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.

A tsunami generated in the Aleutian Islands struck Hawaii in 1946.

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 the waves in the tsunami reach shore, they slow down due to the shallowing sea floor, and the loss in speed is often accompanied by a dramatic increase in wave height. The waves scrunch together like the ribs of an accordion and heave upward. Depending on the geometry of the seafloor warping that first generated the waves, tsunami attacks can take different forms. In certain cases, the sea can seem at first to draw a breath and empty harbors, leaving fish flopping on the mud. This sometimes draws the curious to the shoreline and to their deaths, since the withdrawing of the sea is inevitably followed by the arrival of the crest of a tsunami wave. Tsunamis also flood in suddenly without warning. Tsunami waves usually don't curve over and break, like Hawaiian surf waves. Survivors of tsunami attacks describe them as dark "walls" of water. Impelled by the mass of water behind them, the waves bulldoze onto the shore and inundate the coast, snapping trees like twigs, toppling stone walls and lighthouses, and smashing houses and buildings into kindling.
 

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Tsunami survivor.