What does "Tsunami" mean?
Tsunami is a Japanese word that is the combination of two roots:
(i) tsu means harbor; and
(ii) nami; means wave.
Therefore, tsunami was originally created to describe large-amplitude waves in a harbor.
Nowadays, tsunami is widely used to describe long ocean waves (compared to water depth) generated by submarine earthquakes, volcanic eruption, landslides or meteorite impact.
In the past, tsunamis were sometimes referred to as tidal waves because as they approach land, they exhibits the characteristics of a violent onrushing tide rather than the sort of cresting waves that are formed by wind action upon the ocean (with which people are more familiar). This is misleading since tide results from the extraterrestrial, gravitational influences of the moon, the sun, and planets unlike the tsunamis.
How do tsunamis differ from other water waves?
Unlike wind-generated waves (generally with periods in the order of 10 seconds and wavelengths in the order of 100 meters ), which many of us may have observed on a local lake or at a coastal beach, a tsunami has a much smaller amplitude offshore (from several centimeters to tens of centimeters), and a very long wavelength (often hundreds of kilometers long) compared to water depth (several kilometers). The wave period of a tsunami is typically within the range of 10 minutes to up to one hour. This is also why they generally pass unnoticed at deep sea, forming only a passing "hump" in the ocean.
Tsunamis travel very fast in the open sea. The wave speed can be estimated as the square-root of the product of the acceleration of gravity (9.8 m/s2) and the water depth. In the Pacific Ocean, the water depth is typically 4,000 meters. This will yield a tsunami speed about 700km/hour, comparable to that of a jet plane.
Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances without too much energy losses.