Most tsunamis are caused by earthquakes generated in a , an area where an oceanic plate is being forced down in to the mantle by forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".
As the stuck plate continues to push into the mantle the movement causes a slow distortion of the overriding plate. The result is a build up of energy very similar to the energy stored in a compressed spring. This energy can build up in the overriding plate over a long time - decades or even centuries. Eventually this energy is more than the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back like a released spring. The Pacific plate, which moves at a rate of 3.1 to 3.5 inches per year, dips under Honshu's underlying plate releasing large amounts of energy. This motion pulls the upper plate down until the stress builds up enough to cause a sudden movement, which causes the tsunami - because it gives an enormous shove to the water overhead. At the same time, inland areas of the overriding plate are suddenly lowered.
The moving wave begins travelling out from the earthquake. Some of the water travels out and across the ocean basin, and, at the same time, water rushes towards the land to flood the recently lowered shoreline.
Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. In many tsunamis the shoreline is pounded by repeated large waves. The waves are not very high outat sea and can go unnoticed, but they are extremely long and also grow in height when they reach shallower water. Normal waves have a ‘wavelength’ of about 330ft, but a tsunami in the deep ocean will be about 120 miles long! It travels at over 500 miles per hour!
When the wave enters
shallow water, it slows down
and its height increases.
The wave further slows and amplifies as it hits land. Only the largest waves crest.
Effects of a tsunami
Tsunamis cause damage in two ways: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying all with it, even if the wave did not look large. Both farmland and buildings are damaged or destroyed and many people drown or are killed by the force of the water and moving objects. For example, cars are lifted up and swept away with the rush of water, and people can be hit by them or simply drown. The Japanese has so far officially confirmed 11,938 deaths, 2,876 injured, and 15,478 people missing as well as over 125,000 buildings damaged or destroyed. The number of deaths is expected to reach tens of thousands. ‘’ reports that as many as 100,000 children have been uprooted from their homes, some of whom were separated from their families because the earthquake occurred during the school day.
The earthquake released a equivalent to 9,320 , or approximately 600 million times the energy of the !
"If we could only harness the [surface] energy from this earthquake, it would power [a] city the size of Los Angeles for an entire year," Marcia McNutt, director of the U.S. Geological Survey, said in an interview.
- Structural damage and Transport
The earthquake and tsunami together caused severe structural damage in Japan, including heavy damage to roads and railways as well as fires in many areas, and a dam collapse which caused even more flooding. Whole towns were swept away as well as cars and planes.
It was reported that four passenger trains containing an unknown number of passengers disappeared in a coastal area during the tsunami. Some ports were destroyed. Japan's transport network suffered severe disruptions.
Around 4.4 million households in northeastern Japan were left without electricity. Many electrical generators were damaged.
1.5 million households were left without water. Flooding also contaminates the water supply which is left, causing a health risk, as well as the earthquake damaging pipelines bringing water to houses.
- Oil, Gas and Coal
O were set on fire by the earthquake. An analyst estimates that consumption of various types of oil may increase by as much as 300,000 , as back-up power plants burning try to compensate for the loss of 11 GW of Japan's nuclear power capacity. So this will cause even more environmental damage.
- Nuclear Reactors
At least three were damaged by explosions due to hydrogen gas that had built up within their outer buildings after cooling system failure. The radiation levels are a danger to humans.
Phone services and Internet connections were disrupted.
- Flood Damage
The salty mud from ‘soil liquifaction’ causes damage to houses and other property.
- Lasting Psychological Damage
Apart from the obvious grief suffered by those who lost loved ones and their homes, the bereaved could not even have a normal Japanese funeral, when 99.9% of bodies are and burials are often banned by law. The crematoriums were damaged and the ones remaining could not cope with the thousands of bodies, which were put in mass graves. For many, they will never know what happened to their missing family members and friends, since the bodies have not been found.
The aftermath of the earthquake and tsunami included both a humanitarian crisis and a major economic impact. The tsunami resulted in over 300,000 refugees in the Tōhoku region, and shortages of food, water, shelter, medicine and fuel for survivors. The economic impact included both immediate problems, with industrial production stopped in many factories, and the longer term issue of the cost of rebuilding which has been estimated at 10 trillion yen - about £80 billion. The long term reinvestment in car factories such as Toyota will be hard to find. There will have to be a huge cleaning up process before rebuilding can begin, as well as clearing of farmland before new crops can be planted or animals are able to graze. Some rice farmers are thinking they should retire. The rice paddies on the outskirts of the tsunami-hit cities are ankle-deep in a black, salty sludge. Crumpled cars and uprooted trees lie scattered across them. The ground has to be cleared and the soil washed free of salt. Nothing would grow in such salty mud. Many fishing boats were destroyed, too, and Japan’s fishing industry is important. So, as well as being a nation in mourning for the dead, the Japanese have to find the strength to rebuild their cities, their farming, industries, transport network and tourism.