variations in the number of earthquakes and in their distribution have been observed since the start of the eruption in early July 2018, a continuous seismicity persists. It could weaken the steep submarine slopes of Mayotte, as highlighted by the high-resolution bathymetry data collected during the MAYOBS cruise in May 2019. This could trigger submarine landslides with associated tsunamis.To address the hazards associated with such events, we analyzed geomorphological data to define 8 scenarios of potential submarine landslides with volumes ranging from 11,25.106 to 800.106 m3. We simulated the resulting landslide dynamics as well as generated waves (Poulain et al. 2022). In order to estimate the uncertainty associated to the modeling approach, a hierarchy of different model approximations was tested, spanning hydrostatic, non-hydrostatic and multilayer approaches. A sensitivity analysis was also performed by varying the initial released mass, the rheological parameters describing the landslide, its interaction with the water column, the Manning friction coefficient as well as the resolution of the bathymetry description. The combination of all these elements provides an estimate of the uncertainty on simulation results. We show that, in the context of Mayotte, non-hydrostatic effects have the most prominent influence on simulated water elevation and waves velocity. Other key factors include the friction coefficient within the landslide and the resolution of the bathymetry. These results show that landslide-tsunami models should still be improved as well as the estimates of the parameters involved to reduce the related uncertainties on the water wave calculation (water elevation, velocity) that can exceed a factor two.Poulain, P., et al. (2022). Numerical simulation of submarine landslides and generated tsunamis: application to the on-going Mayotte seismo-volcanic crisis. Comptes Rendus. Géoscience, 354(S2), 1-30.

The Toba volcano super-eruption on the island of Sumatra occurred about 74,000 years ago[1], close to the transition between interglacial Marine Isotope Stage (MIS) 5 and glacial MIS 4. This eruption, called Youngest Toba Tuff (YTT), is currently described as the largest cataclysmic eruption of the Quaternary. However, the impact of this super-eruption on climate is widely debated and its effects on the ocean remains poorly understood.The aim of this work is to estimate its impact on oceanic pH at a site near the eruption center. To do so, we measured δ11B values (pH proxy) on monospecific samples of planktonic foraminifera Globigerinoides ruber and Pulleniatina obliquiloculata from sediment core BAR94-25 (Andaman Sea) using a recently developed method at the Institut de Physique du Globe de Paris (IPGP)[2]. G. ruber is a species that thrives preferentially in surface waters, while P. obliquiloculata lives at the thermocline. Therefore, δ11B measurements on their shells can reconstruct pH variations in surface and thermocline waters, respectively.We selected the interval from 258 to 355 cm, corresponding to an age between 57 and 82 ka. This interval contains two clearly visible tephra layers corresponding to the YTT, at the transition from MIS 5 to MIS 4, and to a post-YTT explosive activity during MIS 4. These layers are correlated with a significant decrease in carbonate content (CaCO3). Our results indicate a complex pH response during the two concerned volcanic episodes. Thermocline seawater doesn’t show significant pH decrease during the volcanic episodes compared to the overall signal recorded throughout the studied interval. Conversely, surface seawater shows a much more important pH decrease during part of the volcanic episodes than during the all studied interval. Such decrease in pH during the transition to a glacial state is particularly surprising because an increase in pH, due to the global reduction in atmospheric CO2, is rather expected, as shown by previous foraminifera δ11B records[3].The coupling of CaCO3 and pH decrease during tephra levels suggests acidification in the Andaman Sea as a consequence of the Toba volcanic eruptive activity. The seawater surface seems much more sensitive to pH changes than the thermocline zone. However, the reduction of carbonate in the two tephra layers may also be due to dilution from ash falling into the sediment. Other analyses, such as measuring the variation of calcification intensity in planktonic foraminifera, are therefore necessary to better interpret these paleo-pH data.[1] Storey et al., 2012, PNAS, 109 (46), 18684-18688[2] Buisson et al., 2021, JAAS, 36, 2116-2131[3] Foster et al., 2008, EPSL, 271, 254-266

We investigate the dynamics and deposits of granular flows and the amplitude of the generated water waves using the depth-averaged shallow numerical model HySEA, both at the lab- and field scales. We investigate the different sources of errors by quantitatively comparing the simulations with (i) six new laboratory experiments of granular collapses in different conditions (dry, immersed, dry flow entering water) and slope angles, and (ii) numerical simulations made with the code SHALTOP that describes topography effects better than most landslide-tsunami models. In the laboratory configurations, at the limit of the shallow-approximation in such models, we show that topography and non-hydrostatic effects are crucial. However, when empirically accounting for topography effects by artificially increasing the friction coefficient and performing non-hydrostatic simulations, the model is able to reproduce the granular mass deposit and the waves recorded at gauges located at a distance of more than 2-3 times the characteristic dimension of the slide, with an error ranging from 1 % to 25 % depending on the scenario, without any further calibration. Taking into account this error estimation, we simulate landslides that occurred on Montagne Pelée volcano, Martinique, Petites Antilles as well as the generated waves. Results support the hypothesis that large flank collapse events in Montagne Pelée likely occurred in several successive sub-events. This result has a strong impact on the amplitude of the generated waves, and thus on the associated hazards. In the context of the on-going seismic volcanic unrest at Montagne Pelée volcano, we calculate the debris avalanche and associated tsunami for two potential flank-collapse scenarios.

