One of the most destructive natural disasters in human history, a tsunami (from the Japanese tsu: port and nami: wave) also called “harbor wave” by the Japanese, is a series of giant and powerful waves that propagate across the ocean. They originate from a large quantity of water caused by a rapid and large-scale disturbance that originates in the ocean or large lakes generated following ground movements mainly due to earthquakes, an underwater volcanic eruption, a landslide or the impact of a meteor in the ocean.

Tsunami waves move very quickly and can reach several meters in height. One of the visible signs of a tsunami is a drop in water level and a retreat of the sea, followed by a rise in water levels of one to several meters. It is important to remember that a tsunami is a very violent phenomenon that destroys everything in its path. The largest tsunamis known in human history reached an altitude of over 30 meters and are extremely destructive, causing numerous casualties.

Tsunamis, origin and characteristics

A tsunami occurs when the seismic source is located at sea and has given rise to an earthquake of magnitude greater than 6.3 with a focus located less than 50 km deep. Its waves are generated by ground movements due mainly to underwater earthquakes. 85% of tsunamis recorded worldwide are due to earthquakes. A tsunami consists of several waves that will flood the coastline generally every 10 to 30 minutes, sometimes for several hours. The first wave is often not the largest. A rapid drop in sea level or the rapid retreat of the sea are natural warning signs of the imminent arrival of tsunami waves.

Powerful waves can also occur following a landslide near the sea or a lake. A lava flow from a volcanic eruption or, more rarely, meteorite falls or other impacts on the sea surface can also cause a tsunami.

Risk predictability, monitoring

The instrument used for tsunami forecasting and warning is called a tide gauge. It measures the change in sea level relative to a reference level. A tsunami warning can be given either by an earthquake, by the sudden rise or retreat of water levels, or by local authorities. At this point, the population has only a few minutes to move away from the shore to higher ground. In the event of a tsunami, people should avoid driving to avoid creating traffic jams, then stay away from the coast and move to higher ground.

Some supporting links :

• https://www.youtube.com/watch?v=QOfS9kvsrjo
• https://www.youtube.com/watch?v=9wcJTliGvqE
• https://www.1jour1actu.com/science-et-environnement/cest-quoi-un-tsunami