Much Uncertainty in Predicting Tsunamis

It happened so often that it finally became a macabre joke around the Coast Guard office in Honolulu where I served as public information officer many years ago. If you wanted to have a big crowd show up at the beach, just issue a tsunami warning.

Sure enough, thousands would show up to see the wave come in, and many even followed the receding shoreline as the approaching tsunami pulled coastal waters out to sea. And then, in every case I witnessed, the tsunami passed harmlessly by, the beach returned to normal, and disappointed tourists headed back for their hotels.

That would be pretty harmless if it always worked that way. But what those tourists didn't know, or chose to ignore, is that in 1946 a tsunami that was triggered by an earthquake in far off Alaska struck the town of Hilo, killing several hundred people. They didn't even know it was coming.

And unfortunately, all these years later, with many advances in technology, it's still very difficult to predict exactly what effect a tsunami is likely to have on any area, even if it's clear that a giant wave is on the way.

"There's a lot of uncertainty," says Arun Chawla, a coastal engineer with the Oregon Health and Science University in Hillsboro. That uncertainty is a tough pill for Chawla to swallow. Part of his task these days is to help develop comprehensive plans that could help Oregon's coastal residents know what to do, and what to expect, if they learn that a tsunami is heading their way.

Most Dangerous Quakes

It's no idle exercise. Some scientists believe the Cascadia subduction zone off the coast of the Pacific Northwest is ripe for a major earthquake that could send a wall of water toward the shoreline, and the tsunami could hit within 20 to 40 minutes after the quake.

Not every earthquake can produce a tsunami. First, the epicenter has to be in the ocean floor, not on land. Some faults move laterally, as in the case of California's notorious San Andreas, and they won't produce a tsunami unless they cause land to slide into the sea.

The most dangerous earthquake from a tsunami perspective is one that results in considerable uplift. That would displace huge amounts of water that will have to go someplace else. And that's exactly the kind of earthquake expected from the Cascadia subduction zone, where the sea floor is being pulled under the North American continent.

Of all the options facing Chawla and other Oregon officials, a Cascadia tsunami is the easiest one for them to predict. So maps have been made, showing where the greatest innundation is likely to occur, and meetings have been held to educate the public. So it would seem that Oregon is pretty well prepared for a major tsunami.

But there's a problem. A different earthquake, thousands of miles away, could send a tsunami toward Oregon, and it could arrive with totally different results than would be expected from an earthquake closer to home.

As soon as this hypothetical tsunami began its long journey, warnings would be issued because of an extensive early warning system that was established after the 1946 Hilo tragedy. But determining the most likely result would be far more difficult. So some false alarms should be expected in the years ahead, possibly resulting in a gradual erosion of public confidence in tsunami warnings. That could have disastrous consequences.

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.