| Scientists can do a very good job of tracking the | | | | as typically happens along the San Andreas fault -- |
| progression of a tsunami and predicting when it will | | | | they won't displace large volumes of water and there |
| arrive, but they have much more difficulty predicting | | | | will be essentially no tsunami. |
| how big it will be. | | | | But if one plate dives under the other, which is |
| That disparity has become apparent following the | | | | probably what occurred early Saturday off the coast |
| massive magnitude-8.8 earthquake off the coast of | | | | of Chile, it will displace "a huge column of water," Yim |
| Chile. Researchers from the Pacific Tsunami Warning | | | | said. |
| Center in Hawaii predicted to within a few minutes | | | | That is precisely what happened with the 2004 Indian |
| when the tsunami -- a massive influx of water rather | | | | Ocean earthquake that devastated parts of |
| like a flash flood -- would arrive at Hilo. They also | | | | Indonesia. |
| predicted that the wave height would range from 2 | | | | Yim noted that researchers would not know exactly |
| to 8 feet, and they were correct. But when the | | | | what happened seismically for a few days, when |
| waves proved to be at the low end of the range, | | | | they can send scanners to look at the ocean floor |
| they were widely perceived as having blown the | | | | and determine how it was reshaped. |
| prediction. | | | | A wave created by an earthquake can travel across |
| Tsunami expert Harry Yeh of Oregon State | | | | the ocean at 450 to 600 mph, about 10 to 100 times |
| University said the forecasters had to err on the side | | | | as fast as a wind-driven wave. In deep water, the |
| of caution. "Even if they knew deep in their heart | | | | wave will only be a few inches to a foot high. |
| that it probably wouldn't be large, it could be large," | | | | Buoys spaced across the ocean monitor these small |
| he said. "If they lost one life [because they | | | | increases in ocean height so researchers can track |
| underestimated the size of the tsunami], that would | | | | the wave's progress and predict when it will strike |
| be a problem." | | | | land. But they provide little or no information about |
| The biggest problem in predicting the size of a | | | | magnitude. |
| tsunami is in determining exactly how much energy | | | | When the tsunami reaches shallower water, the |
| was put into the ocean by an earthquake, and that is | | | | wave slows down, the energy is compressed and |
| something that cannot be determined in the | | | | the wave can get much higher. A large tsunami could |
| immediate aftermath of the event. | | | | be 35 to 40 feet high and move at a speed of 35 |
| Chile's 8.8 quake occurred at the junction of two | | | | mph. |
| massive tectonic plates: the South American plate, | | | | Topography plays a big role in how high the wave will |
| which sits under most of the continent, and the | | | | grow. Hilo harbor "attracts wave energy," Yim said, |
| Nazca plate, which lies under much of the Pacific | | | | because the headlands on both sides of the bay tend |
| Ocean. The Nazca plate is attempting to slip under | | | | to focus the incoming energy. That is why there is |
| the South American plate, but is largely held in place | | | | generally good surfing there. "If not for the |
| by friction. Every so often, however, the geologic | | | | headlands, [the tsunami] would have been even |
| forces overcome friction and the two plates slip. This | | | | smaller," he said. |
| time, the slippage occurred over a stretch of the | | | | Underwater geography off the coast of Japan is |
| juncture about 400 miles long. | | | | capable of focusing the energy even more, and |
| The size of a tsunami depends on how that slippage | | | | experts there were predicting waves of up to 9 feet |
| occurs, said Solomon Yim, also a tsunami expert at | | | | a full day after the Chile quake. Initial waves that hit |
| Oregon State. If the slippage is mostly horizontal and | | | | outlying islands, however, were not threatening. |
| the two plates slide side to side against each other -- | | | | |