Why does the sea receed before the tsunami waves come?

Answer 1

A tsunami is not a wall of water. In deep water there is a rolling effect with a lot of movement under the surface.

As it approaches shallower areas, the water ahead is drawn back under the wave thus causing the effect.

Dont hang around to watch by the way or you might end up closer than you intended.

Answer 2

The wave can be moving up to 500 mph and when it gets in shallow water it slows down and the water starts to pile up much like a car in an accident. This in turn draws water to the wave and away from shore. The receding water is usually attributed to the trough of the wave.