In recent years we have heard about tsunamis in the media, especially after what happened in the Indian Ocean in 2004 and more recently in Japan in 2011 with the tsunami that also caused the accident at the Fukushima nuclear power plant. In general, we know that tsunamis are produced by earthquakes in the sea. But how? We also know that they are dangerous only when they approach the coast: why in the open sea are they harmless? And what happens to them near the coast that they become so destructive? In this article we will answer these questions.
This is how tsunamis are formed
A tsunami is a wave (wave) that propagates in the sea and is caused by an underwater earthquake, a landslide, a volcanic eruption or the fall of a meteorite. As the first cause is the most frequent, we are going to focus on it. The vast majority of earthquakes occur on faults. These are fractures in the earth’s crust that accumulate tension, which is released in the earthquake: something like if we stretch an elastic until it escapes us.
What happens is that the two portions of the earth’s crust separated by the fault slide relative to each other. This slip can be completely vertical, which literally means the fall of one of the sides of the fault, totally horizontal or something in between. The key for an underwater earthquake to cause a tsunami is that the movement has a vertical component; if it is completely horizontal, it will not occur. The movement of the fault is so fast that the “step” that occurs on the ocean floor is instantly reflected on the surface of the sea, which deforms exactly like the bottom. We all know that a “step” in the water is not stable, the surface tends to regain horizontality. The highest water descends and vice versa, which gives rise to a series of waves that propagate in all directions from the site of the earthquake: the tsunami.
How does a tsunami spread?
A characteristic of tsunamis is that their wavelength, that is, the distance between two consecutive waves, when they are generated is typically around 10 to 100 km. This supposes a depth much greater than that of the sea, reason why they are considered “long waves”. As the height of the wave is a few meters, the slope of it is negligible – a few meters of unevenness in tens of km-, the tsunami being totally harmless . Of course, a property of long waves is that the propagation speed grows as the square root of the depth, only depending on it. For example, if the depth is 4 km the wave moves at 700 km / h, the same speed as a jet plane. Bottom line: the tsunami is harmless but it moves at high speed, crossing an ocean in a few hours.
When they reach the coast …
When the tsunami approaches the coast, its speed decreases as mentioned above, as the depth decreases. If for example the depth drops to 30 meters, the speed of the wave will be only 60 km / h. By conservation of energy, if the speed of the wave decreases, its height increases . We can also imagine that the water accumulates when braking, increasing the height of the wave. The wavelength also decreases, with which the slope of the wave – higher in less distance – grows threateningly. The wave can break if the slope exceeds a limit. If the coastline is very gently sloping, the tsunami manifests itself as a “fast tide”, in which the sea level rises very quickly: less than 10 minutes. Sometimes, if the slope of the wave is very large, the tsunami behaves like a “wall of water” that advances at an angle of about 45º with the ground. It is the most destructive but less frequent case.
Simulating the tsunamis of the past
Finally, it remains to comment that tsunamis can be simulated on a computer by numerically solving the equations that describe fluid dynamics. In our group we have carried out simulations to study if a tsunami could destroy the city of Tartesos, in the current Guadalquivir marshes, or if another could be responsible for the “opening of the waters” and destruction of the Egyptian army in the exodus episode. . The responses, published in the Journal of Marine Systems and Marine Science and Engineering , were both negative.
Raúl Periañez is a researcher in the Applied Nuclear Physics group at the University of Seville. Article written in collaboration with the UCC + i of the University of Seville.
