Savage Earth: Waves of destruction!

08 May 2008

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.

Tsunami threatening to 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.

Krakatoa Volcano in Indonesia

On the open ocean, tsunami waves approach speeds of 500 mph, almost fast enough to keep pace with a jetliner. But gazing out the window of a 747, you wouldn't be able to pick it out from the wind-driven swells. In deep water, the waves spread out and hunch down, with hundreds of miles between crests that may be just a few feet high. A passenger on a passing ship would scarcely detect their passing. But in fact the tsunami crest is just the very tip of a vast mass of water in motion. Though wind-driven waves and swells are confined to a shallow layer near the ocean surface, a tsunami extends thousands of feet deep into the ocean.

Because the momentum of the waves is so great, a tsunami can travel great distances with little loss of energy. The 1960 earthquake off the coast of Chile generated a tsunami that had enough force to kill 150 people in Japan after a journey of 22 hours and 10,000 miles. The waves from a trans-Pacific tsunami can reverberate back and forth across the ocean for days, making it jiggle like a planetary-scale pan of Jell-O.

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.