Audrey Michaud-Dubuy receives the 2020 CCR Cat Nat for her PhD thesis entitled: Dynamics of Plinian eruptions: re-assessment of volcanic hazard in Martinique.
The CCR Cat Nat prize rewards a PhD thesis devoted to the knowledge of natural disasters and its application to insurance and risk prevention.
© Audrey Michaud-Dubuy
PhD thesis abstract:
Volcanic plumes produced by explosive eruptions represent a major threat in areas located near volcanoes. Physical models have been developed over the past forty years with an aim of better understanding these eruptions and assessing associated hazards. To test these models, we need robust and detailed field data from past and historical eruptions at active volcanoes. In this PhD work, we revisit the Plinian eruptive history of the Mount Pelée volcano in Martinique (Lesser Antilles) for the last 24,000 years. Our results combining new extensive field studies and carbon-dating measurements allow us to establish a new chronology of past eruptions, consistent with volcanic deposits identified in a deep-sea sediment core. We then reconstruct the dynamical evolution of the newly discovered eruptions of Bellefontaine (13,516 years cal BP), Balisier (14,072 cal BP), Carbet (18,711 cal BP) and Étoile (21,450 cal BP), whose great interest stems from their unusual southward dispersal axis encompassing areas that are considered to be safe in current hazard maps. The strong similarities observed between all documented Plinian eruptions of Mount Pelée volcano allow us to draw an accurate picture of the Plinian eruptive scenario most likely to occur in the future.
This scenario may include a column collapse and the production of deadly pyroclastic density currents; we thus upgrade a 1D physical model of volcanic plume in order to improve its predictions. We first study the impact of the total grain-size distribution on the transition from a stable Plinian plume to a collapsing fountain. The effect of wind is then taken into account using laboratory experiments simulating turbulent jets rising in a windy environment. This new theoretical model, validated by laboratory experiments, is consistent with field data from several major historical Plinian eruptions. We then study the southward dispersal axis of the Bellefontaine and Balisier eruptions using a 2D physical model, in order to better understand this unusual dispersion towards Fort-de- France, capital of Martinique. Our results allow identifying peculiar atmospheric circulations associated to a modification of the subtropical jet-stream path, thus producing northerly winds over Martinique and spreading tephra towards the most populated areas of the island. This integrated approach, combining field studies, theoretical predictions and laboratory experiments, allows us to build a new volcanic hazard map for Martinique by taking into account for the first time the Plinian eruptions of the Mount Pelée volcano of the last 24,000 years, together with monthly variability of atmospheric winds.
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