Question

What event happens underwater to cause a tsunami?

Answer 1

Displacement of water volume caused by a change in the position of the lithosphere: an undersea earthquake.

Explanation:

Underwater earthquakes occur when energy is released from the buildup of friction between two rock surfaces. This can occur at an undersea fault, or at a subduction zone. The subduction zone earthquakes are capable of releasing a lot of energy, and are to blame for the largest tsunamis.

There are other ways that a significant amount of water can be quickly displaced, but these are the most common and generally the most significant.