How does vertical movement along an ocean-floor fault generate a tsunami?

A tsunami is generated when a strong earthquake occurs at or just beneath the ocean floor, causing a sudden, violent vertical displacement of the seabed.12

When one side of an ocean-floor fault thrusts upward past the other, it creates a buckle in the entire volume of water directly above the fault. This sudden raising or lowering of a large body of water acts as the initial impulse that propagates a series of long-wavelength waves across the ocean.123

It is important to note that horizontal movement, where one side of a fault simply slips past the other, typically does not disturb the water column sufficiently to trigger a tsunami. Only the vertical deformation of the seafloor provides the necessary energy to displace the water and initiate the wave train. Once formed, these waves travel across the ocean and increase in height as they reach shallower coastal waters, where they can cause significant destruction.123