The weight of the water in a man-made reservoir may have triggered the massive earthquake that struck China in May killing more than 70,000 people.

Immediately following the quake, scientists began to suspect the Zipingpu Dam may have set off the magnitude 7.9 jolt. Now evidence is starting to emerge that suggests they may have been right.

The 500-foot tall Zipingpu Dam, built to generate hydroelectric power to support growth in the area, holds more than 300 million tons of water behind it and is located less than a third of a mile from the fault that ruptured and 3.4 miles from the epicenter of the Wenchuan earthquake. The added weight of the reservoir may have changed the stress in the area just enough to set off an earthquake that was waiting to happen.

"It's like a house of cards swaying in the wind, and you touch it with a feather and it falls down," said geologist David Schwartz of the U.S. Geological Survey. "It was going to happen anyway."

Earthquakes that are ready to go can be triggered by many different things such as mine activity, or injection of fluid into an oil well to push out the oil. Recently there has been a lot of discussion among geologists about the possibility of tidal triggering of quakes.

Most earth scientists agree that reservoir-triggered earthquakes do happen, but it is hard to definitively point the finger in any given case. And most of these triggered quakes are much smaller, usually less than magnitude 5. One of the largest was a magnitude 6.3 in 1967 thought to be triggered by the Koyna Dam in India.

It would be much more unusual for an earthquake the size of the Wenchuan quake, which ruptured around 175 miles of the fault, to be affected by a reservoir.

Geophysicist Christian Klose of Columbia University recently estimated the stress caused by the weight of the water to be equivalent to 25 years' worth of natural stress buildup on the fault, according to a news report in Science, though this amount is minor in comparison to the natural stress that would have built up over thousands of years since the last major quake on the fault.

But if the stress had built up to the point that a major earthquake was imminent, it is possible that the extra stress from the reservoir was just enough to spark the quake.

"The weight of these reservoirs themselves is insufficient to cause an earthquake," said geophysicist Rob van der Hilst of MIT who has been studying the Wenchuan area for two decades. "It could be that the stress field is perturbed by the reservoir, but how exactly it translates into the onset of an earthquake, we just don't know."

The potential influence of the Zipingpu Dam on the earthquake is clearly a touchy subject. Scientists from the Chinese Academy of Sciences initially dismissed the possibility saying that the quake was far too large to have been triggered by the reservoir, and that if anything, the water helped dampen the energy released during the quake.

They claimed that the frequency and size of small earthquakes in the area was unchanged by the filling of the reservoir between 2005 and April 2008, indicating that the dam was not affecting stress in the area.

But a study published in a Chinese journal last month concludes that the reservoir did affect the local seismicity.

It's unclear whether the Chinese were aware of the fault. Most of the earthquakes in China are ultimately the result of the Indian tectonic plate pushing northward into the Eurasian plate. The fault that ruptured in May is a major boundary between the Sichuan Basin and the Tibetan Plateau. More than 2.5 miles of uplift has occurred along the fault over the last 15 million years as the plateau pushed up above the basin. During the Wenchuan quake, more than 30 feet of slip occurred on the fault in some areas.

"I'm sure we wouldn't have built that dam [in the United States] on or so close to an active fault," Schwartz said. "Would you build a dam across the San Andreas or the Hayward fault? No."

But even if the Chinese were completely aware of the fault, the spot where the Zipingpu Dam was built may have looked like a relatively safe bet in the area. Because the average time between major quakes on that fault is several thousand years, the seismic hazard would have been considered low. Certainly it was lower than other nearby areas.

"There is a lot of concern because it is a very sensitive, almost political issue," van der Hilst said. Hydroelectric power is a very important component of China's development strategy for the area, but "if the reservoir can kill 80,000 people, then of course there will be a major uproar which would make dams harder to build."

The bulk of the scientific community outside of China has not seen the seismic data from the Wenchuan area, and until that happens, questions will remain.

"People will probably start to clamor for the data," Schwartz said.

Van der Hilst's team deployed seismometers in the area from 2003 to 2004, and their research prompted the China Earthquake Administration to install some 300 seismic stations in the area in 2006.

"Now there is an enormous amount of valuable information about the earthquake," van der Hilst said.

The data could also reveal a lot about reservoir-induced seismicity in general, but the Chinese are keeping that information close for the moment. Just three weeks before the earthquake, van der Hilst was in Beijing hammering out an arrangement to work on data from the Chinese seismic array. Now he is one of the few who has access to it.

"We are encouraging them to share it with everyone," he said. "In China that's a very slow process. I am one of the lucky ones."

Critical will be an analysis of the pattern of earthquakes before and after the building of the dam, and as the reservoir level was raised and then as it dropped slightly in the week before the quake — a situation suspected of playing a part in reservoir triggering.

"But that is data I haven't seen," van der Hilst said.

"If this reservoir could have triggered it, it's probably a sensitive subject," he said. "But of course they want to know, so there will be a lot of research."

*Images: 1) Vincent Yu/Associated Press. 2) Geoeye *

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