Tsunamis, like those who started hitting the coasts of the Pacific of Russia and Japan on Wednesday by triggering alerts in dozens of countries, are caused by underwater earthquakes.
• Read also: Tsunami alert in the Pacific after an 8.7 magnitude earthquake near the Russian coast
The earthquake here responsible, which occurred in the night off the Russian Kamchatka peninsula around 11:25 p.m. on Tuesday, was assessed at an 8.8 magnitude by the American Institute of Geophysics (USGS), based in Hawaii.
He has already led to a tsunami of around 30 centimeters in northern Japan, the evacuation of employees of the Fukushima nuclear power plant – rugged by a devastating tsunami in 2011 – and the flood of a port city in the Kouriles.
The authorities warned against possible heights of one to three meters in several countries, notably in Japan and the United States (Hawaii, Alaska).
The tsunami wave, born from the seismic shock, wins in energy each time it strikes the underwater floor.
At its starting point, a tsunami only generates small, very spaced small waves, because the huge water bodies moved by the jerk deep down along the deformations of the sea soil, unlike ordinary waves which only affect the surface of the water.
Toshiyuki Kon / The Yomiuri Shimbun via Reuters Connect
But as these waves progress towards the coasts, at a speed of around 800 km/h, the bottom of the ocean goes up, concentrating the energy conveyed by the tsunami. The waves slow down, get closer and their height increases sharply, up to more than 20 meters.
As during its propagation at sea, a wave loses very little from its energy, it can move over considerable distances to hit the ribs located thousands of kilometers from its origin.
In 1960, in 1960, an earthquake of 9.5 magnitude in Chile sparked a devastating tsunami which reached the coasts of Japan.
The main countries residents of the Pacific coordinate their observations to prevent the dangers of these ocean waves.
If the largest number of tsunamis occur after an earthquake, there are other possible origins: underwater avalanches, sometimes triggered by earthquakes like in Papua New Guinea in 1998 (more than 2,000 dead), the explosion of a volcano like in Krakatoa, a small island between Java and Sumatra (36,400 dead in August 1883) asteroid in water.
Small tidal waves can also be caused by meteorological phenomena, including violent thermal exchanges which cause depressions behind violent winds.
On December 26, 2004, the coasts of a dozen countries in Southeast Asia had been ravaged by a tsunami which had killed 220,000. The power of the earthquake at its origin was equivalent to around 23,000 atomic bombs like that of Hiroshima, according to the USGS.
In March 2011, Japan was the victim of a 9.0 magnitude earthquake followed by a giant tsunami on the northeast coast of the country, a disaster that left some 20,000 dead and disappeared.
However, the tidal waves are not limited to the Pacific. The Atlantic or the Mediterranean have also been affected in the past, as evidenced by the Roman historian Ammianus Marcellus, who attended that of Alexandria (Egypt) in the year 365.
