 The term "tsunami" comes from the Japanese words for harbour ("tsu") and wave ("nami"). The term was created by fishermen who returned to port to find the area surrounding the harbour devastated, although they had not been aware of any wave in the open water.
Tsunamis are natural phenomena that result when water is rapidly displaced on a massive scale. Tsunamis are not subsurface events in the deep ocean; they simply have much shorter waves offshore, and a very long wave length (often hundreds of kilometres long). Ocean-going vessels in their path do not usually notice them. Thus tsunamis generally pass unnoticed at sea, forming only a passing "hump" in the ocean.
High waves occur when shallow water is reached. The shallow water acts as a brake on the bottom part of the wave, sending the fast-moving water upward. Earthquakes, landslides, volcanic eruptions, and the impact of extraterrestrial bodies such as meteorites all can generate tsunamis, which can rapidly and violently inundate coastlines.
The undersea earthquake of Dec. 26, 2004 displaced massive volumes of water, resulting in tsunamis with waves up to 15 m / 50 feet. The tsunami took anywhere from 15 minutes to 7 hours (Somalia, 4500 km / 2,800 miles away) to reach land.
Despite a lag of up to several hours between the earthquake and the impact of the tsunamis, nearly all of the victims were taken completely by surprise. This is largely because there is no tsunami warning system in the Indian Ocean to warn the general populace. No warning system exists in that ocean due to the rarity of tsunamis there; the last major one was caused by the Krakatoa eruption in Java in 1883.
Tsunamis have been recorded in all the major oceans of the world; but they are mainly restricted to the Pacific basin, an area surrounded by volcanic island arcs, mountain chains and subduction zones earning the nickname the "ring of fire." The region is the most geologically active area on the planet. Due to earthquake action in the Pacific Ocean, tsunamis usually occur there. Thus, there has long been an effective tsunami warning system in place there.
Tsunamis were historically referred to as tidal waves because as they approach land they take on the characteristics of a violent onrushing tide, rather than the sort of cresting waves that are formed by wind action on the ocean. They are not actually related to weather or tides.
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