An earthquake underwater can create a tremendous sudden displacement of ocean water with a circular surge of energy much like a rock falling into a pond.

These outward spreading waves travel quickly when in open water.

However, arriving at shallow coastal areas their speed slows and their height is magnified greatly by the upward sloping land beneath.

Just before a destructive tsunami wave hits, the water at a coastal area such as the depicted town recedes and exposes tidelands rarely if ever seen.