Monday, March 4

When and where will the next eruption be on La Palma? A risk assessment tries to solve the unknown

The perception of volcanic risk in the Canary Islands has increased considerably in the last year after the eruption that took place in La Palma. The sights are set on the recovery of the affected area. And it’s normal. But from now on, the islands must think about how they will work to mitigate an impact of such magnitudes when lava and pyroclasts sprout from the earth again, as Nemesio Pérez, from the Canary Islands Volcanological Institute (INVOLCAN), has defended on numerous occasions, and now also Joan Martí, volcanologist at the Higher Council for Scientific Research (CSIC).

The allegation of the geographers of the Canary Islands: “We must rethink the future taking into account the volcanic risk”

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The latter, together with expert colleagues from Chile and Granada, has published a Article in which a volcanic risk assessment methodology on the island of La Palma is detailed, where hazard maps and possible scenarios are exposed. The study was conducted before the recent eruptive activity at Cumbre Vieja, but has now been published in the journal Journal of Volcanology and Geothermal Research. The researchers tried to predict where and when the next volcano on Isla Bonita would explode from 2015. According to Martí, “the probabilistic model with what happened has an 87% coincidence.”

Similarly, the scientists took this risk analysis to Iceland, where they also applied it before the eruption of the Fagradalsfjall volcano in March this year, and obtained a very high similarity percentage of 80%. “[En La Palma], we were able to identify the most probable scenarios for a relatively narrow forecast window, as well as the spatial probability of the occurrence of future fumaroles and the different volcanic risks that could be involved”, highlights the research. To try to predict another eruptive episode in both territories, it would only be necessary to update the data.

Martí jokes and describes his work as if he were a volcano meteorologist. From data and historical records, he notes, “you realize that, deep down, there may be a repetition. They are all different eruptions, yes, but they have something similar. And when you deal with that type of data, you can see if there are patterns of behavior. So in some way we can afford to study what the next eruption would be like in terms of size, intensity, products it could generate…”.

The methodology used by Martí and company has an artisan part, such as field work and data collection. And another more technical one that involves statistics and mathematical calculations. According to him, this assessment must be based on a thorough reconstruction of the geological and historical records of the volcanic area. In the Canary Islands, only Tenerife has a island action plan against volcanic risk (PAIV) and, therefore, updated hazard maps, while that of La Palma is still in the homologation phase.

“Everywhere you have to do an analysis of this type. Then the risk study. It is a pending issue that the Canary Islands have, because those problems are going to repeat themselves, that’s for sure,” says Martí. “And the question no longer only concerns the residents of the Islands, but also the number of visitors it receives, which is extremely high. In Iceland it is exactly the same problem. Here there is a population of 350,000 inhabitants, but more than one and a half million tourists come every year. In a crisis of this type, all of this must be managed. And it’s not easy.”

A three-step methodology

In the study, Martí and the rest of the team describe a long-term risk assessment methodology, exposed for the first time in a Master’s thesis, with four well-differentiated steps. In the first, it is about obtaining the susceptibility or spatial probability, including the recognition of the most representative volcanic elements. In the second, a temporal analysis is carried out based on historical volcanism. In the third, the possible development of eruptive scenarios enters the scene. And in the fourth, obtaining a hazard map.

For the spatial analysis, the researchers used geological maps of La Palma produced by the Spanish Geological and Mining Institute (IGME), scientific literature and remote sensing, as well as field work. In total, they found 529 emission centers that at some point ejected lava on the island, about 75% located in the south. And they also identified 126 eruptive fissures, 76% framed in Cumbre Vieja.

“This part is crucial. It is the most fundamental of all. If the eruption on La Palma had been in the eastern part of the island, for example, the scenarios would have been totally different. We work with all the data we collect and apply probabilistic methods to recognize which areas are most likely”, explains Martí.

Next, the temporal analysis took into account all the historical eruptions reported on La Palma, including that of Tacande or Montaña Quemada, which some documents date from 1470 and others from 1492. Another model, in this case HASSET, was used to measure whether there would be an eruptive event within two decades from the year 2015. The calculation gave a 74% probability of a new volcano emerging. It only took six years to do it.

The third chapter, for its part, comes to specify the possible scenarios of the eruption. Where will the lava flow, in which areas will ash fall and what types of material will the crater expel. All this by analyzing the topography of the place, digital terrain models, the thickness of the magma, future forecasts of gusts of wind, etc.

“Here we go back to study all the previous eruptions. In this case, in La Palma, they have basically been with lava, with a little pyroclast, ash, etc. But we also want to know where the lavas flowed, what type they were, how many kilometers they traveled… All the characteristics. And then we reproduce those lavas with simulation models that we have developed”, explains Martí.

Even so, there is always uncertainty, clarifies the volcanologist: “Volcanoes are not linear systems. They depend on many factors that we do not control. Therefore, when we say ‘volcanoes of the same type’, I think that is a mistake. There are no volcanoes the same. There are those that look alike. And the eruptions are all different.

For each hazard that has been explained, the study draws a risk map. But these have been combined to represent in a single illustration the volcanic hazard map of La Palma with four degrees: very low, low, medium and high hazard. The result is an image of the island that could scare more than one.

“The ideal thing would have been to make these maps before planting the first house. That way you have the map and you can assess which area has more or less danger. By interposing the scientific information that we now have on the reality that we have built, of course it is scary,” acknowledges Martí. The expert considers that this type of information should be in the hands of those who make decisions and thus carry out cost-benefit analysis on each project.

“One would have to ask how we can manage this area in particular so that it is as viable as possible to live here. Because we can’t build anywhere, since the losses are humiliating, but we can’t go thinking that perhaps one day we will have those losses when the frequency of eruptions is so many years. Take stock of all this, in short”, emphasizes the CSIC scientist.

A presumable step to discuss these issues is to approve, not only the island action plans against volcanic risk, but also a Canarian strategy for risk reduction in this context. Nemesio Pérez, from INVOLCAN, is the one who has been making this claim for months. The Government of the Canary Islands, for its part, does not have it on the table and refers to the Special Plan for Civil Protection and Emergency Attention (PEVOLCA) that coordinated the management during the La Palma volcano.

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