A Different Kind of Wave
A tsunami is one of the most powerful and dangerous events the ocean can produce: a series of huge waves capable of striking coastlines with tremendous force. It is important to understand that a tsunami is not an ordinary wave. The waves we usually see at the beach are made by wind. A tsunami is created by something far more powerful.
A Sudden Push of Water
The cause of a tsunami is a sudden, large displacement of a great volume of ocean water. Something has to push or move an enormous amount of water all at once. When that happens, the displaced water cannot stay piled up. It spreads outward in every direction as a series of long, powerful waves, racing away from the source across the open ocean.
The Most Common Cause
By far the most common cause of a tsunami is an undersea earthquake. Earthquakes happen when the Earth's tectonic plates suddenly move. If a strong enough earthquake occurs under the ocean and causes the seafloor to abruptly rise or drop, the entire column of water above it is moved up or down with it. This sudden vertical shift of the seafloor displaces a huge mass of water and sets a tsunami in motion. Not every undersea earthquake does this; it generally needs to be large and to move the seafloor vertically.
Other Causes
While earthquakes cause most tsunamis, they are not the only trigger. Anything that suddenly displaces a large volume of water can do it. Underwater landslides, where masses of rock and sediment slide along the seafloor, can generate tsunamis. So can violent volcanic eruptions near or under the sea. In very rare cases, a large object striking the ocean from space could do the same. In every case, the principle is identical: a sudden, massive displacement of water.
Source
This article was written using information from Wikipedia.