How can an undersea earthquake create a tsunami?

An undersea earthquake creates a tsunami when it causes a sudden, violent displacement of the seabed, which in turn raises or lowers a large volume of water.12

For this to occur, the earthquake must typically be strong and located at or just beneath the ocean floor. If the movement along a fault is purely horizontal, very little water is disturbed, and no tsunami is generated. However, if one side of the fault thrusts violently upward, it creates a buckle in the water column above it. This initial disturbance propagates outward as a series of long-wavelength waves.123

While these waves can travel across entire ocean basins at high speeds, they are often imperceptible in deep water due to their small height and long periods. As they approach a coast, friction with the rising sea bottom slows the waves, causing their wavelengths to shorten and their heights to increase dramatically, sometimes resulting in destructive run-ups of up to 30 meters.231