Tsunami << tsoo NAH mee >> is a series of powerful ocean waves produced by an earthquake, landslide, volcanic eruption, or asteroid impact. Tsunami waves can travel great distances and still retain much of their strength. They differ from common ocean waves, which are caused by wind. The word tsunami is a combination of Japanese words meaning harbor and wave.
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Tsunami waves are much longer than common ocean waves. In the open ocean, the water may take from 5 minutes to over 1 hour to reach its highest level and fall back again as a tsunami wave passes. By contrast, a common ocean wave causes the water level to rise and fall in 5 to 20 seconds. Tsunami waves in the open ocean usually raise and lower the water level by 3 feet (1 meter) or less. Because the change happens gradually, tsunamis frequently go undetected by ships.
The deeper the water is, the faster a tsunami wave travels. In the Pacific Ocean basin, where depths average about 13,000 feet (4,000 meters), tsunami waves can travel up to 600 miles (970 kilometers) per hour, as fast as a jet aircraft. As a tsunami wave approaches land, its speed drops to about 20 to 30 miles (30 to 50 kilometers) per hour. As the wave’s speed decreases, its height usually grows by at least three times. The resulting flood of water can surge more than 2/3 mile (1 kilometer) inland and pile up in certain places to reach elevations higher than 100 feet (30 meters) above sea level.
Scientists strive to predict tsunamis so that endangered coastal areas can be evacuated. One method uses devices called seismographs to measure seismic waves (waves of vibration generated by earthquakes). By analyzing seismic waves, scientists can determine when and where an undersea earthquake has occurred and calculate its strength. As a result, they can then estimate the size of a possible tsunami and the time its waves will reach land. Seismic waves travel through the ground much faster than tsunami waves travel through the water. For this reason, scientists can sometimes warn people several hours before tsunami waves strike.
Other forecasting methods use pressure sensors placed on the ocean floor. When a sensor detects pressure from a large tsunami, it relays the information to a buoy. The buoy then transmits the data to a warning center. Scientists are also developing methods to detect tsunamis using radar signals from satellites.
In late 2011, researchers released evidence showing a “merging” or “double” tsunami triggered by a March 2011 earthquake off the coast of Japan. Such waves form when a single wave front breaks up into several smaller wave fronts. A wave front is the leading edge of a moving wave. Researchers believe the wave front breaks apart as it travels over undersea ridges and mountains. The smaller wave fronts may then collide, combining into a larger “double” wave.
