A tsunami is a very large ocean wave, which is caused by an underground earthquake. The word tsunami means 'harbor wave' in Japanese. A tidal wave is different. It is caused by the force of the moon, sun or planets or by the wind along with it. The water flows straight during a tsunami. This is different from the way ordinary waves flow. They flow in a circular pattern. This straight movement of the tsunami causes a great deal of damage to the land.
Tsunamis begin due to a movement of the seafloor. They can be caused by an earthquake, a volcano, a meteorite or a big landslide into the ocean or under it. They can occur along a fault line in the earth's crust where there is a movement of the plates below the sea. These plates can crash together producing large forces of water. Usually two tsunamis a year occur around the world near the source of the earthquake or another movement. About every fifteen years, a huge tsunami happens which can affect the waters worldwide.
About ten to fifteen percent of earthquakes, which cause damage near their source, are called strike-slip earthquakes. They occur when two blocks of land move horizontally along a fault plane. A fault is a place in the earth's crust where the plates of the crust have slid past or over one another. The strike-slip earthquake does not cause a tsunami as often as those with a vertical motion.
The speed of the tsunami depends on how deep the water is. Tsunamis travel approximately 475 mph in 15,000 feet of water. In a much less deep amount of water, the speed drops way down. A tsunami wave can travel from Portugal to North Carolina in eight and one-half hours!
The size or height of a tsunami wave can range from an inch to several feet high. It usually cannot even be seen by a ship at sea. When a tsunami comes near land, however, the height can multiply by even ten times. How far inland a tsunami goes can depend upon the features of the shoreline. A tsunami can go inland as much as one mile if the coast is a very flat piece of land at or below sea level.
The tsunami will look like a quickly moving tide coming in or going out from the shore. A wall of water may be seen or large breaking waves. Waves caused by the wind usually have periods between five and fifteen seconds. Waves produced by a tsunami occur between five to sixty minutes. Waves caused by winds also break on the shore. The tsunami wave rolls right in over the land in a flood. The height of a tsunami wave is a rise in the sea level by that much.
Signs of an incoming tsunami occur a very little amount of time in advance. A sudden drop in sea level may occur. The ground may shake for twenty seconds or more. Loud, booming noises may be heard.
After an earthquake, scientists can determine the exact time of arrival of a tsunami. These predictions may not be accurate right near the source. Tsunamis farther away may be predicted more accurately using mathematical models from past tsunamis.
A major tsunami occurred in 2011 in Japan. It began in the Pacific Ocean. In 2004, a major tsunami began in the Indian Ocean and affected Asia. Major tsunamis begin along the coast of Alaska every few years. They have had many smaller local tsunamis also. The west coast of the United States and Canada had local tsunamis in 1812, 1873, 1878, 1927, 1930, 1946 and 1949. A great number of tsunamis hit the west coast, which had been generated in other regions. The latest was in 2010. The United States and Canada Atlantic coast has felt the effect of tsunamis also, along with Puerto Rico and the Virgin Islands.
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