Dams and Earthquakes

Quakes jolt Three Gorges area as huge reservoir fills

Kelly Haggart – Three Gorges Probe news service

October 29, 2006

The strongest earthquake to hit China’s Hubei province in two decades shook an area near the Three Gorges dam on Oct. 27, the same day the project’s rising reservoir reached the 2006 target of 156 metres above sea level.

State media said no one was seriously hurt in the magnitude 4.7 quake, which struck at 6:52 p.m. The tremor damaged thousands of houses and forced 5,860 people to leave their homes, the Beijing Morning Post reported. Local governments were said to be providing tents and quilts.

The quake was centred in Suizhou city’s Sanligang township, 200 kilometres (120 miles) northeast of the Three Gorges project, but it rocked buildings in Yichang city near the dam, China News Service reported.

More than 50 aftershocks were recorded over the weekend, including a magnitude 4.2 quake on Saturday.

Seismologists monitoring the situation have “ruled out” the possibility of another, stronger quake hitting the region, the official Xinhua news agency said Sunday.

Just hours before the quake struck, Xinhua announced that the Three Gorges reservoir had reached the 156-metre mark at 9:50 that morning.

The operation to raise the reservoir from 135 metres above sea level began on Sept. 20. Small tremors have jolted the area in the past few weeks, including a magnitude 3.0 quake on Oct. 15, Xinhua reported.

The Three Gorges dam is situated near two major fault lines, and project officials say the structure has been built to withstand powerful tremors.

However, the danger of impounding such a vast reservoir in a seismically active region worries experts, and has been a key argument raised against the dam. The Three Gorges reservoir will eventually stretch for 660 kilometres (400 miles) behind the dam.

Around the world, earthquakes have both damaged dams and been caused by them. “Reservoir-induced seismicity” has been recorded even in areas with no previous history of tremors.

In one such case, a reservoir in western India triggered a magnitude 6.3 quake in 1967 that killed 200 people and seriously damaged the Koyna dam.

More than 1,000 minor quakes were detected after the Three Gorges reservoir was filled to the 135-metre level in June 2003, with the biggest recorded at around magnitude 3.0, Three Gorges Corp. deputy general manager Cao Guangjing was quoted as saying by Hong Kong newspaper Wen Wei Po on Sunday.

An increase in seismic activity had been anticipated, Mr. Cao said, and he expressed confidence that the frequency of tremors in the region would return to normal in time.

Wen Wei Po also quoted Chen Yuntai, director of the Institute of Geophysics at the Chinese Earthquake Administration, as saying: “We can’t be sure that the earthquakes were caused by the filling of the reservoir, and we need to undertake advanced monitoring and analysis.”

The Three Gorges reservoir is set to rise a further 19 metres to 175 metres above sea level before the dam becomes fully operational, probably by the end of 2008.

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