[gurpal (OP)]

how do tsaumamis occur?

[Bass]

Most commonly, earthquakes cause dislocations on the ocean floor which vertically displaces the overlying water creating the massive waves (tsunamis).  But anything that displaces the water can cause tsunamis- submarine volcanic eruptions, landslides, explosions, etc.

[Naufal the B. S.]

Earthquakes in the sea, it makes big erosion at the beach, and the erosion makes big explode water