KEZAKO: Comment se forme un tsunami?


How is a tsunami formed ? The surface of the Earth is a huge jigsaw, made of only 7 great pieces and 7 smaller, plus a few little pieces here and there. Those pieces are the tectonic plates. They can be more than 10 000 km wide, but sometimes only 5 km thick. The jigsaw pieces accurately fit, but they rest on an unstable base, the Earth’s mantle. The Earth’s mantle is unstable because of the huge differences of temperature (several thousands of degrees) there are between the top and the bottom of the mantle. Because of this difference of temperature, convection movements are created, exactly as a room heated by a radiator: the hot air goes up at the contact of the radiator, spread on the ceiling as it is cooling down and then goes back to the ground and comes back towards the radiator. Within the Earth’s mantle, we have the same phenomenon: the hot rocks rises to the surface of the mantle, just under the tectonic plates, spread under the plates while cooling down, and end up by going back down into the depths. The tectonic plates are carried away by the movement of the mantle, but in such a disordered way that they do not all move in the same direction or at the same speed. Some move away from each other, while others come closer to each other. But as they well fit together, there is no room for these movements. Let’s take the example of two plates that comes closer. At the point of collision (this point is called the fault), they will withstand the pressure for a moment, as it is the case for instance when one tries to crush a soda can: first, nothing happens and, suddenly, the can is crushed in one go. In the case of two tectonic plates, it is the same: the pressure might increase for years and then, suddenly, something yields, the plates move along a few metres in a go: it is the earthquake. In most cases, this brutal movement does not happen only horizontally. Often, one of the plates slides under the other and, consequently, the extremity of the upper plate rises, whereas the lower plate subsides. If the collision between both plates happens deep down the ocean, the rising or the settling of the plates carries away water that is above the fault, therefore small waves are formed on the sea surface: it is the tsunami. These waves are really different from those formed by the wind. Wind only sets the ocean surface in motion, the deep underwater is not moving. On the contrary, tsunami waves are related to the whole height of water, from the deep bottom of the ocean, where the rock has moved around, up to the surface of the ocean. So there are huge quantities of water that have moved around. Besides, the distance between two waves is much more important, sometimes several hundreds kilometres. And waves spread at a gigantic speed, up to more than 800 km/h, which means the speed of an airliner. But if you are on a boat, on the surface of the ocean, you won’t notice anything: a series of a few metres high-waves but of several km long will lift the boat, and it typically last for one hour per wave. So, you will always feel like the sea is flat and that it slowly raises up of a few metres. But as the wave spread on the whole height of the ocean, the way it moves along is related to this height and, therefore, to the ocean depth. In particular, its propagation speed decreases as the depth decreases. This means that when the bottom of the ocean goes back up, the back of the wave is going faster than the front of the wave. Consequently, the crest of the wave will be closer and the heigth will increase. On the shore, when there is almost no more depth, the wave height – that can reach several dozens of metres – becomes bigger than the depth of the sea. Thus, the wave is transformed into an horizontal current seeping through the lands with a several dozens km/h speed, and causing the destructive effects we know. Production: Unisciel/ University of Lille 1 (SEMM) Conception/Production: Maxime Beaugeois, Damien Deltombe and Daniel Hennequin Editing/Special effects: Benoît Leleu Music: Sébastien Ride, « Thunder Chacha » (SR Music) Presentation: Maxime and Nina Beaugeois Graphic design/Credits animation: Michaël Mensier.

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