To the high ground!
Tsunamis can be deadly, but they don’t have to be. Early warning and early action are effective tools to protect people, save lives and prevent the threat from becoming a disaster. To be effective, tsunami early warning systems must cover all people at risk, they must be multi-hazard, and communities must be prepared to be able to act quickly.

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In December 2015, the UN General Assembly established 5 November as World Tsunami Awareness Day.

The creation of World Tsunami Awareness Day is an original idea from Japan, which, unfortunately, has been exposed to these disasters repeatedly over the years. Japan has extensive experience in areas such as tsunami early warning, public action and post-disaster reconstruction to reduce future impacts. The UN Office for Disaster Risk Reduction is responsible for promoting the observance of the Day, in collaboration with relevant organisations of the UN system.

What is a tsunami?
The word “tsunami” (or tsunami) is formed from the Japanese words “tsu” (port) and “nami” (wave). A tsunami is a series of giant waves caused by an underwater disturbance, usually associated with earthquakes occurring at or near the bottom of the ocean.

Volcanic eruptions, underwater landslides and coastal rock falls can also generate a tsunami, as can the impact of a large meteorite in the ocean. Tsunamis originate from a vertical movement of the seabed, with the resulting displacement of the water mass.

Tsunami waves often look like walls of water and can reach the coast and be dangerous for hours. The time between waves can range from five minutes to an hour. Usually, the first wave is not the largest; often it is the second, third or even fourth wave. After a wave floods, or floods inland, the sea recedes, exposing large areas of the seabed. A new wave then rushes ashore in a matter of minutes, carrying with it the many pieces and damage caused by the previous flows.

What are the effects of tsunamis?
Earthquakes
An earthquake is a shaking and trembling of the earth. Its origin is mainly due to the collision of tectonic plates. Most strong earthquakes occur in subduction zones, where an oceanic plate dives under a continental or younger oceanic plate.

Not all earthquakes cause tsunamis. There are four conditions necessary for an earthquake to cause a tsunami:

The epicentre of the earthquake is located under the ocean or near the coast.
It has a large magnitude, at least a magnitude of 6.5 on the Richter scale.
There is a breaking of the Earth’s surface and it occurs at shallow depths, less than 70 km below the Earth’s surface.
The earthquake causes vertical movement of the seabed, which can reach up to several metres in height.

Landslides
A landslide occurring close to the coast can generate waves that can reach a significant amplitude due to the volume of cubic metres of land sunk. It can cause the movement of large quantities of water in the sea, cause water disturbance and generate a tsunami. Submarine landslides can also result in tsunamis when the detached material moves violently, pushing the water. which can reach a significant amplitude due to the volume of cubic metres of land submerged.

Volcanic eruption
Although relatively rare, volcanic eruptions can also cause tsunamis. They can result in the displacement of a large volume of water and the generation of extremely destructive tsunami waves.

One of the largest and most destructive tsunamis ever recorded occurred on 26 August 1883, following the eruption of the Krakatoa (Krakatau) volcano in Indonesia. This explosion generated waves of up to 41 metres and destroyed coastal towns and villages along the Sound Strait on the islands of Java and Sumatra, killing 36,417 people.

Extraterrestrial collision
Tsunamis caused by extraterrestrial collisions (i.e. asteroids, meteorites) are extremely rare. Although no tsunamis caused by meteors or asteroids have been recorded in recent history, if these celestial bodies were to fall into the ocean, a large volume of water would undoubtedly be displaced to cause a tsunami.