BEIJING - PRESSURE from a dam, its reservoir's heavy waters weighing on geologic fault lines, may have helped trigger China's devastating earthquake last May, some scientists say, in a finding that suggests human activity played a role in the disaster.

The magnitude-7.9 quake in Sichuan province was China's worst in a generation, causing 70,000 deaths and leaving 5 million homeless.

Just 550m from the fault line and 5.5km from the epicentre stands the 156m high Zipingpu dam, the area's largest. The quake cracked Zipingpu, forcing the draining of the reservoir.

Fan Xiao, a chief engineer at the Sichuan Geology and Mineral Bureau, said on Wednesday that the immense weight of Zipingpu's waters - 315 million tonnes - likely affected the timing and magnitude of the quake. Though earthquakes are not rare in the area, one of such magnitude had not occurred for thousands of years, Fan said.

'I'm not saying the earthquake would not have happened without the dam, but the presence of the massive Zipingpu dam may have changed the size or time of the quake, thus creating a more violent quake,' Mr Fan said in a telephone interview.

Seismologists recognize that large bodies of water may exert pressure on fault lines deep in the earth, leading to earthquakes.

The pressure can push the sides of fault lines harder together increasing friction or cause the fault lines to slip apart. Scientists have recorded smaller earthquakes caused by reservoirs. A magnitude-6.4 quake near India's Koyna dam killed at least 180 people in 1967 and is thought to have been induced by the reservoir.

The Sichuan quake was much larger. Fan is among a number of experts who have voiced concerns in recent months about the likelihood that Zipingpu may have contributed to the quake. Their concerns were reported last month in Science magazine.

If proven true, the findings would sit uncomfortably with a Chinese government that has portrayed the Sichuan quake as an unavoidable natural disaster and that has promoted the building of large dams to meet energy needs and reduce flooding. The Three Gorges Dam, the world's largest hydroelectric project, was built to end flooding in the Yangtze River and provide a clean energy alternative to coal, but has instead been plagued with problems, from resettlement to landslides.

Many scientists are unconvinced that the Zipingpu dam caused the Sichuan quake, even if it may have been a factor. Lei Xinglin, a geophysicist at the government's China Earthquake Administration, said reservoirs increase seismic activity but will not cause an earthquake. He called for further investigation.

'A reservoir in the region will have positive and negative effects on a potential earthquake, but it is ridiculous to say an earthquake was caused by the dam,' Lei said. 'In order to gain more knowledge, we still need to carefully research this topic rather than jumping to conclusions.' With Zipingpu, Dr Lei said a fall in the reservoir's waters between December 2007 and the time of the earthquake and the penetration of water into the fault line were 'major factors' in the quake.

Roger Musson, a seismologist with the British Geological Survey, said at best Zipingpu may have accelerated the timing of the quake.

'But the scale of the Wenchuan earthquake (300km, of rupture) indicates that it was a true tectonic event which would have occurred with or without the Zipingpu dam,' Dr Musson said in an e-mail. 'It is thus only a question as to whether stresses from the reservoir advanced the timing of the earthquake.'

Also calling for further investigation is Christian Klose, a geophysical hazards research scientist from Columbia University in New York. An abstract of a paper he presented at the American Geophysical Union meeting in San Francisco in December said that the added weight weakened the fault below Zipingpu.

Mr Fan, the Chinese engineer, said he was so convinced of Zipingpu's potential dangers that he strongly opposed its construction in 2003, worried that a disaster would devastate the Min River valley below. He said he began pointing to the dam as a possible cause just a month after the quake.

Still many large dams continue to be built. Mr Fan said he has continued to write letters to government officials voicing concerns about dams being built on the Dadu and Jinsha rivers to the west and north-west of the quake zone. -- AP