Sudden movements of under sea tectonic plates,
When huge under water plates make jerking movement after building pressure over long periods the huge quantity of water gets displaced-fast.
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Think about this as dropping a huge rock in a pool. The movement of the rock going down causes water to move upwards creating waves. This wave creation is easily observed in a small pond.
It is not as obvious to see the creation of the Tsunamis waves in the ocean. But when these fast moving large waves hit the shore the damage can be devastating .