Giant Chinese Dam May Cause Earth to Move
In a project reminiscent of the Great Wall, the Chinese are building another of the world's largest structures. When they finish the Three Gorges Dam, the Chinese will have built a wall across the third largest river in the world, created a reservoir almost 300 miles long, and tapped an electrical source equal to 18 nuclear power plants.
Water held by the dam also may trigger earthquakes that could threaten millions of people. Two scientists at the Geophysical Institute are working together to help the Chinese assess the earthquake risk of Three Gorges Dam. Born in the mountains of the Tibetan Plateau, the Yangtzi River flows almost 4,000 miles to the ocean, making it the third longest river in the world after the Nile and the Amazon (the Yukon is half the length of the Yangtzi). Hoping to harness the power of the river, the Chinese government began building the dam a few years ago, expecting to finish by 2009. When the mile-wide, 600-foot high dam is complete, the flooding upstream will begin. As the water rises, it will drown more than 1,400 rural towns and villages abandoned earlier by government decree. The water rising behind the dam will power 26 huge turbines to provide electricity, and will allow people to control a river that has killed 300,000 people by flooding during the 20th century.
No one knows how local seismic faults will react to the incredible mass of water behind Three Gorges Dam. Like heavy snow on an overloaded roof, the weight of water blocked by dams can cause existing cracks in Earth's crust to slip, resulting in earthquakes. Faults tend to slip more often when a nearby giant reservoir is filled with water. The largest was a magnitude 6.5 triggered by the Konya reservoir in Turkey. That earthquake killed 200 people in December 1967.
NASA funded Jeff Freymueller and Shusun Li of the Geophysical Institute to help the Chinese determine the seismic risk of Three Gorges Dam. Millions of people downstream from the dam are at risk should an earthquake damage or destroy it. "A catastrophic failure of the dam would be perhaps the single most destructive event in human history," said Freymueller, a professor of geophysics. He added that the chances of the dam being destroyed by an earthquake are small, but because the consequences are so severe any seismic activity induced by the reservoir has to be taken seriously.
Freymueller and Li will study movement of Earth's crust around the dam site before and after the dam is built. Freymueller will use global positioning satellites, the same tool he used to determine that Seward and Homer are creeping in opposite directions by a few centimeters a year. GPS receivers placed at various points in the Yangtzi River basin by Chinese scientists will tell Freymueller how much the ground has subsided due to the weight of the reservoir. Li, a professor of remote sensing, will use a different type of satellite to view the river basin. A synthetic aperture radar satellite sends radio pulses to the ground and records the time it takes them to return. The satellite will allow scientists to get a wider view than that allowed by the GPS receivers on the ground. Combining the technologies may allow Freymueller and Li to measure the sinkage of the new river basin, which will probably be less than 20 centimeters, about the length of a man's hand.
Both researchers hope their findings on ground subsidence around the dam will allow an accurate assessment of how seismic faults will react to the load of water. Li has added incentive to find the seismic effects of the Three Gorges Dam. His hometown, Shanghai, is downriver.
