A tsunami (pronounced sue-nahm-ee) is a series of huge waves that can cause great devastation and loss of life when they strike a coast. The word tsunami comes from the Japanese word meaning “harbor wave.” Tsunamis are sometimes incorrectly called “tidal waves” — tsunamis are not caused by the tides (tides are caused by the gravitational force of the moon on the sea). Regular waves are caused by the wind. Tsunamis are caused by an underwater earthquake, a volcanic eruption, an sub-marine rockslide, or, more rarely, by an asteroid or meteoroid crashing into in the water from space. Most tsunamis are caused by underwater earthquakes, but not all underwater earthquakes cause tsunamis – an earthquake has to be over about magnitude 6.75 on the Richter scale for it to cause a tsunami. About 90 percent of all tsunamis occur in the Pacific Ocean.
After the huge volume of water has moved, the resulting wave is very long (the distance from crest to crest can be hundred of miles long) but not very tall (roughly 3 feet tall). The wave propagates (spreads) across the sea in all directions; it can travel great distances from the source at tremendous speeds. Tsunamis have an extremely long wavelength (wavelength is the distance between the crest (top) of one wave and the crest of the next wave) — up to several hundred miles long. The period (the time between two successive waves) is also very long — about an hour in deep water. In the deep sea, a tsunami’s height can be only about 1 m (3 feet) tall. Tsunamis are often barely visible when they are in the deep sea. This makes tsunami detection in the deep sea very difficult. A tsunami can travel at well over 970 kph (600 mph) in the open ocean – as fast as a jet flies. It can take only a few hours for a tsunami to travel across an entire ocean. A regular wave (generated by the wind) travels at up to about 90 km/hr.
As a tsunami wave approaches the coast (where the sea becomes shallow), the trough (bottom) of a wave hits the beach floor, causing the wave to slow down, to increase in height (the amplitude is magnified many times) and to decrease in wavelength (the distance from crest to crest). At landfall, a tsunami wave can be hundreds of meters tall. Steeper shorelines produce higher tsunami waves. In addition to large tsunami waves that crash onto shore, the waves push a large amount of water onto the shore above the regular sea level (this is called runup). The runup can cause tremendous damage inland and is much more common than huge, thundering tsunami waves.
Reference:
Col, Jeananda. Enchanted Learning. https://www.EnchantedLearning.com 1996