Tsunami

History
"Tsunami" is the Japanese word meaning tidal wave. A tidal wave is a large sea wave caused by a submarine earthquake or volcanic explosion. When the ocean floor is tilted or offset during an earthquake, a set of waves is created. These waves are similar to the concentric waves generated by an object dropped into the water.

Usually tsunamis move entirely across an ocean to the shore. A tsunami can have wavelengths of 60 to 120 mi and may reach speeds of 800 km/h. When the wave enters shallow waters, the wave, which may have been half a meter high out at sea, grows rapidly. When the wave reaches the shore, it may be 50 ft. high or more. Tsunamis have incredible energy because of the great volume of water affected. They bring waves of destruction capable of killing thousands of residents along the coast. Towering walls of water have struck populated coastlines with such fury that entire towns have been destroyed. In 1896 a population of 20,000 in Sanriku, Japan were wiped out. Tsunamis have resulted in waves as high as 135 ft. above normal sea level.
When a tsunami strikes the shore, it creates a number of waves with troughs that are lower than normal sea level. Each following wave is higher than the one before it. The period between waves is 10 to 30 minutes. This usually gives people ample time to escape to high ground after the first wave.



Big waves
Following the world's biggest recorded earthquake (measuring 9.5 on the Richter Scale) off the coast of Chile in 1960, a series of waves created havoc around the Pacific Rim. It caused 56 deaths in Hawaii, 32 deaths in the Philippines, and 138 deaths in Japan - 10,000 miles (16,000km) away.



Tsunamis
A tsunami (pronounced tsoo-nah-mee) is a chain of fast moving waves caused by sudden trauma in the ocean. They can be generated by earthquakes, volcanic eruptions, or even the impact of meteorites. Tsunami are also incorrectly known as tidal waves, but unlike tidal waves they are not caused by changes in the tides.

They are most common around the edge of the Pacific, where more than half of the world's volcanoes are found. These seismic surges can assault coastlines, often with little or no warning. Rocks weighing as much as 20 metric tonnes have been plucked from sea walls and carried 180m inland.

Wave power
Tsunamis aren't like wind-generated waves that rhythmically roll onto a beach. A tsunami can have a wavelength (ie distance between wave crests) in excess of 100km (60 miles) and there may be an hour between them. They travel at great speeds across an ocean with hardly any energy losses and are barely noticeable out at sea.

Over the deep Pacific Ocean, a tsunami travels at about 800kph (500mph). If an earthquake happened in Los Angeles, a tsunami could hit Tokyo quicker than you could fly between the cities by jet. The tsunami caused by the earthquake off the coast of Indonesia in late 2004 travelled 4,500km (2,800 miles) to Somalia in East Africa in just seven hours.

As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it behaves like a normal wave - only with more muscle.

Shallow water slows the tsunami and its height grows. Tsunamis batter the coast with tremendous amounts of energy. They can strip sand from beaches, tearing up trees, and even obliterating whole towns. Some have been known to reach as much as 30m above sea level.