NOAA explains why tsunamis slow down near the coast

A tsunami can travel at around 800 kilometres per hour in the deep ocean and roughly 30–50 kilometres per hour near shore. A NOAA representative explained the difference to RIA Novosti after the Panama earthquake.

The comments were published on October 9 amid reports of a tsunami threat to Latin American countries. The representative explained that waves move faster in deeper water. In the open ocean, a tsunami can cross an ocean basin in less than a day.

As it approaches land, its speed drops to roughly that of a moving car, around 30–50 kilometres per hour. Those figures describe typical tsunami behaviour and are not a calculation of when a particular wave will reach the coast.

NOAA's reference materials confirm the mechanism. In shallow water, wavelengths shorten, wave heights increase and currents become stronger. Slower movement alone therefore does not mean less danger to a coastal area. A current warning for a specific location should be distinguished from this explanation of the waves' physical properties.