From a single tectonic plate slip, waves radiate outwards in all directions moving away from the earthquake. Once a tsunami forms, its speed depends on the depth of the ocean.
In the deep ocean, a tsunami can move as fast as a jet plane, over 800 km/h, and its wavelength, the distance from crest to crest, may be hundreds of miles. Sailors at sea will not normally notice a tsunami as it passes beneath them - in deep water, the top of the wave rarely reaches more than three feet higher than the ocean swell.
A tsunami only becomes hazardous when it approaches land. As a tsunami enters shallow water near coastal shorelines, its speed decreases to 30 to 50 km/h, slowed by greater friction with the seafloor. Its wavelength decreases but its height (amplitude) consequently increases - as the wave’s energy is compressed into a smaller space, the ocean water is propelled into a wall of water as it comes ashore.
Tsunami height: the height of the tsunami wave as it hits the shore
Flow depth: the depth of the water from a tsunami, measured on shore in different locations
Run up height: the elevation reached by seawater measured relative to the mean sea level
Inundation distance: the horizontal distance inland that a tsunami penetrates
Shoaling: the effect by which waves entering shallow water change in wave height
The 2004 tsunami in Indonesia was the most fatal tsunami in recorded history. The tsunami, generated by an earthquake 250 km offshore Sumatra, occurred along a 1600 km long fault line and so inundated a vast geographical expanse of low-lying areas throughout the Indian ocean basin. In many places, the waves reached as far as 2 km inland. Within 20 minutes of the earthquake, the first of several tsunami waves hit the shoreline of Sumatra with wave heights up to 30 meters and run up elevations of 10 meters. Eight hours later and 8,000 km from its Asian epicenter, the tsunami hit the coast of South Africa. As there was no adequate tsunami warning system in place in the region at the time, the tsunami claimed over 227,000 lives in 14 countries. Consequently, there is now an internationally coordinated Tsunami Warning System.