Journal: News & Comment

Sunday, December 26, 2004
# 6:43:00 PM:

Tsunami info

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NOTE: I've combined all my various writings about tsunamis from December 2004 and January 2005, including this entry, into a single tsunami article, which might make a good introductory reference.

About 15 years ago, during an oceanography course that was part of my marine biology degree, I wrote a paper on tsunamis—huge surface waves created in the ocean, usually by earthquakes or other seismic events. Tsunamis are often far more dangerous than the earthquakes that generate them, and today's widespread Indian Ocean disaster shows why.

A magnitude 8.9 or 9.0 earthquake is massive on its own: this morning's event was the fourth largest ever recorded. Tsunamis aside, it killed hundreds, perhaps thousands, of people on the northwest coast of Sumatra as structures collapsed. Nothing has shaken the earth's crust as hard since 1964, when a 9.2 quake took place in Prince William Sound in Alaska. Four years earlier than that, the largest quake known, of magnitude 9.5, hit Chile.

But tsunamis resulting from such earthquakes are, in many respects, the worse danger, because while even a huge earthquake is a local event, if it generates a tsunami, the effects can spread more than half way around the world. The 1960 Chile quake caused lethal tsunamis along the coast of South America, in Hawaii, and as far away as Japan; the 1964 Prince William Sound event generated waves that killed people (though not nearly as many) and did extensive damage down the west coast of North America, from Alaska to Port Alberni to the Oregon Coast and California. For most of today's victims, there was little or no warning of the waves, either because there was no time or because there is no coordinating tsunami warning system for the Indian Ocean,.

Think of yourself sitting in a bathtub. If you suddenly lift or drop your body, a large wave sloshes along the tub, and might spill over the rim, even if the tub is far from full. A tsunami is essentially the same thing, with an earthquake or other event moving the ocean floor up or down and displacing titanic quanitities of water. In a bathtub, you might move a couple of dozen litres of water with such a slosh, but sea floor movements have in the past displaced more than 100 cubic kilometres of water—billions of bathtubs.

The physics of water waves means that (in general) the longer the wavelength, the faster the wave travels in the open sea. While the length of typical beach waves might be a few dozen or a few hundred metres, a tsunami might be 100 km long. Away from shore, ocean waves don't actually move water very far either, which is why ships, logs, seabirds, and anything else floating can bob up and down in even a large sea swell and not travel anywhere if there isn't a wind or current. Tsunamis are the same: since it doesn't push water ahead of it as it crosses the ocean, the energy of a tsunami wave can travel at frightening speeds—up to 800 km/h, which is as fast as a jet plane.

In the middle of the ocean, a tsunami might also be very shallow, perhaps a metre or less in height—you'd never notice it among all the other, taller swells unless you had a sea-level gauge to tell you it was passing by. But as it approaches shore and the ocean floor squeezes it (like typical surf waves), a tsunami gets shorter and taller. With its stupendous energy, a tsunami can get very tall indeed, depending on the shape of the bottom. Today's earthquake generated tsunamis 10 m (30 feet) high in some places; some tsunami waves from the 1964 Alaska quake reached 24 m (80 feet).

The stereotype of a tsunami is of a wall of water crashing down, like the giant waves surfers ride off the north coast of Oahu. Some are like that—but they also come ashore and don't retreat right away, driving roiling rivers of swirling water far inland. Other times, they act more like rising tides that just keep rising, or like rivers overspilling their banks. Often, as the wave approaches, the waterline will retreat much farther from shore than usual, and some people who die are exploring the rarely-exposed sea floor there when the big wave comes in. Tsunamis can also oscillate, so that one wave will come in, destroy things, then retreat. As residents return to begin cleaning up, another wave might arrive, perhaps bigger and more violent than the first, and maybe even hours later.

Most disheartening, tsunamis are beyond human control, and sometimes even prediction, more than nearly anything. They are not a consequence of the weather, or climate change, or any kind of human activity—or of any living thing. They're not even affected by the sun or moon. Earthquakes and tsunamis are simply the result of this humungous planet cracking and popping as it goes about its business, as it has for billions of years, and will for billions more.


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