This work focuses on the use of thin-layer models for simulating fast gravitational flows for hazard assessment. Such simulations are sometimes difficult to carry out because of the uncertainty on initial conditions and on simulation parameters. In this study, we aggregate various field data to constrain realistic initial conditions and to calibrate the model parameters. By using the SHALTOP numerical code, we choose a simple and empirical rheology to model the flow (no more than two parameters), but we model more finely the geometrical interactions between the flow and the topography. We can thus model both a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow.Using the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we focus on extreme events with a high potential to impact populations and infrastructures. We use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analysis to define realistic simulation scenarios and determine the main characteristics of documented events. The latter are then reproduced to calibrate rheological parameters. With a single rheological parameter and the Coulomb rheology, we thus model the emplacement and main dynamic characteristics of a recent rock avalanche, as well as the travel duration and flooded area of a documented high discharge debris flow. Then, in a forward prediction simulation, we model a possible 1.9x106 m3 rock avalanche, and the instantaneous remobilization of the resulting deposits as a high-discharge debris flow. We show that successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus do not influence the initial conditions of the subsequent debris flow simulation. A progressive remobilization of the materials slows down the debris flow and limits overflow, in comparison to instantaneous release. However, we show that high discharge debris flows, such as the one considered for model calibration, are better reproduced with an instantaneous initiation. The range of travel times measured for other significant debris flows in the Pr\^echeur river is consistent with our simulation results, with various rheological parameters and the Coulomb or Voellmy rheology.

Since May 2018, Mayotte island has experienced an important seismic activity linked to the on-going sismo-volcanic crisis. The epicenters of the seismic swarms are located between 5 and 15 km east of Petite Terre for the main swarm, and 25 km east of Petite Terre for the secondary swarm. Although variations in the number of earthquakes and their distribution have been observed since the start of the eruption in early July 2018 [Lemoine A.(2020), Cesca et al.(2020)], a continuous seismicity persists and could generate several earthquakes of magnitudes close to M4 widely felt by the population. This recurrent seismicity could weaken the steep submarine slopes of Mayotte, as highlighted by the high resolution bathymetry data collected during the MAYOBS cruise in May 2019 (Feuillet et al.,submitted) and trigger submarine landslides with associated tsunamis.To address the hazards associated with such events, we analyzed morphological data to define 8 scenarios of potential submarine slides with volumes ranging from 11,25.106 to 800.106 m3 and we simulate the landslide dynamics and generated waves. We use two complementary numerical models: (i) the code HYSEA to simulate the dynamic of the submarine granular flows and the water wave generation, and (ii) the Boussinesq FUNWAVE- TVD model simulate the waves propagation and the inundation on Mayotte. The effect of the time at which the models are coupled is investigated.The most impacting submarine slide scenarios are located close to Petite Terre at a shallow depth. They can locally generate a sea surface elevation more than a meter in local areas especially at Petite Terre. The various simulations show that parts of the island are particularly sensitive to the risk of tsunamis. Indeed, some scenarios that does not cause significant coastal flooding still seems to cause significant hazards in these exposed areas. The barrier reef around Mayotte has a prominent role in controlling the wave propagation towards the island and therefore reducing the impact on land. It should be noted that the arrival of tsunamis on the coastline is not necessarily preceded by a retreat from the sea and the waves can reach the coasts of Mayotte very quicky (few minutes). Cesca, S., Letort, J., Razafindrakoto, H.N.T. et al. Drainage of a deep magma reservoir near Mayotte inferred from seismicity and deformation. Nat. Geosci. 13, 87–93 (2020). https://doi.org/10.1038/s41561-019-0505-5Feuillet, N, Jorry, S. J., Crawford, W, Deplus, C. Thinon, I, Jacques, E. Saurel, J.M., Lemoine, A., Paquet, F., Daniel, R., Gaillot, A., Satriano, C., Peltier, A., Aiken, C., Foix, O., Kowalski, P., Laurent, A., Beauducel, F., Grandin, R., Ballu, V., Bernard, P., Donval, J.P., Geli, L., Gomez, J. Guyader, V., Pelleau, P., Rinnert, E., Bertil, D., Lemarchand, A., Van der Woerd, J.et al. (in rev). Birth of a large volcano offshore Mayotte through lithosphere-scale rifting, Nature.Anne Lemoine, Pierre Briole, Didier Bertil, Agathe Roullé, Michael Foumelis, Isabelle Thinon, Daniel Raucoules, Marcello de Michele, Pierre Valty, Roser Hoste Colomer, The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption, Geophysical Journal International, Volume 223, Issue 1, October 2020, Pages 22–44, https://doi.org/10.1093/gji/ggaa273

