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Because of the physics that governs the propagation of water waves, the speed of a tsunami depends only on the water depth. In the Pacific Ocean where the average depth is 4500 m, the speed of a tsunami is 750 km/h.
In comparison, large ocean swells travel much slower; a typical swell with a 15 second period travels across the ocean at 40 km/h.
Learn more: https://www.spc.int/pccos