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Hidden Underwater in the Ocean, Tsunamis Grow Fearsome Near Land

By Kathy Dobson


On December 26th, fishermen from Indonesia, Thailand, Southeastern India, Sri Lanka, and even Somolia may have been astonished when they returned home to their coastal villages and found them ravaged by gigantic waves. The tsunamis that had hit the coasts earlier in the day would have passed their boats undetected.

Tsunamis are sea waves that are generated by the displacement of water in the ocean. This displacement is most often caused by earthquakes, but also sometimes by landslides, volcanic eruptions, and objects from outer space.

As a tsunami moves toward the shore and the ocean becomes shallower, water displaced by the propagation of the tsunami is pushed upward, creating a large wall of water. This wall sucks in surrounding water, and often the arrival of a tsunami will be preceded by a rapidly receding tide.

The velocity of a tsunami is directly proportional to the square root of the depth of the undisturbed ocean. So, regardless of the intensity of the initial disturbance, a tsunami will always travel at the same speed for a given ocean depth. Therefore, as a tsunami approaches a coast, the speed of the wave will slow down. In the deepest parts of the ocean, tsunamis can reach 1000 km/hr (about 600 mi/hr).

A tsunami is dangerous if it carries a lot of energy when it reaches the coast, which depends on the disturbance that caused it.

Paul Whitmore, the scientist in charge at the West Coast and Alaska Tsunami Warning Center, said that there are three major ways through which waves can lose energy as they travel: friction with the sea floor or air, viscosity, and spreading. A tsunami traveling across the deep ocean usually dissipates very little energy.

However, tsunamis can lose a lot of energy when they break, said Paul Sclavounos, a professor of Ocean Engineering. This is what makes islands and other land masses with steep shelves more prone to risk, since their topography makes it less likely that an approaching tsunami would break offshore, said Raffaele Ferrari, a professor of earth, atmospheric, and planetary sciences at MIT. Also, the shape of a coast can either focus or spread the oncoming waves; a focused wave causes more devastating effects.

The December 26th tsunami was especially devastating for several reasons. The magnitude of the earthquake that caused the tsunami, at 9.0 on the Richter scale, made it one of the strongest quakes ever recorded. Hence, it generated the largest tsunami in over 40 years, according to the National Weather Service.

Since the rupture was a large crack, close to 1000 km in length, the resulting tsunami did not spread out radially, as it would have with a point source. As a result, the waves that crashed on the coasts close to the epicenter were more focused and especially strong.

Although a tsunami can be as high as 30 meters by the time it reaches the coast, its small amplitude and long wavelength in the deep ocean are such that vessels far off shore will not sense the ensuing disaster. “If you’re on a ship, you won’t even notice,” said Ferrari.

Despite the immense damage caused by the earthquake and resulting tsunami, perhaps the safest place for boats to be on the morning of December 26th was not at port, but rather near the epicenter of the quake.