The tsunami, a tidal wave of seismic origin

Tsunamis, like those that began hitting the Pacific coasts of Russia and Japan on Wednesday, triggering alerts in dozens of countries, are caused by underwater earthquakes.

The earthquake responsible for this, which occurred overnight off the Russian peninsula of Kamchatka at around 23:25 GMT on Tuesday, was assessed at a magnitude of 8.8 by the Hawaii-based USGS. It has already triggered a tsunami of about 30 centimeters in northern Japan, the evacuation of employees from the Fukushima nuclear power plant - damaged by a devastating tsunami in 2011 - and the flooding of a port city in the Kuril Islands.

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Authorities have warned of possible heights of one to three meters in several countries, including Japan and the United States (Hawaii, Alaska).

The tsunami wave, created by the seismic shock, gains energy each time it strikes the seafloor. At its origin, a tsunami generates only small, widely spaced waves because the enormous masses of water displaced by the shock cascade down along the deformations of the seafloor, unlike ordinary waves, which only affect the surface of the water.

But as these waves move toward the coast, at a speed of about 800 km/h, the ocean floor rises, concentrating the energy carried by the tsunami. The waves slow down, come closer together, and their height increases sharply, reaching over 20 meters.

Because waves lose very little energy during their propagation at sea, they can travel considerable distances to strike coasts thousands of kilometers from their origin. For example, in 1960, a magnitude 9.5 earthquake in Chile triggered a devastating tsunami that reached the coast of Japan. The major Pacific Rim countries coordinate their observations to warn of the dangers of these ocean waves.

Although most tsunamis occur after an earthquake, there are other possible causes: underwater avalanches, sometimes triggered by earthquakes, such as in Papua New Guinea in 1998 (more than 2,000 deaths), the explosion of a volcano, such as in Krakatoa, a small island between Java and Sumatra (36,400 deaths in August 1883), and the fall of an asteroid into the water.

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Small tidal waves can also be caused by meteorological phenomena, in particular violent thermal exchanges which lead to depressions which cause violent winds.

On December 26, 2004, the coasts of a dozen Southeast Asian countries were devastated by a tsunami that killed 220,000 people. The force of the earthquake at its origin was equivalent to approximately 23,000 atomic bombs like the one dropped on Hiroshima, according to the USGS.

In March 2011, Japan was hit by a magnitude 9.0 earthquake followed by a giant tsunami on the northeast coast of the country, a disaster that left some 20,000 people dead or missing.

Tidal waves, however, are not limited to the Pacific. The Atlantic and the Mediterranean have also been affected in the past, as evidenced by the Roman historian Ammianus Marcellus, who witnessed the one in Alexandria, Egypt, in 365 AD.