Have you ever imagined what will happen in the ocean when an earthquake happens underground? If you've ever felt a big earthquake or seen one on television you know that it has the power to shake the ground and move large buildings. When a great earthquake happens under the ocean floor, the earthquake's large energy is transferred (转移) to the water above it, creating a series of water waves called a tsunami (海啸). The word comes from the Japanese words tsu (harbor) and nami (wave).
Normal ocean waves have an average wavelength (波长) of about 100 meters and an average height of about 2 meters, while a tsunami can have a wavelength of 193 kilometers or more. Travelling at up to about 800 kilometers per hour in the deep ocean, a tsunami might be at a height of as little as 0.9 meters, which makes it almost impossible to be noticed, even for a ship in the area.
As a tsunami approaches (靠近) a coastline, things begin to change greatly. As it approaches shallow (浅的) waters, the wave speed drops below about 80 kilometers per hour. The wavelength also decreases much, but amplitude—the height of the wave—increases greatly, leading to the huge wall of water along coastlines, which causes great damage to coastal areas.
About 80% of tsunamis happen in the Pacific Ocean. Tsunamis cannot be prevented, but areas that experience tsunamis regularly have developed tsunami warning systems to give as much advance warning as possible to people who live along the coastline. For example, there are tsunami warning signs and warning sirens (警报器) on the top of nearby hills along Japanese coastlines. And many Japanese coastal communities have built tsunami walls, floodgates and others to stop or change the direction of water from incoming tsunamis.
