Tsunami from Deep to Shallow

In the deep ocean, a tsunami wave moves fast but stays low and stretched out. As it reaches shallow coastal water, it slows down, bunches up, and rises into a tall wave.

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In the picture

Low, broad crest in deep water: small height and wide spacing.
Ship at the surface: it floats calmly while the long wave passes beneath.
Upward-sloping seafloor: the water becomes shallower near the coast.
Tall curling crest at the shore: the wave slows, shortens, and grows high.

Examples

A long swell crossing the open sea after a distant undersea earthquake.
A wave in a bathtub that rises when it reaches the shallow end.

Key takeaways

Tsunami waves can cross deep ocean quickly while remaining barely noticeable at the surface.
When the seafloor shallows, the same wave slows, compresses, and can rise into a destructive wall of water.