The world’s earthquake experts have identified tectonic plate movements as the cause of this week’s earthquake in southwestern China. But the question now is did the filling of the massive Three Gorges reservoir, which reaches the southeastern part of the Sichuan Basin, trigger seismic activity in what has always been an earthquake-prone region?

Engineers have already linked the massive weight of water behind the Three Gorges dam to increased seismic activity since its filling began in 2003.

“Whether reservoir-induced seismicity is behind this week’s earthquake should be urgently investigated before the Three Gorges reservoir is filled to its maximum height,” says Patricia Adams, executive director of Probe International, a Canadian group monitoring the Three Gorges dam since the 1980s.

A recent article in Scientific American, explained that the reservoir sits on two major faults: the Jiuwanxi and the Zigui–Badong. According to Fan Xiao, a geologist at the Bureau of Geological Exploration and Exploitation of Mineral Resources in Sichuan province, changing the water level will strain the fault lines.

“When you alter the fault line’s mechanical state it can cause fault activity to intensify and induce earthquakes,” he said.

Engineers in China blame dams for at least 19 earthquakes over the past five decades, ranging from small tremors to one near Guangdong province’s Xinfengjiang dam in 1962 that registered magnitude 6.1 on the Richter scale—severe enough to topple houses.

Since the reservoir began to fill in 2003, the Three Gorges dam has induced seismic activity within the reservoir area. The respected 21st Century Business Herald reported in 2003 that, according to Xu Guangbin, director of the Hubei Seismological Monitoring and Prevention Centre, “as many as 1,000 micro-earthquakes have occurred in the Three Gorges reservoir area since June 7, with the biggest recorded at 2.1 on the Richter scale,” but that “these minor tremors have had no significant impact on the dam or reservoir, and have caused no damage.”

Three Gorges officials, meanwhile, insist the dam is designed to withstand earthquakes of magnitude 7 and poses no danger to the millions of people living close by and downstream. (Monday’s deadly quake registered a 7.9 at its epicentre.)

Mr. Xu said the 2003 seismic activity was concentrated in the Badong area, about 80 kilometres upstream of the Three Gorges dam. However, as the reservoir rises, more frequent and bigger shocks could be anticipated, the 21st Century Business Herald report said.

And there were more shocks. In October 2006, just as the reservoir reached 156 metres above sea level, the strongest earthquake to hit China’s Hubei province in two decades shook an area near the Three Gorges dam reaching a magnitude 4.7. The tremor damaged thousands of houses and forced 5,860 people to leave their homes. The quake was centred in Suizhou City’s Sanligang township, 200 kilometres northeast of the Three Gorges project, but it rocked buildings in Yichang City near the dam.

For seven months after the reservoir-level increase in 2006, Chinese Academy of Engineering scholar Li Wangping says the Three Gorges area registered 822 tremors. So far, none have been severe enough to cause serious damage. But, later this year when the dam’s water level is set to rise to its full 175-metre capacity, the increase in water pressure, in water fluctuation and in land covered by the reservoir, Fan says, makes for a “very large possibility” that the tremors will worsen.

Back in 1990, U.S. engineer Philip Williams alerted the world to the fact that the official feasibility study for the Three Gorges dam underestimated the risk of reservoir-induced seismicity (RIS) in Damming the Three Gorges: What Dam Builders Don’t Want You To Know [PDF] (Probe International).

Chinese scientists Li Ping and Li Yuanjun, both civil engineering professors at Wuhan University, believe the seismicity of the Three Gorges reservoir area is a cause for concern and were calling for more resources to be put into investigating the region’s seismic problems as early as 2003.

Their research focuses on two major zones of seismic activity, the Enshi-Badong and Xiannushan faults.

“We are particularly concerned that both of these strong seismic fault lines lie near the dam site, passing beneath the reservoir,” they wrote.

“A medium or strong earthquake would set off a chain of events in the reservoir area, with a series of landslides and riverbank collapses being triggered near the epicentre. The consequences could be dreadful to contemplate, quite unimaginable in fact.

“One of the most pressing issues now is to gain a thorough understanding of the relationship between seismic activity and other geological disasters, and then to determine key technical parameters for prevention and control projects … equally important, further studies are needed to explore whether there are more strong seismic faults in the reservoir area between Badong and Chongqing.

“In the past two years, the government has invested four billion yuan [US$485 million] in projects to prevent and control geological disasters in the Three Gorges area,” they wrote, but argue that more funds and personnel are urgently needed to undertake vital seismic research in the region.

For further information see:

http://earthquake.usgs.gov/eqcenter/eqinthenews/2008/us2008ryan/#scitech