Much Uncertainty in Predicting Tsunamis

Risks of Crying Wolf

Accurate predictions are hard because the impact of a tsunami depends so much on the lay of the land, or more precisely the sub-sea, near-shore topography of the ocean bottom, and the direction from which the tsunami is coming, Chawla says.

Major earthquakes can happen in many areas of the world, he notes, and no one knows exactly when or where the next one will strike.

"That is the biggest uncertainty that we have," Chawla says. "Nobody knows yet how to predict an earthquake."

Much progress has been made in recent years in mapping the offshore areas, and that's some help because certain features are required for a tsunami to cause massive damage. An offshore canyon that can funnel the energy of the wave toward a specific area, for example, is cause for concern.

"But it depends on the direction that the waves are coming from," he says.

An offshore depression, for example, can either "focus or de-focus the energy of the wave," Chawla says.

Most tsunamis are deflected around the Hawaiian islands by a high undersea ridge, he adds, thus sparing them from the impact of most giant waves. But when the waves approach a larger land mass, there isn't anywhere else for the water to go but up the beach, and that is what has happened in the tragic events in southern Asia.

Several offshore islands also sustained major damage from the tsunami, but the situation there is quite different from Hawaii.

The water depth around the islands in the northern Indian Ocean is deep, and there is no protective ridge, so "the waves don't see the land until they are quite close," Chawla says. "They just go right over it."

The results could have been diminished considerably if an early warning system had been in place in the Indian Ocean, he adds. At least the people could have known the tsunami was coming, even if they couldn't have known how bad it was going to be.

The tragedy of it is they wouldn't have had to do much to have been safe. Even a giant tsunami slows down when it enters shallower water, and as a result its reach isn't long.

Chawla says that even a few feet of elevation can make a difference, so if more people had run just a little ways inland, more lives would have been spared.

So despite all the research, and all the sensors and monitors that are now available, Chawla's advice is still pretty basic.

When a tsunami warning is issued, even if there have been many false alarms in the past, "please don't go to the beach," he says.

Chances are everybody knows that now, on the heels of this tragedy, but will they still remember a few years down the road?

Lee Dye's column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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