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September 18, 2004
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Sinking City - Low-lying, coastal cities around the world are in a battle with nature that threatens to wash them away. (12/19/02)

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Pacific Tsunami Museum

The next big earthquake… : Information about earthquakes and tsunamis from the University of Alaska - Fairbanks

Tsunami Community: This web site, funded by the US National Science Foundation, represents the breadth of research and knowledge in the tsunami community for scientists, policy makers, and disaster managers.

The WWW Tsunami Information Resource

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It's a disaster that can kill thousands of people without warning.

But as this ScienCentral News video reports, researchers studying the movement of water are now creating better ways to predict where and how tsunamis can affect us.

Modeling Waves

In movies like Paramount’s “Deep Impact” and MGM’s “Meteor”, a huge wave caused by a comet or meteor crashing into the ocean wipes out entire cities and kills millions of people. But is such a wave possible?

In real life, a wave like that might be called a tsunami. Tsunamis (pronounced: "SOO-nah-MEEs") are usually caused by earthquakes, explains Stephan Grilli, a hydrodynamicist in the Department of Ocean Engineering at the University of Rhode Island. Grilli, who studies the movement of water, says, “Earthquakes shake the ocean bottom—move it up and down—and that produces waves on the surface. But earthquakes can also produce underwater landslides by shaking sediment—loosening up sediment…. Those underwater landslides can produce waves on the surface, and those waves, if they occur very close to shore, can be even more damaging" than waves caused by earthquakes.

The word “tsunami” is Japanese for “harbor wave”, because of the devastating effects of these waves on low-lying Japanese coastal areas. But because tsunamis occur most often in earthquake-prone regions, many areas other than Japan are also at risk. For instance, tsunamis occur in the Pacific Ocean, along the coasts of Asia, Japan, and North and South America. “These are regions with very frequent earthquakes, and therefore very frequent tsunamis,” says Grilli.

Not only are many areas at risk for tsunamis, but the waves occur far more frequently than one might think. According to Grilli, “Many small tsunamis are occurring on any given day, but very few of those will be damaging.”

In order to forecast the effects of future tsunamis, Grilli created a computer program that predicts what a landslide-generated tsunami would look like in particular conditions. He says, “The computer model actually gives us numbers that correspond to the motion, the speed, the elevation of a tsunami.”

The computer model, which took many years to develop, can be used to study both past and future tsunamis. “A computer model can help us simulate tsunamis in many situations," he says. "So we can actually reproduce historical tsunamis, as well as [simulate] future tsunamis that could happen in an area where the potential for a landslide has been identified.”

But how does Grilli know his computer model is accurate? He uses real data from two different sources: One is a wave tank, and the other is history.

“The wave tank is like a large pool in which we can produce all sorts of waves,” says Grilli. In fact, he created a scale model of a landslide—a device that he calls the “flying saucer”—that slides down a slope in his wave tank. That sliding motion produces waves on the surface that look and act just like a miniature tsunami. By measuring these small tsunamis and comparing those numbers to his computer data, he verifies the predictions of his computer model.

Grilli’s use of historical accounts of tsunamis also helps verify his computer predictions—and at the same time helps him investigate how they occurred. For example, he and some of his colleagues have studied a tragic tsunami that occurred in Papua New Guinea in 1998. In that event, about 15 minutes after an average size earthquake occurred, very large waves hit the country’s northern coast. This tsunami killed more than 2000 people, destroyed three villages, damaged 4 others, and left 12,000 people homeless. The waves were much bigger and they occurred later than what was expected for such an earthquake. So what caused the tsunami? Researchers have been searching for the answer for many years.

Grilli says, “Looking at waves, we can tell by experience where the source was likely to be located, and it was about 40 miles offshore.” Additionally, underwater sound recordings taken during that time revealed some rumblings that could have been caused by a landslide. So some of Grilli's colleagues created sonar images of the ocean bottom in that area and found evidence of a landslide. Using that data, soil samples, and his computer model, Grilli and his colleagues were able to piece together what happened. Grilli says the earthquake “produced a sizable landslide about 40 miles off the coast, and about 15 minutes later, very large waves reached the shore, and those were about 50 feet tall.”

How did this help Grilli verify the accuracy of his model? Grilli’s simulation of the tsunami matched the actual data—the size of the waves, the direction that they traveled, and the timing between the landslide and the waves hitting the coast—better than other computer models.

Grilli hopes that in the future he and others can use his computer model to help prevent such loss of life or the destruction of property that occurs. He has made his computer program available to the public via the Internet so others can use it to assess the risk from landslide-generated tsunamis in different areas. Grilli hopes the information can be used to warn people, and perhaps help prevent destruction from tsunamis. For example, offshore barriers could be built that could block or break up an incoming tsunami before it reaches shore; or underwater sensors could be used in an advanced warning system.

Grilli’s research appeared in Engineering Analysis with Boundary Elements, and is funded by the National Science Foundation.

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