A tsunami is a geological phenomenon mainly generated by seismic ruptures or underwater landslides, linked to plate tectonics.
Tsunamis are among the most studied natural phenomena to understand how the Earth releases its energy. Beneath our feet, the ground is constantly moving even if we don’t feel it. Sometimes it starts under the sea. The word “tsunami” comes from the Japanese “tsu” which means “port” and “nami” which means “wave”. It thus defines a “port wave”. The term was coined by Japanese fishermen. When they returned from fishing, they observed significant damage in the port while the sea was calm offshore. They named these destructive waves after the place where their effects were most visible: the port. The term “tsunami” established itself in scientific vocabulary in the 1960s and 1970s. It has long been confused with that of “tidal wave” but it has no link with tides and weather conditions. But with what then?
A tsunami is the result of a sudden geological event that disturbs the ocean at rest. Most often, it is an underwater earthquake, sometimes an underwater landslide, more rarely a volcanic eruption or even a meteorite. Let’s take the most common case of an earthquake. The terrestrial crust which envelops the Earth is made up of tectonic plates, fitted one inside the other, which move. Some slide against each other; others deviate or overlap. In so-called “subduction” zones, it can happen that one plate slides under another. The crust heats up, deforms, breaks. The energy stored by the plates is suddenly released, causing the shaking characteristic of an earthquake. The water begins to move: waves first, then waves.
Initially, the waves are only a few centimeters but the closer they get to the coast (the speed of the waves can reach nearly 800 km/h), the less depth there is and the more the wave rises and gains height. A wave less than a meter high in the ocean can exceed several tens of meters at the shore. This is where the tsunami causes the sea to overflow and sometimes causes flooding far inland.
According to the Atomic Energy and Alternative Energies Commission (CEA), for an underwater earthquake to cause a tsunami, it must occur at a shallow depth (less than 50 or 100 km) and have a force or “magnitude” of at least 6.5. From magnitude 8, an earthquake can generate a potentially devastating tsunami at ocean level. On a daily basis, scientists observe land movements to spot tsunamis and warn countries in danger. Stations measure sea level in real time (“tide gauges” along the coast, “tsunamimeters” offshore) and can quickly identify a tsunami. On the ground, seismometers are used to measure vibrations and calculate the magnitude of earthquakes.
The Pacific area is the most exposed to tsunamis because it has many subduction zones, but there can also be some in the Mediterranean. In France, the risk exists, especially in the South. If this happens, experts recommend moving away from the sea immediately. A tsunami is not always visible before impact: ground shaking and/or a sudden retreat of the sea are warning signs. In this case, you have to flee without returning to look for anything, because the first wave is not always the strongest. You must then take refuge on the heights of the city, at least 10 meters above sea level.
