WHEN THE EARTH TOUCHED THE SKY
Although on a very large scale, the first modeling of the impact, carried out by Brandon C. Johnson, lecturer from Purdue University, Indiana, gives a rather striking idea of the consequences of the impact of the asteroid. From the first seconds, the ocean and the earth’s crust rise well beyond the scale of the graph, more than 40 kilometers away, almost higher than the stratosphere.
“We can see that ejecta are projected at very high altitude. Below, the surface deforms and materials move, creating the wave. Other debris also falls, and forms projection waves,” explains Arbic. “Ten minutes after the impact, we get this wave of waves that propagates. 80% of the energy of the tsunami is due to this wave. The remaining 20% is due to the water returning to the crater formed. »
The wave resulting from the impact then propagated across the ocean, as shown in the second model. Unlike a classic wave, which is caused in the open sea by the displacement of the upper layers of the ocean, a tsunami affects the entire water column.
Initially made by plotting the impact model data around a single axis, the first propagation model (not shown in the article) allowed the researchers to get a better idea of the displacement of the wave wave in high sea.
“We managed to bring together many co-authors. For this article, we wanted to make sure that we would be able to model and verify everything,” says Molly Range, a student of Ted Moore, co-author of the study and responsible for carrying out the first ocean modelling. “We did not expect such a success. [Réaliser cette étude] was fun not only for us, but also for other people. This is a fascinating subject for many people. »
The second model was then made from the first thanks to the Vasily Titovoceanographer and expert modeler of the tsunamis of the Pacific Marine Environmental Laboratory (PMEL), but also from the Method of Splitting Tsunami (MOST) model, designed for tsunamis and regularly used for modern disasters.
Using this model, the researchers were also able to estimate the size and amplitude of the waves produced as well as their impact on the coasts of the continents as they were at the time. Several elements could then be deduced, such as the size of the waves offshore, before they hit the coast.
“What may seem odd is that the Titov model shows waves 5 meters above ocean level at their strongest, which doesn’t look that big,” explains Arbic. “However, during the 2004 tsunami in Indonesia, which was particularly destructive, the waves were only 60 centimeters in the open sea, right next to the epicentre. They were almost undetectable. »
The amplitude of the waves generated by the impact, or the distance between the two crests, is another impressive criterion. Like the waves generated when a stone is thrown into the water, several waves are indeed formed, both in the case of an earthquake than in that of the crash of the asteroid. The giant waves of Nazaré, in Portugal, can reach several tens of meters in height, and have an amplitude of about thirty meters. In the case of the waves from the Chicxulub asteroid, on the other hand, the distance extended over several hundred kilometers.
“Because of this huge amplitude, there was a huge amount of energy,” adds Brian Arbic. “Tsunamis always increase in size as they approach the coast. When they reach the earth, they compress and rise in huge waves. »
More than a wave of 5 meters, it was therefore blocks several meters high and several tens of kilometers long, followed by troughs of similar dimensions. Once they arrived on the coast, they would then have formed waves 1.5 kilometers high.
According to the study, when the Cretaceous tsunami reached the coast of New Zealand, several hundred kilometers from the impact, it was still ten times more powerful than that of 2004 a few seconds after the quake.