The Cumbre Vieja volcano in La Palma has just erupted. If many images show the lava fountains and flows emitted by the stratovolcano of this island, part of the Canary Islands archipelago in the Atlantic, some fear the occurrence of a devastating megatsunami evoked for 20 years in the event of destabilization from the side of this volcano. Is this fear founded?
20 years agoand geophysicist Simon Day and Steven N. Ward published in an article – which was to generate much ink – in which the two men claimed that the western flank of the Cumbre Vieja volcano in La Palma is unstable, and that a landslide could occur. As a result, a capable of causing very significant damage on the other side of the Atlantic in particular was to be feared.
Recall that the island of La Palma is part of the Canary Islands archipelago which is a series of islands approximately aligned along a west-east axis, and which extends north-east by a series of mountains.. Off the coast of Morocco, these islands are the product of what geologists call a .
La Palma is subject to substantial seismic and volcanological monitoring because there is aon which there is an active volcanic fissure at the level of the Old Summit, Spanish toponym which literally means in French “Old summit”. The eruptions in La Palma, effusive, have been known since the Spanish colonization of the island around 1480, an eruption which had been followed so far by six other eruptions, the last of which had occurred from October 26 to November 18, 1971.
Cumbre Vieja, a volcano located in the Canaries, has erupted. It had been dormant since 1971. © euronews (in French)
However, the seismic activity since September at the level of the Cumbre Vieja volcano worried. As the island of La Palma had not experienced such a phenomenon since October 1971, one could fear a . Tiltmeter measurements used to measure minute changes in the horizontality of a level also indicated that was under because the island had been bulged six centimeters in the seismically active zone.
The researchers’ concerns proved to be justified because on Sunday, September 19, an eruption did indeed start in Cumbre Vieja at 3:15 p.m. local time (2:15 p.m.). Television footage, especially during the night, showed several impressive lava fountains as well as several flows which necessitated the evacuation of several villages and reached a road below. Thousands of people have been evacuated at this stage but these eruptions being effusive, without and without powerful explosions, one should not fear casualties.
To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. Subtitles in Spanish should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © GranCanariaTv.com
A megatsunami crossing the Atlantic?
There remains of course the question of the occurrence of a megatsunami. In fact, as Futura already explained in 2005 in the previous article below, it is highly unlikely. This fear is a sea serpent that has regularly surfaced in the media for years and is greatly exaggerated so that it has beento .
The fact remains that Luis González de Vallejo, director of theCanary Islands Volcanological Institute (INVOLVCAN), felt compelled to. Here are some excerpts from a translation of his update:
« The recentvolcanic activity in La Palma has again sparked debate over the stability of the island’s western flank … a recurring issue for decades, causing concern in society.
For 20 years … the possibility of a greatfrom the western flank of the old summit and as a result, the generation of a megatsunami was broadcast … Although this hypothesis has been disputed and dismissed by many researchers afterwards, surprisingly some media continue to speak of this catastrophic event.
For the flank of Cumbre Vieja to meet conditions close to instability, it would simultaneously be necessary tofrom unusually high and a large-scale explosive volcanic eruption … the probability of an eruption with a high explosive rate and both of a large is extremely low, according to the geological record of this type of events on the island. Cumbre Vieja is therefore stable, even under the effects of eruptions similar to those which have occurred in the last tens of thousands of years. »
The eruption of the Cumbre Vieja volcano on the island of La Palma in the Canaries threatens the tourist coastal areas that have been evacuated. Lava flows could cause fires. © euronews (in French)
Several direct available onshow footage of the eruption for several hours and through the night of September 19-20.
Tsunami in the Atlantic? About the La Palma volcano in the Canaries
Article by Michel Olagnon, Ifremer, published on 01/18/2005
Following the tidal wavesdevastating events of December 26, 2004, the question of the danger of encountering in the Atlantic has resurfaced. In particular, questions are raised about the possible collapse of the Cumbre Vieja volcano in La Palma in the Canaries. The next few lines present my personal analysis of this case.
In September 2000, researchers at University College London contacted the British Minister of Science Lord Sainsbury to warn him that a collapse of a volcano in the Canaries could send aof water several hundred meters high sweep the Atlantic Ocean. The Caribbean and the US east coast would suffer most of the damage, but large parts of the west coast of Great Britain would be affected. This warning came on the occasion of Lord Sainsbury’s receipt of a report on the threat of NEOs (Near Earth Objects), prompting the United Kingdom to take the leadership of a warning system on planetary risks.
Dr. Simon Day, of UCL’s Benfield Greig Hazard Research Center, said the western flank of the Cumbre Vieja volcano in La Palma is unstable, and a landslide could occur. A Swiss model would have indicated that the landslide would create a mega-tsunami with an initial amplitude of 650 meters, whosewould be 720 km / h, and which could, despite mitigation, create damage up to 20 km inland in the US.
Let’s take these points in detail:
An unstable flank
The western flank of the Cumbre Vieja volcano in La Palma is undoubtedly unstable, but half a dozen top eruptions can still occur before it collapses (Prof. Bill McGuire). Moreover, it is extremely unlikely that then everything will collapse at once as the researchers postulated.
A Swiss model would have indicated that the landslide would create a mega-tsunami. There are limits to what we know how to model correctly. Likewise, when performing basin tests on a reduced model, the rules of similarity must be observed. Unfortunately, the similarity can only be respected for some of the phenomena involved, assuming that the effects of the others remain negligible. Beyond scale factors of 50, this is rarely the case.
In addition, I by chance assisted someone on a code for calculating the effect of submarine landslides, this one or his. This is the kind of program that no longer works as soon as you change the version of the compiler, , or user. A computer scientist must know how much credit can be given to his results.
650 meters of initial amplitude
When you know that the highest wave ever reached is 524 meters, you can believe it. But not if we know the conditions. The 524 meters of Lituya Bay in Alaska occurred in a nearly closed basin, where the collapse on one side produced a run-up on the other. In the open sea, you cannot physically create the necessary vertical acceleration. Moreover, tsunamis are practically undetectable offshore by their height alone (one or two decimetres max), the crew of a ship feels it like thean underwater explosion or a grounding.
A displacement at 720 km / h
A tsunami spreads like a lonely wave ofinfinite, i.e. the wave seems to move in one block, and at a speed given by the root formula (gd) as a function of the and depth, which gives 200 m / s, or 720 km / h for a of 4000 m. Although it is called the solitary wave, the phenomenon is generally made up of a succession of waves. In Hilo, on the Big Island of Hawaii, more than half of the victims of the April 1, 1946 tsunami were people who rushed to help after the first wave, and were drowned by the following ones.
What risk in the Atlantic?
Note that of the 157 tsunamis of the last 20 years of the twentieth century, 138 occurred in the Pacific, 2 in the Indian Ocean, 9 in the Mediterranean, 5 in the Caribbean, 1 in the Gulf of Aqaba, 1 in the sea. from South China, 1 in the Sea of Marmara. The Atlantic is not the hardest hit.
Everyone knows that there is no such thing as zero risk. An honest scientist cannot therefore rule out the risk of a tsunami, or even a mega-tsunami in the Atlantic. On the other hand, the same honesty should lead him to prioritize funding for the most probable risks and for populations in immediate need because they have been hit by a disaster, to the detriment of uncertain contingencies that may occur in the future. worse once in the next few tens of millennia.