Beijing  - The devastating Sichuan earthquake that killed more than 80,000 people in May might have been partly caused by a huge dam and reservoir, a Chinese expert said.

"The likelihood is very big," said Fan Xiao, chief engineer of the investigation team of the geological office of Sichuan province.

The waters of Zipingpu reservoir, which reach depths of more than 120 metres, put too much pressure on the geological faults underneath, Fan said. Furthermore, water entering crevices was weakening the rock layers, he said.

"The tension is lowered, and the fault gets active sooner, which makes an earthquake happen faster," he said in a telephone interview.

Around the world, a number of quakes were caused by dams, experts said. Owing to Zipingpu's size, holding 1.12 billion cubic metres of water, the likelihood that the reservoir could trigger a quake is estimated around 30 to 40 per cent.

While the case needed further examination, "the possibility was very big, according to preliminary results," Fan said.

The danger is not at its height, however, when the reservoir is bursting full. "It often happens when the water rises or falls quickly," Fan said.

The reservoir on the Min River filled over the course of two years, but water levels dropped drastically just before May 12. On that tragic day, the earth shook along the nearby Longmenshan fault zone stronger than ever before, reaching magnitude 8 on the Richer scale.

Whole cities and villages were destroyed, and tens of thousands of people were buried under the rubble of their homes. The quake was even felt in Beijing, 1,500 kilometres away.

Geophysical hazards scientist Christian Klose of Columbia University in New York said he believes the reservoir's waters increased the strain on the earth's crust 25 times more than the normal tectonic activities.

"Different geophysical observations show that the quake was triggered by changes in tension on the earth's surface," Klose said, summarizing his analysis that is soon to be published.

"The changes in tension were induced by the waters in the Min River valley," he concluded.

When the fault ruptured, it moved in the direction in which it was pushed by the pressure emanating from the reservoir.

While the analyses of other Chinese geologists also hinted at the reservoir as a cause of the quake, the thesis that the catastrophe was manmade bears great risks in China, not last because of political responsibilities.

Therefore, Fan has been alone in his demands of closer scrutiny of the causes. Newspapers reporting on his findings have been criticized by China's censors, and state-controlled media later only quoted scientists disagreeing with his theory.

Fan criticized that necessary data for analysing the reservoir's effects had been suppressed.

"The data are not released," he said. "The data should be offered to different scientific departments to share. This should be open. But until now it is not."

He saw China's powerful energy sector - which pushed ahead with the construction of Zipingpu and other dams in seismic active south-western China despite concerns - as being behind the stonewalling.

"Many people oppose my ideas, mainly from water-resources and hydropower-engineering departments", he said.

Even when the dam was constructed, the dangers of an earthquake were not taken seriously.

"In the calculation, the risk was described as lower, whether by accident or on purpose," Fan said.

To receive a construction permit, the maximum strength of a quake for the dam to withstand was put at 5.5 despite the fact that quakes with magnitude 6 had occurred there in the past, he said.

"We think they made this out of their own interest, not from a authentically objective point of view," Fan said.

After the quake, the dam showed dangerous cracks, and probably only its low water levels prevented a catastrophic flooding of the 600,000-inhabitant town of Dujianyan a short distance downriver.

According to official information, the earthquake damaged 2,830 - mostly small - reservoirs in the region, but in Sichuan alone, at least 69 dams were in danger of breaking.

As China continues to put its focus on hydropower, improved knowledge about the connections between reservoirs and earthquakes would be an important step to adapt running projects and ensure the safety of future ones. (dpa)