How Do Tsunamis Travel So Fast?

Unlike wind-generated surface waves that affect only the top few metres, a tsunami is a shallow-water wave — its wavelength (100-300 km) is much greater than the ocean depth (average 3.7 km), so the entire water column moves. The wave speed follows the formula v = sqrt(g * d), where g is gravitational acceleration and d is water depth. In the open ocean, this yields speeds of 600-900 km/h. Despite this enormous speed, tsunami wave heights in deep water are typically less than 1 metre with wavelengths of hundreds of kilometres — a ship at sea would barely notice one passing. As the tsunami enters shallow coastal waters, friction with the seabed slows the front of the wave while the back continues at speed, compressing the wave energy upward. Wave heights can build to 10-30 metres or more, and the immense volume of water behind the wave front is what causes the devastating flooding — a tsunami is not a single breaking wave but more like a rapidly rising tide that keeps coming.