How do tsunamis get bigger?

Answer 1

The Tsunami waves got bigger (or higher) as they approach land because the swells of water rise up as the seafloor gets shallower.

They start as an underwater wave (see related question below for what causes the wave) usually very deep on the floor of the ocean and moves (they can move in speeds up to hundreds of miles an hour) outward from the initiation point. If they move toward land, as the water gets more shallow with the increased height of the ocean floor in a harbor or coastal area, the wave gets higher as the sea floor gets closer to the water's surface. They have enough force and speed that they do not stop or get smaller at that point, but continue to move into water that is more and more shallow and get bigger and bigger (taller) and rise out of the water, since there is no place to go but up.

Answer 2

Tsunamis can get bigger as they travel across the ocean due to a phenomenon called wave shoaling, where the wave energy compresses and the wave height increases. Additionally, when a tsunami approaches shallow coastal areas, the shape of the seafloor can amplify the height of the wave, contributing to its size.