Introduction Return to index page
What does "tsunami" mean?
Tsunami is a Japanese word meaning "harbor wave". Represented by two characters, the top character, "tsu" which means harbor, while the bottom character, "nami", means "wave". In the past, tsunamis were sometimes called "tidal waves" , and "seismic sea waves" by the scientific community. The term "tidal wave" does not match with the phenomenon; even if a tsunami impact upon a coastline depends on the tidal level when a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. Indeed "seismic" implies a link with an earthquake mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact.
What is the phenomenon?
A tsunami is a series of waves, or a wave train, of extremely long wavelength and period. It is generated by a disturbance that vertically displaces the water column from its equilibrium position. Earthquakes, landslides, volcanic eruptions, explosions, and meteorites, can generate a tsunamis. Generally speaking, tsunamis generated from these mechanisms excepted those caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source, however when they reach the coast they bring dramatic devastations. Many tsunamis, like the Pacific-wide tsunamis caused by some earthquakes, can savagely attack coastlines and also cause huge damages and loss of lives. Such tsunamis can travel for thousands of kilometers accross the sea and still carry enough energy to bring devastation.
Tunamis 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 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.