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.

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