The phenomenon we call a tsunami is a series of waves of extremely long wavelength and period generated in a body of water by an impulsive disturbance that displaces the water. Although tsunamis are often referred to as "tidal waves" by English-speaking people, they are not caused by the tides and are unrelated to them.
Tsunamis are primarily associated with earthquakes in oceanic and coastal regions. When an earthquake occurs, the energy travels outward in all directions from the source. This can be illustrated by throwing a pebble into a small, still pond. The pebble represents a meteorite or some other energy source, and the pond represents the ocean. The ripples that travel out in all directions from the focus, or the point where the pebble hit the water, represent the energy that creates a sea wave. Notice how the waves become larger as they reach shore, where the water is shallower.
Detecting tsunamis is a very difficult thing to do. When a wave begins in the deep ocean waters, it may only have a height of about twelve to twenty-three inches and look like nothing more than the gentle rise and fall of the sea surface. An example of how easy tsunamis are to overlook is the Sanriku tsunami, which struck Honshu, Japan, on June 15, 1896.
Fishermen twenty miles out to sea didn't notice the wave pass under their boats because it only had a height at the time of about fifteen inches. They were totally unprepared for the devastation that awaited them when they returned to the port of Sanriku. Twenty-eight thousand people were killed and 170 miles of coastline were destroyed by the wave that had passed under them.
Tsunamis in deep water can have a wavelength greater than 300 miles (500 kilometers) and a period of about an hour. This is very different from the normal California tube, which generally has a wavelength of about 300 feet (100 meters) and a period of about ten seconds. (The period of a wave is the time between two successive waves.)
Tsunamis are shallow-water waves, which means that the ratio between water depth and wavelength is very small. These shallow-water waves move at a speed equal to the square root of the product of the acceleration of gravity (9.8m/s/s) and the water depth. The deeper the water, the faster and shorter the wave is. For example, when the ocean is 20,000 feet deep, a tsunami travels at 550 miles per hour. At this speed, the wave can compete with a jet airplane, traveling across the ocean in less than a day.
Another important factor in considering tsunamis is the rate at which they lose energy. Because a wave loses energy at a rate inversely related to its wavelength, tsunamis can travel at high speeds for a long period of time and lose very little energy in the process.