The Prêcheur river is located to the West of Montagne Pelée, in the Northern part of Martinique island. For several decades it has produced numerous lahars that directly threaten the Prêcheur village, at the mouth of the river. In recent years, the most important lahars have been correlated to massive collapses of the Samperre cliff, 9 km upstream from the sea, that create a reservoir of loose material at the bottom of the cliff. In 2010, a lahar started from this reservoir, destroyed a bridge and inundated part of the Prêcheur village. A new major period of collapses of the Samperre cliff started on 2 January 2018, involving more than 4 × 106 m3 of material. In the following days, intense rainfalls triggered several lahars that reached the Prêcheur village but remained confined in the river bed. Since then, lahars and collapses have continued to occur, even though their frequency has decreased with time and their intensity is smaller compared to the onset of the crisis. One single lahar overflowed the river bed on 22 February 2018 without significant impact on infrastructures.In this study, we test different possible scenarios to model the first and most important phase of the collapse of the Samperre cliff, that occurred in early January 2018, with the shallow-water model SHALTOP. We constrain the collapse geometry with photogrammetric 3D models and LIDAR topographic surveys, acquired in 2010 and in late January 2018. We also consider an intermediate volume to take into account a possible retreat of the cliff face between 2010 and 2018. The modeled traveled distances are compared to field observations. Finally, we use geomorphological and geological observations to identify potentially unstable structures within the cliff, and model the associated collapses.These simulations provide insights on the possible geometry (extent and depth) of the debris reservoir at the bottom of the cliff, after a major collapse episode. This is of prior importance in order to estimate the location and volume of future lahars. In order to investigate their dynamics, we model the major 2010 lahar, for which the initial debris reservoir volume is known (about 2 Mm3). We first simulate the progressive remobilization of the reservoir by water with the r.avaflow numerical code. In a second test, we impose instead a constant flow discharge upstream until the same volume has been released. We test different parameters to identify which ones have the most significant influence on the lahar travel time, from its initiation until it reaches the Prêcheur village.

The early development and growth of seamounts are poorly known as the birth of a volcano on the sea bottom has been rarely observed. The on-going Mayotte seismo-volcanic crisis is associated with the formation of a new seafloor volcano at a water depth of 3300 m and provides the opportunity to study its early development.Four oceanographic cruises, MAYOBS 1 to 4, were carried out between May and July 2019 aboard the French R/V Marion Dufresne. High resolution bathymetry and backscatter data as well as sub-bottom profiler, gravity and magnetic profiles were collected during each cruise. A dense network of profiles has been achieved over the new volcano at different epochs, allowing to assess its detailed morphology and the evolution through time. During MAYOBS4, a deep-towed underwater camera provided sea bottom videos and photos on the volcano.First results indicate that the new volcano is still growing at the end of July 2019. Repetitive surveys in May, June and July 2019 allow to document the morphological evolution of the volcano, to estimate the volume of material emplaced between each epoch and to discuss the emitted lava rate.The new volcano has a starfish shape and is now 820 m high. Steep slopes are observed close to the summit and several radial ridges developed from its central part, displaying hummocky morphology similar to the ones observed along mid oceanic axial volcanic ridges. At the bottom, flat areas with high backscatter could indicate channelized lava flows emplaced at higher effusion rates. The morphological analysis combined with video imagery brings constraints to the eruptive processes yielding to the formation of a nascent volcano.

The aragonite shell–bearing thecosome pteropods are an important component of the oceanic plankton. However, with increasing pCO2 and the associated reduction in oceanic pH (ocean acidification), thecosome pteropods are thought to be particularly vulnerable to shell dissolution. The distribution and preservation of pteropods over the last 250,000 years have been investigated in marine sediment cores from the Caribbean Sea close to the island of Montserrat. Using the Limacina Dissolution Index (LDX), fluctuations in pteropod dissolution through the most recent glacial/interglacial cycles is documented. By comparison to the oxygen isotope record (global sea ice volume), we show that pteropod dissolution is closely linked to global changes in pCO2 and pH and is, therefore, a global signal. These data are in agreement with the findings of experiments upon living pteropods, which show that variations in pH can greatly affect aragonitic shells. The results of this study provide information which may be useful in the prediction of future changes to the pteropod assemblage caused by ocean acidification.