Addressing La Palma megatsunami fears

- Janine Krippner

Originally published on 20/9/2021 here: http://janinekrippner.weebly.com/la-palma when we did not have access to our blog.

I am honestly angry about how misinformation about a small eruption has put so many people in a state of pure fear across multiple countries. There is enough stress and fear in the world and I truly feel for everyone being impacted by this. It is incredibly sad to see people losing their homes and livelihoods to this eruption, and on top of that is a tsunami of utter lies. We all need more compassion. We all need to be careful of what we share. We all need to be kinder. We all need to think more critically when we are afraid (this is a tough one). This is a bad case of the implications of people acting without these traits.

People want us, the experts, to say that this scenario is impossible to ease fears, but that is something that is extremely difficult for a scientist to say. There are many scary things in this world that will not happen on the scale of humanity, like mass extinction-causing impacts, but we can't say these things are impossible. Unfortunately, that leads people to dismiss everything else we say. Once people are worked up about an idea, it is very hard to slow that momentum.

I hadn't met Dr Alexandre Paris until he reached out to me on Twitter with his research on tsunamis. I suggested that he write a plain-language summary to help people understand what the research says and he happily jumped at the chance to assist. I am not a tsunami expert, so here are the words of someone who is.

Based in France, his research compared two types of computer (numerical) models for the simulation of landslide-triggered tsunamis. He numerically reproduced two recent events: the Karrat Fjord landslide tsunami on June 17, 2017 in Greenland and the collapse of the Anak Krakatau volcano on December 22, 2018 in Indonesia. Importantly for this event, he also studied a potential collapse of La Palma and the tsunami that could be generated. It is clear that he is truly fascinated about landslide tsunamis around volcanic islands. This is a very multidisciplinary field and it brings together people from different background who usually work separately, as these large topics often require.

These are his words and I want to thank him for speaking during an event that has people attacking the truth. I helped with editing, but this is his science and his expertise.

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Recently an eruption began at La Palma volcano in the Canary Islands. While this has so far been a small eruption typical of this volcano, it has hurt local communities and has hit global headlines. During these events rumors exploded about this eruption triggering a megatsunami with a large part of the island collapsing into the sea, destroying parts of the USA and Europe. This idea comes from a BBC show in which scientists presented their research about a potential La Palma collapse of about 500 km3 and the tsunami that would be generated. Since then, this research has been debated and strongly dismissed by multiple studies, but the damage has been done.
 
Before talking about the idea of a megatsunami hitting the western coast of the USA, it is important to keep in mind that this kind of event, with such a huge volume, is extremely rare. The return period is around 10,000 years, which means that every year, the probability of an event is around 0.0001%. Moreover, a landslide volume of 500 km3 would signify a collapse en masse, all at once, which is highly unlikely. Such a volume would probably collapse gradually, in succeeding smaller landslides and therefore not generating a megatsunami. Basically - it is not easy to get that much land to move at once.
 
In a paper published in 2020, Abadie et al. reassessed the impact of a collapse of La Palma on the US and European coasts. The initial volume around 500 km3 was ignored and only smaller,  but still significant volumes (from 20 to 80 km3) were considered. The objective of the paper was not to produce an accurate simulation of a realistic landslide on La Palma, but the comparison between different computer models that scientists use to study tsunamis, and to assess the impact of a potential, although unlikely, landslide. This helps to understand the wave behavior or to identify the areas which would require attention in the case of such an event, but it does not mean that it will ever happen. As scientists it is our duty to study all phenomena from every day scenarios to scenarios that are extremely unlikely, and those that may never happen. This is a case of the latter.
 
Whatever the volume envisaged, there are many uncertainties regarding the landslide characteristics, such as the collapse behavior as mentioned earlier. The main uncertainty is about the landslide rheology, i.e. how the material moves. For example, in Abadie et al. (2020), the landslide is modeled as a Newtonian fluid, which means that its behavior depends on a unique parameter, the viscosity (or how runny it is). To simplify, the lower the viscosity, the more easily the mass will slide, thus the material will move faster and will have a greater impact on the water surface, generating higher waves. Considering a volume of 80 km3, which is still unlikely, waves around 1 or 2 m could be expected. This is far from the 50 m wave announced by the BBC show for Florida, New York or Boston. As is usually the case, this sort of exaggeration is repeated until it is stated as fact.

In any case there would be hours of warning for an ocean-crossing tsunami. This volcano is being closely monitored and is even being viewed by many around the world through livestream videos. We are all watching this eruption in real-time. Yes, a La Palma landslide of some kind is possible, but highly unlikely and we can’t tell neither the volume or the collapse behavior or when it will happen. We do know that these large events are very rare. If a collapse was about to occur, there would be precursory signs that would be captured by the networks of instruments and satellites monitoring the eruption. In the case of a tsunami, the different tsunami warning centers would issue an alert. How do we know this? Because of the hoards of research done on this topic looking at likely to unlikely events.
 
Étienne Klein, a French philosopher of sciences, said "People who speak without nuance give the impression that they are right.". That’s the problem when scientists tell beautiful stories to the media without nuance, or the nuance is removed by the media. Science is about nuance.

 
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Alexandre on Twitter: https://twitter.com/Rollalex

Janine on Twitter: https://twitter.com/janinekrippner

Link to the paper discussed: https://nhess.copernicus.org/articles/20/3019/2020/

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