Why does undersea earthquakes causes a “tsunami/one giant wave” instead of multiple small waves disruption in all direction?

Question

Why does undersea earthquakes causes a “tsunami/one giant wave” instead of multiple small waves disruption in all direction?

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Answer 1

An undersea earthquake causes a slip in the floor of the ocean. Think of it like a crack in the floor, and one side of the crack goes down 5 feet/meters/whatever. That long strip of ocean is now unsupported, and gravity causes water to fall into the corresponding chunk of floor. The result is a strip of water with a huge amount of energy in it. That strip starts a wave, and when the energy gets into shallower water the height of the wave gets higher.