Seismic waves (tsunamis), such as those affecting the northeastern coast of Japan off the coast of Sendai, and threatening numerous coastal areas in the Pacific where an alert is issued, are caused by submarine earthquakes similar to Japan.
The earthquake in Japan has all the conditions for the creation of such gigantic waves: a very large magnitude, estimated at 8.9, according to the US Geoscience Institute (USGS), and a submarine origin at 24.4 kilometers deep.
Wave wave, generated by seismic shock, increases in energy each time it hit the submarine plateau.
At its training point, a tsunami generates only small waves spaced a great deal apart. But as these waves head off to the shore at a speed of approximately 800 kilometers, the ocean floor rises, concentrating the tsunami-driven energy. The waves slow down their speed, they are approaching and their height increases very much, reaching over 20 meters.
Because when it propagates in the water, a wave loses very little energy, it can travel over considerable distances and can hit shores located thousands of kilometers away.
Thus, in 1960, an earthquake measuring 9.5 magnitude in Chile triggered a devastating tsunami that hit Japan's coasts.
The main Pacific Ocean countries coordinate their observations to prevent the dangers these ocean waves pose. A tsunami alert center centralizes information to Hawaii in the United States.
Most seismic waves occur after an earthquake, but there are other possible origins: submarine avalanches, sometimes triggered by earthquakes such as Papua New Guinea in 1998 (2,000 dead), the explosion of a volcano like that of Krakatoa - the island between Java and Sumatra (36,400 deaths in August 1883), and the fall of an asteroid into the water.
Smaller waves can also be caused by weather phenomena, especially violent temperature changes.
On December 26, 2004, the shores of about ten countries in Southeast Asia were devastated by a seismic wave that killed 220,000 people. Its power was the equivalent of about 23,000 atomic bombs like Hiroshima, according to the USGS.
The huge waves are not limited to the Pacific. The Atlantic Ocean or the Mediterranean Sea have been affected by such waves in the past, as the Roman historian Ammianus Marcellus, who witnessed at Alexandria in Egypt,