Tsunami

A tsunami is a large wave event which can cause huge devastation to the areas it hits. The name tsunami is derived from the Japan and means harbour (tsu) wave (nami)

A tsunami is an event caused by the vertical displacement of a large volume of water, generally this is under the sea but it has been known to happen in very large lakes. The displacement is due to a rapid change caused by an earthquake, landslide or a volcanic eruption. Sometimes nuclear testing can lead to a tsunami. Tsunamis are not linked to tides, and have in the past been called ‘tidal waves’ which is incorrect.

Today most tsunamis which are reported on the news are associated with earthquakes, linked to the displacement of the sea floor. Commonly these occur at destructive margins where the oceanic crust is being subducted beneath the continental crust. If the pressure becomes too high, the overriding continental plate can begin to bulge, which when the pressure it released causes subsidence, forcing a rapid change in the water level. This sudden release of energy causes a shockwave which leads to a tsunami forming.

Even though a tsunami can travel between 500km and 800km an hour, a boat out in the deep ocean wouldn’t really notice a tsunami passing beneath them because the wave amplitude is about 30cm above the normal wave surface. They become more deadly when they reach the shore. The increase in the angle of seabed causes the waves to slow down and increase in height. A wall of water forms which can be anything from a few metres in height up to 50+m. Sometimes the sea can be drawn back rapidly exposing coastline usually buried under water. People along the coastline become curious as to why this has happened and run out to explore areas which are usually submerged and to pick up fish, unaware of what is about the happen.

Tsunamis are still very powerful as they reach land and can travel a large distance across the land. Because of the power of a tsunami, everything in its path can be destroyed. Houses are smashed to pieces, trees are pulled up from their roots and beaches can be stripped of their sand. Quite often a tsunami can be followed by other tsunamis, as the sea bed settles. The first wave may not always be the largest.

In recent years, we have seen a number of devastating tsunamis such as the Indian Ocean tsunami which occurred on 26th December 2004. This was caused by an earthquake off the coast of Sumatra, Indonesia. The tsunami killed over 230,000 in fourteen countries.

On the 11th March 2011 an earthquake occurred 130km off the east coast of Japan, near Sendai. The magnitude 9.8 earthquake is one of the largest recorded since records began in 1900 and caused one of the most devastating tsunamis to hit to coast of Japan. The tsunami occurred minutes after the earthquake. Although a tsunami warning was issued, the lack of time meant that many people were unable to reach safety before the 10m high wave hit the coat around Sendai. Waves travelled as far as 10km inland causing huge amounts of damage and wiping out some villages completely. Tsunami waves travelled across the Pacific Ocean hitting places such as Hawaii and the west coast of South America, but the wave height wasn’t high enough to cause any damage. The scale of this disaster is huge with over 2,500 confirmed deaths and many thousands of people still missing in Japan.

Since the 2004 Indian Ocean earthquake warning systems have been set up in the Indian Ocean and the Pacific. Japan have their own early warning system. These can issue warning within minutes of an earthquake occurring, and predict the speed and energy of the tsunami. Both regional and international systems are in place depending on where the tsunami is predicted to travel. The only downside to these warning systems is that they can only warn of a tsunami once an earthquake occurs, giving people close to the coast near where the quake occurred very little time to escape. It is more beneficial for those living hundreds of miles away which won’t be affected by the tsunami for a number of hours, allowing them time to take action and evacuate.

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