Tsunamis (translated from Japanese for “a big wave in a harbor”) are a series of waves caused by violent wave movements on or near the sea floor. The generated waves have extremely long wave length and wave period.They can reach speeds of up to 800 km/h, grow to over 30 m in height and travel right across the Pacific.
The wave velocity is decreased near the coastline because of shallower water and the slowing of the wave by the roughness of the bottom
- Ordinary wind waves affect only the surface of the ocean; the water movement rarely exceeds 150 m below the surface. They are caused when wind blows over the surface of the water and causes the water particles to move. Tsunamis, because of their long wave length, involve the movement of water all the way to the ocean floor and, as a result, their speed is controlled by the depth of the sea.
- A Tsunami event may be induced by a bottom earthquake, a submarine landslide, underwater volcano eruption, atmospheric disturbance, cosmic body fall or the decomposition of a gas hydrate at the shelf. These gravity waves at the water surface occur in the sea as the result of a large- scale fast-acting disturbance on the water basin.
Most tsunamis occur in the Pacific Ocean because the Pacific basin is surrounded by the most tectonically active zone. Often referred to as the Pacific Ring of Fire, the Pacific basin is bounded by the edges of tectonic plates.
- The typical length of a tsunami wave is from 1 to 1000 km, the event`s duration may run between 5 and 100 minutes, the propagation velocity is from 1 to 200 m s-1, and the wave height may reach up to 10 – 30 m above the coastline.
Causes of tsunamis
1.Generation of a tsunami by earthquake
Many large underwater earthquakes, whose epicenters are dislocated at the bottom of ocean or sea, are able to generate tsunami waves. These events, so-called tsunamigenic earthquakes (i.e. tsunami-making), are characterized by high energy, and the magnitude on the Richter scale are M>7.0.
2.Tsunamis induced by submarine landslide and landfall.
A landslide-created tsunami, as a rule, has a local character. However, in the case of an extensive landslide involving the bottom sediments of the continental shelf or at the fall of a marine glacier into sea, the tsunami source may reach a size of several kilometers.
An landfall of caving coast, a break-away and the sliding of rock/ice blocks into the sea, occurrence of the bottom landslide and turbid flows, snow avalanches, or failure of harbor constructions may be responsible for tsunami generation.
A landslide motion process is usually caused by long-term accumulation of sediments at some ocean bottom areas, submarine slopes of basins, into the river deltas.
3.Volcanoes-Volcanic eruptions underwater can also cause tsunamis. In these cases, the force of the explosions (when magma mixes with water, steam explosions occur), volume of rock erupted and associated earthquakes can all cause tsunamis. The blast can also cause parts of the volcano itself to collapse. This collapsed material can fall into lakes, causing tsunamis.
4.Meteorites and Asteroids
As the two-thirds of the earth is covered by water, chances are that when an asteroid or a meteorite actually hits the earth it will land in the ocean. This would be devastating for the most coastal areas due to a devastating tsunami, but the same impact on land would create a dust cloud, which could possibly block the sun for many months
Once a tsunami starts, it cannot be stopped. This is why the focus of preventing the damage from tsunamis is still on warning systems: the best method of surviving a tsunami is running to high ground. However there are ways to reduce risks.
1.Buildings can be built on reinforced concrete ‘stilts’ to raise them out of flood waters after a tsunami.
2.Tall platforms can be constructed along coastlines for people to reach high ground quickly.
3.Planting trees along coastlines help as the trees help to break apart the wave before it reaches homes.
4.Education is key. If people know how to react to a tsunami warning, they are much more likely to be safe. This is why the location of the tsunami source is so crucial to survival, if the cause is far out to sea, people can have hours to prepare but if it is just off the coast, there may be only minutes.