Pressure on a fault line caused by water amassed in Zipingpu dam's reservoir in the southwestern province of Sichuan may have caused the disaster that killed and left missing 87,000 people, some Chinese researchers say.

Fan Xiao, 54, a chief engineer at the government-run Sichuan Geology and Mineral Bureau for the past 14 years, is one of the theory's proponents.

"The Zipingpu reservoir was built right on the earthquake fault area, so it was very easy for Zipingpu to have had an impact on the fault," Fan told AFP Thursday.

The phenomenon, well known within the science community, goes by the name of "reservoir induced seismicity" and reservoirs in several parts of the world have caused smaller scale tremors.

But if true in the case of the Sichuan earthquake, this would be the first time that a reservoir caused a large scale, 8.0-magnitude tremor.

Zipingpu, a 156-metre-high dam finished in 2006, and its reservoir, which can store up to 1.1 billion cubic metres (38.5 billion cubic feet) of water, is located just five kilometres (3.1 miles) from the quake's epicentre.

Fan said the location was an important factor, as was the fact that the huge tremor happened at a key moment for the reservoir when its water level was falling at a rapid pace.

"The most dangerous period (for reservoir-induced quakes) is after the water level in a reservoir has reached its highest point, and it changes and starts going down," he said.

That sudden change can greatly destabilise a fault, according to Fan.

"And Zipingpu's water level started to change and go down rapidly just before the earthquake happened."

Lei Xinglin, a geophysicist at the government's China Earthquake Administration, also published a report in December saying the process of storing water in Zipingpu had an impact on faultlines in the area.

However other experts in China have rejected the theory, insisting the earthquake was an entirely natural phenomenon.

Wu Faquan, a researcher at the Institute of Geology and Geophysics at the Chinese Academy of Sciences, another government-run body, said the quake was triggered by natural underground forces.

"After several studies and research, the majority of Chinese scientists have concluded that the earthquake was mainly triggered by the earth movements," he told AFP.

Pan Jiazheng, a well-known hydraulic engineer involved in the Three Gorges Dam project, also rejected the theory in an article published by Science Times, a Chinese magazine, in December.

"There has never before been a case of a reservoir triggering an 8.0-magnitude earthquake in the world," Pan said in the article.

So far, there have been at least four earthquakes of magnitude six or above in the world that have been widely recognised as having been triggered by a reservoir, including one in the southern Chinese province of Guangdong.

But Fan argued that these were in areas where previous seismic activity had been much lower, and that the area around the Zipingpu dam had already experienced seismic activity of 6.5 magnitude.

"So because previous seismic activity in the area was so strong, it (Zipingpu) could have induced an even stronger tremor," he said.

Fan said quake prevention should be a top priority when repairing some of the many dams that had been damaged by the Sichuan quake, and some should not even be re-built.

"But some are already being re-built, and the likelihood of stopping that is slim," he said.

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