Just minutes after a pair of powerful earthquakes jolted China’s northwest Gansu province in late October, workers raced to release massive amounts of water stored behind two damaged dams.

The tremors, measuring 6.1 and 5.8 on the Richter scale, hit the Zhangye region at 8:41 p.m. and 8:48 p.m. on Oct. 25. By 9 p.m., water was already being discharged from the two reservoirs in Minle county.

The quakes “threatened the safety of two dams,” China Daily reported. Cracks five centimetres wide had opened in the walls of the Shuangshuzhi reservoir, while the Zhaizhaizi reservoir had developed a fissure one centimetre wide and 410 metres long, the newspaper reported.

Tuo Xingfu, head of Minle county, said water was drained from the reservoirs over the course of the next two days, reducing the stored amounts to safe levels. He added that a third reservoir, in Nangu township, had also sustained serious damage.

Ten people and 16,000 head of livestock died in the quakes. More than 14,000 houses collapsed and 45,000 others were damaged, rendering thousands homeless in freezing temperatures. People huddled overnight inside wheat stacks until tents and other emergency aid could reach the area.

It was a horrendous event. But many lives were no doubt saved by the dam authorities’ swift response, which may well have averted a much worse catastrophe.

How safe is the Three Gorges dam, which has been constructed in another of China’s seismically active regions? The issue was addressed wholly inadequately in the Canadian-funded feasibility study for the dam, U.S. hydrologist Philip Williams concluded after reviewing that study. (His critique is contained in the Probe International/Earthscan book Damming the Three Gorges: What Dam Builders Don’t Want You To Know.)

Lianziyan, Zigui county: One of the most dangerous potential landslide sites in the Three Gorges reservoir area

Around the world, earthquakes have damaged dams, and have also 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.

“Earthquakes caused by big reservoirs are not unprecedented,” Xin Zhiguo, a professor at the Chongqing Environmental Science Research Institute, was quoted as saying in July. “There have been 102 induced earthquakes throughout the world, and China has suffered 16 of them. Therefore, this is a very important problem.”

As many as 1,000 micro-quakes occurred in the Three Gorges area in early June while the reservoir was being filled to the 135-metre level, Xu Guangbin, director of the Hubei Seismological Monitoring and Prevention Centre, told 21st Century Economic Report (Ershiyi shiji jingji baodao).

“These minor tremors have had no significant impact on the dam or reservoir, and have caused no damage,” Mr. Xu was quoted as saying. “It’s normal and to be expected because of the filling of the reservoir.”

Bigger quakes, with magnitudes of 6 to 6.5, are to be expected once the reservoir is filled to its final level of 175 metres in 2009, the newspaper said, adding that these tremors should pose no risk to the dam structures, which are designed to withstand earthquakes of magnitude 7.

Despite these assurances, experts are worried that strong quakes could have disastrous consequences in the geologically fragile Three Gorges region. For instance, an earthquake could destabilize old landslides and send masses of rock and mud cascading down onto towns or crashing into the reservoir. (In July, a huge landslide fell into the Qinggan River in Zigui county, killing at least two dozen people and completely blocking that Yangtze tributary. Explosives had to be used a week later to clear the river for navigation.)

Li Ping and Li Yuanjun, both civil engineering professors at Wuhan University, believe the seismicity of the Three Gorges reservoir area is a real cause for concern. Writing on the website of the Chinese Academy of Engineering, of which Li Ping is a member, they call for more resources to be put into investigating the region’s seismic problems.

Riverbank-strengthening project in Badong county

Their own 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 write.

“The north end of the Enshi-Badong fault extends as far as the new county seat of Badong, while the Xiannushan fault is located just 10 kilometres upstream of the dam. 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. We therefore urge the government and project authorities to pay more attention to this issue.”

The authors write that a magnitude-5.2 earthquake occurred along the Xiannushan fault on March 8, 1961, and another, measuring 5.1, took place along the Enshi-Badong fault on May 22, 1979. The epicentre of the latter was just 10 km from the site of the new county seat of Badong, 80 km upstream of the dam. They say that an even stronger quake, probably between magnitude 6 and 7, was recorded in 1856 in the Daluba area at the south end of the Enshi-Badong fault, on the border between Chongqing municipality and Hubei province.

“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,” they write. “Equally important, further studies are needed to explore whether there are more strong seismic faults in the reservoir area between Badong and Chongqing.”

The authors cite official statistics that categorize 2,490 locations in the Three Gorges area as threatened by landslides or riverbank collapses. Before the reservoir was filled in June, residents were moved away from 232 of the most dangerous places, while remedial work was undertaken at 198 sites.

“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 write, but argue that more funds and personnel are urgently needed to undertake vital seismic research in the region.