these systems have been very successful at saving lives. For example, before the Japanese warning system was established, 14 tsunamis killed over 6000 people in Japan. Since the establishment of the warning system 20 tsunamis have killed 215 people in Japan.
Like all warning systems, the effectiveness of tsunami early warning depends strongly on local authority's ability to determine that their is a danger, their ability to disseminate the information to those potentially affected, and on the education of the public to heed the warnings and remove themselves from the area.

A tsunami is a series of long wavelength water waves caused by a sudden vertical displacement of the ocean's surface. When the waves reach the shore, they rear up and break, and can be many feet high. Tsunamis can reach far inland. Most are caused by underwater earthquakes at or near subduction zones. Rare but much larger tsunamis can be produced by landslides into the ocean or underwater landslides. Any impulsive event that produces a localized surface displacement of an ocean or large lake can produce tsunamis, such as underwater volcanic eruptions, asteroid impacts and nuclear explosions.

Tsunamis have wavelengths that are hundreds of miles long, much larger than the depth of the ocean. As a result, they move at the shallow water wave velocity, which is hundreds of miles per hour and is determined by the depth of the ocean. In the open ocean away from the source, they are only a few feet high and are undetectable except to sensitive sea floor instruments. When they reach shore, they slow down, grow in heigh and break like surf, overrunning low lying areas. At the shore, tsunamis can be small and undetectable or hundreds of meters high.

The Indian Ocean tsunami of 26 Dec 2004 was produced by an earthquake along a subduction zone where the Burma plate overides the Indian plate.