Tephra fallout and dispersal modeling of phreatic eruptions: Implications for volcanic hazard assessment in Martinique and Guadeloupe (Lesser Antilles)
Résumé
A phreatic eruption is the most probable future scenario for the onset of eruptive activity at Montagne Pelée (Martinique) and La Soufrière (Guadeloupe) volcanoes. Despite the potential mortality from this type of non-magmatic eruption, the phreatic scenario is often not considered during hazard assessment. Both Montagne Pelée and La Soufrière are in volcanic alert level yellow (vigilance). Here we used the tephra fallout dispersal model HAZMAP to create hazard maps for tephra fallout during phreatic events in Martinique and Guadeloupe. We first simulated a single scenario based on the historical eruptions of May 2-3, 1902 at Montagne Pelée, and July 8, 1976 at La Soufrière. We then used a multi-scenario approach using three sets of eruptive conditions combined with the 43-year wind ERA5 reanalysis database. The latter maps show that the entire islands of Martinique and Guadeloupe could be impacted by tephra fallout from a future phreatic eruption. We then used a Monte Carlo method to sample a series of eruptive scenarios associated with three different grain-size distributions that we simulated using the complete 43-year wind database. The resulting probabilistic maps for Montagne Pelée and La Soufrière allow us to assess the maximum extent of phreatic tephra dispersal and compare it to historical eruptions of both volcanoes. Calculated probabilities of exceeding impact thresholds for key infrastructure on both islands suggest that Guadeloupe has a higher exposure than Martinique to a future phreatic eruption. The single-scenario, multiple-scenario and probabilistic hazard maps were all shared and evaluated with the local authorities in Martinique in order to improve the efficacy and relevance of the final map. These discussions led to the incorporation of an integrated phreatic scenario considering multiple phenomena into the newly revised 2022 volcano emergency plan for Montagne Pelée. Our results will be integrated in the future revised emergency response plan (ORSEC, Préfecture de Guadeloupe) for La Soufrière.