Though it's true that tsunamis are ocean waves, calling them by the same name as the ordinary wind-driven variety is a bit like referring to firecrackers and atomic warheads both as "explosives." Triggered by volcanic eruptions, landslides, earthquakes, and even impacts by asteroids or comets, a tsunami represents a vast volume of seawater in motion -- the source of its destructive power.

The Japanese characters for tsunami mean "harbor wave," and many people commonly refer to them as tidal waves, but in reality tsunamis have little to do with tides. They are creatures of the open ocean, trains of giant waves that can travel for thousands of miles across the sea and still pack enough energy to smash towns and drown the unwary.

Toss a stone in a pond and you create a series of concentric ripples. A tsunami is just like those ripples, except the disturbance that sets them in motion is of a much greater magnitude. Undersea landslides and the collapse of oceanic islands into the sea make tsunamis. Volcanic eruptions can also do it. In fact, the most deadly tsunami in recorded history followed the eruption and virtual obliteration of Indonesia's Krakatoa Volcano in 1883. An estimated 36,000 people died as a result of the eruption, the majority of them from the tsunamis.

Impacts by comets or asteroids can also generate giant tsunamis. No one has actually witnessed such an event, except perhaps in films like DEEP IMPACT. But computer simulations show that the giant tsunamis unleashed by Hollywood special effects wizards -- large enough to swamp the Manhattan skyline -- are possible and have almost certainly happened in the distant past. Scientists at the Los Alamos National Laboratory in New Mexico calculated that if an asteroid three miles across hit the middle of the Atlantic Ocean, the tsunami would swamp the upper East Coast as far inland as the Appalachian Mountains and drown the coasts of France and Portugal.

But by far the most frequent tsunami-maker is the buckling of the seafloor caused by an undersea earthquake. Tsunami earthquakes happen at subduction zones, places where drifting plates that make up Earth's outer shell, or lithosphere, converge, and the heavier oceanic plate dips below the lighter continents. There are subduction zones off Chile, Nicaragua, Mexico, and Indonesia that have generated killer tsunamis in the past decade. In the Pacific, there were 17 tsunamis from 1992 to 1996, and they took nearly 1,700 lives.

As a plate plunges down into Earth's interior, it moves in fits and starts -- sticking for awhile, then slipping. When it's stuck against the edge of a continental plate, stresses build up. When the locked zone gives way, parts of the seafloor may snap upward like a diver's springboard as the tension is released; other areas may sink downward. In the instant after the quake, the shape of the sea surface mirrors the contours of the seafloor below. But, just as quickly, gravity acts to return the sea surface to its original shape. As the rumpled sea flattens out, ripples race outward. A tsunami is born.

For more on Tsunamis try the PBS site  Savage Earth