Tsunamis: Japan Faces Multiple Emergencies

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  • Topic: 2004 Indian Ocean earthquake, Tsunami, Pacific Tsunami Warning Center
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Posted 17 March 2011

Devastated by tsunamis, Japan faces multiple emergencies
Japan, a world leader in earthquake engineering, has been paralyzed by a series of giant waves that followed one of the most violent earthquakes in a century. Video: Russia Today
Residents of the port town of Kamaishi in Iwate prefecture watch in horror as the first huge tsunami waves sweep away cars and buildings. Although the magnitude 9.0 quake on Mar. 11, 2011, apparently did not collapse high-rise buildings, the ensuing tsunamis flattened vast areas along the northeast coast. The death toll is swelling steadily as bodies wash in on the surf, and citizens and Japan’s Self Defense Forces scour a landscape turned upside down by inconceivably powerful waves. The news recalls the estimated 250,000 people who perished, mainly on the Indonesian island of Sumatra, in the 2004 “Christmas tsunami” that followed a huge, offshore quake. (Both Japan and Indonesia are volcanic lands in the Ring of Fire, which partly surrounds the Pacific Ocean in a giant series of subduction zones and volcanoes.) Shortly after Japan stopped shaking at 2:46 pm local time on Friday, March 11, we began hearing about troubles at a series of nuclear plants. After the reactors automatically shut down during the quake, emergency systems for removing heat still being generated in the reactors were routinely switched on. But because the electric grid was down and the standby generators were damaged — perhaps by seawater — the emergency cooling failed. By Tuesday, March 15, three reactors had exploded, a fourth was burning, radioactive material was airborne, reactor workers were being evacuated, electricity was growing short in Tokyo, and the crucial containment vessels were under severe threat if not already breached. With the first nuclear meltdowns since Chernobyl, in 1986, under way, global stock markets were crashing. ENLARGE

Photo: U.S. Navy
A helicopter flies over the city of Sendai, as it delivers more than 1,500 pounds of food donated by citizens of Ebina City, Japan, to survivors of the earthquake and tsunami. What causes tsunamis?

As Japan licks its wounds, The Why Files wants to know what causes tsunamis. How do they travel across the ocean? How they have impacted coastal people through history? Can we reduce our vulnerability to nature at its most cataclysmic?

Graphic: Anthony Liekens
Movement of the sea floor translates into waves at the surface. Tsunamis — once slangily called tidal waves — are extremely powerful waves caused by large undersea disturbances. (“Tsunami” derives from Japanese for “harbor wave,” reflecting the fact that harbors can concentrate their energy. True tidal waves are the slow oscillations that drive ocean tides in response to solar and lunar gravity.) Although landslides and volcanoes cause some tsunamis, probably 95 percent result from underwater earthquakes that contain a strong vertical motion. Such quakes often occur where one of Earth’s tectonic plates dives, or “subducts,” beneath another. Like the Sunda trench near Sumatra, the subduction zone in the Japan trench is notorious for large earthquakes, says Timothy Masterlark, an associate professor of geological science at the University of Alabama. Although the timing is always uncertain, he says, “The history was known, big earthquakes were known, and even though the people and government went to great lengths to prepare, at some level … there is simply nothing they can do.” Lessons from Sumatra

Masterlark, who has studied the giant, 2004 earthquake and tsunami in Sumatra, says the magnitude 9.0 earthquake in Japan likely broke a fault stretching at a shallow angle from the sea floor roughly 150 kilometers beneath Japan, along a trench several hundred kilometers in length. We asked Masterlark how, if the slip was mainly horizontal,...
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