What is the typical speed of a tsunami in deep ocean?

In the deep ocean, a tsunami typically travels at about 800 km/h. Its speed depends mainly on water depth: the deeper the water, the faster the tsunami moves. In a Pacific Ocean depth of roughly 5,000 metres, the shallow-water wave formula gives a speed of about 806 km/h.

That speed does not mean a tsunami looks like a giant breaking wave in the open sea. A tsunami can have a wavelength of up to 200 kilometres and an amplitude of roughly one metre in deep water, so ships may barely notice it passing beneath them. Its energy is spread across a very long wave.

As the tsunami reaches the continental shelf, the water becomes shallower. The wave slows, its wavelength shortens, and its height increases—a process called shoaling. Near shore, its speed may fall below 80 km/h, but the wave becomes far more destructive.

The 800 km/h figure is therefore an approximation for deep water, not a universal speed. Small variations in ocean depth produce different speeds, and the same tsunami can move at dramatically different rates along different coastlines.