About Tsunami


Japanese (日本語版)


What is a Tsunami?

It is a large wave that moves the entire seawater from the ocean floor to the sea surface and spreads out around it, with wavelengths ranging from several to several hundred miles. Tsunamis are mainly caused by “non-weather phenomena” and can occur anywhere where large amounts of water accumulate.
The difference between a tsunami and an ordinary wave is whether a weather phenomenon is involved or not. In other words, if it is a change in the sea or lake surface that does not involve a meteorological phenomenon, it is called a Tsunami.

However, as explained above, Tsunami can occur even without an earthquake, because they are caused by changes in sea and lake levels due to phenomena other than weather. 
Tsunamis can be caused by.

  • A major earthquake lifts the entire seawater
  • Sea level changes due to air shaking, etc. caused by large volcanic eruptions
  • Large waves caused by undersea or lake bottom landslides
  • Large waves occur when large amounts of sediment generated by mountain collapses flow into oceans, lakes, etc.

This shows that tsunami are never solely caused by earthquakes.


In addition, the JMA will issue tsunami information in the following cases when The details of tsunamis are also available on the JMA website.

*Created based on JMA test telegrams, not an actual event.

Source: Created based on the JMA website


Tsunami Hazard

Tsunami are different from ordinary waves in that they come at you as a mass of water, and if you are caught in them, you are almost certainly in danger of losing your life. If an ordinary person is caught in a tsunami, even by a few dozen centimeters, the tremendous weight of the water can knock him or her off his or her feet, and he or she may be washed away.Also, you can never swim because tsunamis can engulf many things, including sediment and debris. Among them, research has shown that the “black tsunami” generated by “The 2011 off the Pacific coast of Tohoku Earthquake” was mixed with sludge that had settled on the seafloor, and that it was stronger than ordinary tsunami.

When a large-scale crustal movement occurs on the seafloor, such as the occurrence of a huge earthquake, there are cases in which it intrudes more than several miles inland, and even points that were some distance from the coast may not be taken off guard. The most dangerous example of a tsunami is a river tsunami, in which a tsunami travels up a river and strikes at a maximum distance of approximately about 31 miles after “The 2011 off the Pacific coast of Tohoku Earthquake”. It is clear that the earthquake caused a tsunami that traveled up to 35 km up the Arakawa River in the Kanto region. In other words, if a very large crustal movement occurs on the seafloor, even at a point several tens of miles from the coast, the proximity to rivers increases the risk.

Tsunami generated by huge earthquakes that occur overseas can also hit Japan. The most famous example is the 1960 Chilean earthquake and tsunami, which is said to be the largest earthquake in the world’s recorded history, with a M9.5, exceeding that of “The 2011 off the Pacific coast of Tohoku Earthquake”. This earthquake triggered a tsunami that struck even in faraway Japan, killing many people in the Tohoku region.

Tsunami caused by geological phenomena that occur overseas are particularly troublesome in Japan when they occur on the Pacific Ocean side of a foreign country, because there is little or no terrain separating the tsunami and they also have the characteristic of lasting a long time.

This is why it is important to evacuate when a “major tsunami warning, tsunami warning, tsunami advisory” is issued by the JMA even if the area is not shaking.


Evacuation Points

The most important thing to remember when evacuating from a tsunami is not only to stay away from the shore and the mouth of the river, but also to take shelter in the most sturdy and elevated place possible. In addition, evacuation by car is strictly prohibited in principle, and those who can escape by their own feet should try to do so by means that do not rely on vehicles as much as possible.The best recommended high place to evacuate is a tall structure that is as sturdy and open as possible. Typical examples include self-propelled multi-story parking garages, which are highly advantageous locations for disaster prevention.
In addition, as we will discuss in a later article, research has shown that when humans are confronted with rare and massive disasters, they tend to have a stronger psychological tendency to think, “I will be fine,” or “I have never been hit by a tsunami so far, so I will be fine here,” etc. This psychological tendency is called “normality bias” in psychology. If normality bias is mishandled during a disaster such as a tsunami, the possibility of loss of life will surely increase, so the existence of such psychology and how to handle bias should be well understood.