Tsunami magnitude difficult to predict

Scientists can do a very good job of tracking the progression of a tsunami and predicting when it will arrive, but they have much more difficulty predicting how big it will be, experts said Saturday.

That disparity has become apparent following the massive magnitude-8.8 earthquake off the coast of Chile. Researchers from the Pacific Tsunami Warning Center in Hawaii predicted to within a few minutes when the tsunami -- a massive influx of water rather like a flash flood -- would arrive at Hilo. They also predicted that the wave height would range from 2 to 8 feet, and they were correct. But when the waves proved to be at the low end of the range, they were widely perceived as having blown the prediction.

Tsunami expert Harry Yeh of Oregon State University said the forecasters had to err on the side of caution. “Even if they knew deep in their heart that it probably wouldn’t be large, it could be large,” he said. “If they lost one life [because they underestimated the size of the tsunami], that would be a problem.”

The biggest problem in predicting the size of a tsunami is in determining exactly how much energy was put into the ocean by an earthquake, and that is something that cannot be determined in the immediate aftermath of the event.

Chile’s 8.8 quake occurred at the junction of two massive tectonic plates: the South American plate, which sits under most of the continent, and the Nazca plate, which lies under much of the Pacific Ocean. The Nazca plate is attempting to slip under the South American plate, but is largely held in place by friction. Every so often, however, the geologic forces overcome friction and the two plates slip. This time, the slippage occurred over a stretch of the juncture about 400 miles long.

The size of a tsunami depends on how that slippage occurs, said Solomon Yim, also a tsunami expert at Oregon State. If the slippage is mostly horizontal and the two plates slide side to side against each other -- as typically happens along the San Andreas fault -- they won’t displace large volumes of water and there will be essentially no tsunami.

But if one plate dives under the other, which is probably what occurred early Saturday off the coast of Chile, it will displace “a huge column of water,” Yim said.

That is precisely what happened with the 2004 Indian Ocean earthquake that devastated parts of Indonesia.

Yim noted that researchers would not know exactly what happened seismically for a few days, when they can send scanners to look at the ocean floor and determine how it was reshaped.

A wave created by an earthquake can travel across the ocean at 450 to 600 mph, about 10 to 100 times as fast as a wind-driven wave. In deep water, the wave will only be a few inches to a foot high.

Buoys spaced across the ocean monitor these small increases in ocean height so researchers can track the wave’s progress and predict when it will strike land. But they provide little or no information about magnitude.

When the tsunami reaches shallower water, the wave slows down, the energy is compressed and the wave can get much higher. A large tsunami could be 35 to 40 feet high and move at a speed of 35 mph.

Topography plays a big role in how high the wave will grow. Hilo harbor “attracts wave energy,” Yim said, because the headlands on both sides of the bay tend to focus the incoming energy. That is why there is generally good surfing there. “If not for the headlands, [the tsunami] would have been even smaller,” he said.

Underwater geography off the coast of Japan is capable of focusing the energy even more, and experts there were predicting waves of up to 9 feet a full day after the Chile quake. Initial waves that hit outlying islands, however, were not threatening.

thomas.maugh@ latimes.com