What Causes Tsunamis?

Tsunamis, also called seismic sea waves or, incorrectly, tidal waves, generally are caused by earthquakes, less commonly by submarine landslides, infrequently by submarine volcanic eruptions and very rarely by a large meteorite impact in the ocean. Submarine volcanic eruptions have the potential to produce truly awesome tsunami waves. The Great Krakatau Volcanic Eruption of 1883 generated giant waves reaching heights of 125 feet above sea-level, killing thousands of people and wiping out numerous coastal villages.

Not all earthquakes generate tsunamis. To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. All oceanic regions of the world can experience tsunamis.

Regular waves usually travel at a brisk pace, but nothing compared to the insane 500 mph of a traveling tidal wave over deep water. Over deep ocean, however, the tsunami is not much larger than the rest of the normal waves. It runs just several feet high, but there is good reason for this. Normal ocean waves only affect the water at the surface, but tidal waves act as shockwaves, often moving thousands of feet deep. Tsunamis are rarely singular, but travel in sets. That is, there are a series of waves following each other, usually of similar magnitude. 

When the waves reach shore, however, it is a different situation. In the open sea, the wavelength (horizontal distance between identical points on two successive waves) of the tsunami set is very large; sometimes hundreds of miles. When the waves hit shallow water, they slow down dramatically due to friction. Since the waves have less depth to travel through, they become much larger at the surface and begin looking the way we picture tidal waves. As the front waves slow down, the back ones catch up so that the wavelength now becomes much shorter than before. 

Often, the trough can come before the crest. In tsunamis, this can be particularly disastrous. When the trough comes, sea level drops and can completely empty harbors. With fish flopping on the ground and boats sunk in mud where there has always been water, people rush to the shore to see what has happened. It is a fatal mistake as a massive wall of water rushes over the area. This is the only warning the wave gives before it makes landfall and it's a tricky one. When the crest comes first, the only warning it gives is a loud roar as it crashes over you and a big shadow. When tidal waves hit, they aren't like your baywatch surfer waves with nice curls. If you've ever seen a wave break on the beach, picture that times several hundred or thousand. Just a wall of rushing water.