The great magnitude of the recent Japanese earthquake, its shallowness and proximity to the Japanese coast produced large tsunami ranging from 4-15 metres high. While these are very high waves, there are a number of locations throughout the world where higher waves regularly occur. The surf break 800 metres from the north shore of Maui, known as Jaws, a favourite place for big wave surfers, has recorded waves of 21 metres, yet these high waves do not damage the island.
This is because there are major differences between ocean waves and tsunami.
Most ocean waves are created by wind. As a breeze blows over water, the surface tension breaks and ripples are created, the wind pushes the back of the ripples and eddies form at the front. The particles of water are pushed into a circular movement that reinforces the shape of the wave. Waves may look like a movement of water, but actually they are a movement of energy and have very little forward movement of the water particles themselves. Once a wave makes contact with the seabed, friction slows the bottom of the wave, but the crest continues at the same speed and then spills over and releases its energy onto the shore. The profile of the seabed determines if a wave surges, plunges or spills.
The size of the wave is determined by the fetch or distance, over which the wind is blowing. The highest ocean wave ever accurately recorded was 34 m, which was measured by USS Ramapo in 1933.
Tsunami are created by the displacement of immense volumes of water caused by phenomena such as earthquakes, volcanic explosions and events such as meteorite impact. Unlike an ocean wave, a tsunami is a moving wall of water extending from sea floor to sea surface.
On 11 March 2011 a magnitude 9.0 earthquake occurred off the east coast of Japan. Under the sea, stress built up along the subduction boundary between the Pacific and Eurasian tectonic plates. When the boundary ruptured, energy was released and the seafloor was thrust vertically upward displacing a huge volume of seawater, which rushed away to the Pacific Ocean and Japan. In the open ocean the tsunami travelled at an estimated 500 kms/hour and were only about 1 metre high. As they approached the Japanese coast the tsunami slowed, and shoaled, a term which describes the bunching up and increase in wave height. Tsunami do not lose much energy when they travel across the ocean, so they retained tremendous force which inundated the coast and hinterland with huge, swift and powerful surges of water.
The highest wave ever recorded was a tsunami in Lituya Bay Alaska where an earthquake displaced millions of tonnes of rock, earth and ice creating a 524 metre wave, which washed over the headland.