During an explosive volcanic eruption, ash falls can produce dramatic effects in the area nearby the volcano: collapse of buildings, lahars, floods, damage to aircraft engines, major disruptions to water, electricity and wastewater distribution networks… At a distance of several tens or hundreds of kilometers, ash fallout, even moderate, can also have significant impacts: respiratory problems, asthma exacerbations, pollution of water reservoirs, nuisance on the functioning of mechanical, electrical or electronic devices (due to the abrasive power of these ashes). It is therefore necessary to know as soon as possible the quantity of ashes emitted into the atmosphere and their characteristics (size, shape and mineralogical/chemical composition).
On April 9, 2021, the Soufrière of Saint-Vincent begins a series of about thirty explosions and on April 12, 2021, weak ash fallout is observed in Martinique. It is necessary to mobilize to obtain data, especially if the eruption lasts. Under the supervision of the Observatoire volcanologique et sismologique de Martinique (OVSM-IPGP) and within the framework of the PREST project, several initiatives were launched to monitor and characterize the ash falls in Martinique.
Six homemade but standardized ash collectors were constructed by the OVSM technical team using commercial plastic bottles and deposited in Martinique (Figure 1). These efficient and quick to set up collectors were built on the design of those deployed in Ecuador by volcanologists from the Quito Geophysical Institute during the Tungurahua eruption in 2012. The ash fallout of April 12, 2021 will be the only event reaching Martinique and these will ultimately have only a minor impact. But the exercise is very instructive.
The health situation does not facilitate travel, a citizen participation is proposed, via the Facebook account of the OVSM. Thus, nearly 25,000 people are touched by this call, several thousand reactions and more than a hundred testimonies are processed. Participants are asked to take a photo of an ash-covered surface at home before touching it, and then a photo with the location, date and time of the observation written on their finger (Figure 2). Together with abacuses, these photos allow scientists to approximate the thickness of ash. Participants can also collect a small amount of material in a bag, noting the location, date, time, and type of surface, and send it to OVSM for analysis of ash size, shape, and composition. It is also recommended that the surface be cleaned to be available for further ash fallout (which was not the case). The samples received at the OVSM and recovered in the ash collectors were quickly transmitted to the IPG-Paris and are currently being analyzed. Their study will allow a better understanding of the geochemical composition of the volcanic plumes of St-Vincent (sulfur, halogens, metals..) and their environmental impact. The analysis of the testimonies collected on social networks allowed the scientists to establish a map of ash dispersion of this episode (Figure 3). This map shows quantitatively the gradation of ash fallout from the north to the south of Martinique and offers a robust constraint to test and validate a posteriori the physical models of volcanic eruptions that are routinely used, in particular within the PREST WP4, for the prediction of ash fallout from other Caribbean islands.
Other ash samples were taken in the English-speaking islands (St. Vincent and the Grenadines, Barbados, St. Lucia) under the direction of the Seismic Research Center of the University of the West Indies (SRC-UWI) and the installation of other types of collectors. A dialogue is engaged with the actors of the prevention of natural hazards in the West Indies (SRC-UWI, Montserrat Volcano Observatory, OVS-Martinique, OVS-Guadeloupe, IPG-Paris) to coordinate this type of field and laboratory measurements, which are essential to calibrate volcanic eruption models, to assess the impact of ash, and to implement the prevention tools.
David Mélezan, Technician at the OVSM-IPGP
Fabrice Fontaine, Scientist in charge of the OVSM-IPGP
Guillaume Carazzo, Volcanologist at the IPGP
Jean-Christophe Komorowski, Volcanologist at the IPGP
Audrey Michaud-Dubuy, Volcanologist at the IPGP
Jean-Bernard de Chabalier, Geophysicist a the IPGP
Pascale Besson, Volcanologist at the IPGP
Benjamin Bernard, Volcanologist at the Institut Géophysique de Quito