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Despite advances, science of forecasting a tsunami is inexact

By Joel Achenbach and Rob Stein
Washington Post Staff Writer
Sunday, February 28, 2010; A01

Saturday was a tense day in and around the Pacific Ocean. There was a wave on the loose. It was reputed to travel at the speed of a jet airplane. Beyond that, this tsunami was a mystery. No one knew precisely how big it would be when it came ashore. So went a very long day, full of anxious waiting and much staring at a sea that did little to signal its intentions.

The sirens sounded at dawn in Hawaii, where more than 144,000 were told to head to higher ground. Tsunami warnings were posted from Panama to Japan, from Ecuador to New Zealand. Australia made the tsunami-warning list. So did Antarctica.

Authorities told Californians to get out of the water to avoid being swept away by strong currents. The forecasts showed the waves reaching Nome, Alaska, more than 24 hours after the huge earthquake off the coast of Chile.

By Saturday evening, the calamity had not materialized. Although reports were still coming in, it did not appear to have been a killer tsunami like some in the past.

Jenifer Rhoades, tsunami program coordinator for the National Weather Service, said officials would rather err on the side of warning people about the worst-case scenario than play down the risk.

"Forecasting tsunamis is a relatively new science. We learn a lot every time we have an event like this," she said. "We dodged a bullet this time, but since tsunami science is not exact, we erred on the side of caution."

The tsunami when it reached Easter Island on Saturday measured only about a foot above sea level. Ditto on Tahiti. On the Marquesas Islands, it reached six feet. Talcahuano, a coastal town in Chile, reported a 7.7-foot wave. There were reports of three people drowned and more missing on the islands of Juan Fernandez, off the Chilean coast.

By the time the tsunami reached Hawaii, the waves topped out at three feet, and warnings soon were lifted for most of the Pacific. Even though Hawaii was spared, other areas might remain in danger, Nathan Becker, an oceanographer at the Pacific Tsunami Warning Center in Honolulu, said Saturday night.

"It's still moving across the Pacific, and could go all the way to Japan," Becker said. "Hawaii may not be having big waves, but there might be somebody further downstream who does. Japan might, or perhaps Guam."

Indeed, early Sunday, Japan was evacuating coastal areas, still fearing waves of more than 9 feet.

If nothing else, this was a dramatic test of the Pacific tsunami warning system, which uses buoys sprinkled across the ocean to detect tsunamis in real time. The Indian Ocean lacks such a system, which might have minimized the casualties from the catastrophic tsunami of 2004.

Even with scientific measurements, forecasts, alarms and civil defense measures, Saturday's events showed that tsunamis are unpredictable. And even if a wave is measured precisely in the open sea, its effect can be greatly magnified by shallow bays and harbors.

"Harbors are like musical instruments. They have specific pitches, if you wish, specific frequencies," said Emile Okal, a professor at Northwestern University who has spent his career studying tsunamis. "The structure of a harbor is such that for some of these periods it can amplify the wave."

Okal, as it happens, was in Tahiti, in a laboratory, when his instruments sounded an alarm signaling a major earthquake. The initial reading showed an 8.5-magnitude quake in Chile, he said. It was in the early evening, Tahiti time. He would not get more than 40 minutes of sleep during the night.

He knew what tsunamis could do. He knew the 2004 tsunami not only killed hundreds of thousands in Indonesia and about 30,000 in Sri Lanka, but it also crossed the breadth of the Indian Ocean and killed 300 people on the African coast in Somalia.

Over short distances, tsunamis can create waves of stunning dimensions. A tsunami in 1946 created a wave 138 feet high that obliterated a lighthouse in the Aleutian Islands where five people were stationed, he said.

"At some point they found some human remains a few days later, but not much to speak of," Okal said.

In 1755, a major earthquake destroyed much of Lisbon. Fearing that homes would collapse in aftershocks, the authorities told residents to assemble on the beach, where nothing could fall on them. An hour or so later, the tsunami rolled in. The death toll is estimated at 70,000, Okal said.

Earthquakes cause tsunamis by suddenly lifting the sea floor. The waves produced can be deceptively subtle in open ocean -- a boat in deep water will barely register it. But when the waves come on shore, they can toss boats around as if they were plastic beach toys.

"Once you've seen a tsunami, it's probably too late to outrun it," Eric Geist, a geophysicist with the U.S. Geological Survey, said as the waves neared Hawaii.

A tsunami as it comes ashore is not a normal wave, one that breaks once in a mass of foam and dissipates. Rather it is a mass of water that has no back side.

"A tsunami is a six-foot wall of water that hits the shore like a freight train," said Susan Hough, a U.S. Geological Survey seismologist.

"Six feet was some of the inundation heights for the [Indonesia] event in 2004. It's very significant and dangerous," Rhoades said.

Hawaii has been hard hit by tsunamis in the past. In 1960, 61 people were killed when a 33-foot-tall wall of water roared through Hilo Bay. That, too, was caused by a huge quake off the coast of Chile, a 9.5-magnitude tremor -- the most powerful ever measured on the planet.

This one, at 8.8 magnitude, may not have been as powerful, but it still vaulted onto the list of the 10 strongest quakes on record since 1900. Based on history, the Chilean earthquake appeared likely to trigger a major tsunami, experts said.

"All the evidence indicated it would be larger," said Barry Hirshorn, a geophysicist at the Pacific Tsunami Warning Center.

"This was a very large earthquake in a part of the world that tends to produce very large tsunamis. We need to walk a line between a false alarm and missing something that's dangerous and could kill people. Since the science is not exact, we chose to err on the side of not killing people by missing something."

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