Why a Dam May Have Been the Cause of China's Deadly Earthquake Last Year

Last year's devastating Sichuan earthquake, which took at least 69,000 lives, may have been unleashed by the huge Zipingpu Dam. New scientific evidence links the impoundment of the Zipingpu reservoir to the activation of a fault line near the dam site. A thorough scientific assessment is needed before China builds more dams in earthquake-prone areas.

It is well established that large dams can trigger earthquakes through what is called reservoir–induced seismicity. There is evidence linking earth tremors and the raising and lowering of reservoirs for more than 70 dams. Reservoirs can both increase the frequency of earthquakes in areas of already high seismic activity and cause earthquakes to happen in areas previously thought to be seismically inactive.

Zipingpu is a 156-meter-high dam on the Min River, a tributary of the Yangtze. The project, which displaced 33,000 people, was completed with Japanese funding in 2006. Fan Xiao, a chief engineer with the Sichuan Geology and Mineral Bureau, had warned about Zipingpu's seismic risks since before the dam was completed. After the disaster, he explained that "Zipingpu has all conditions that provoke reservoir-induced earthquakes," and said that "we cannot rule out the possibility that building the Zipingpu Dam induced the earthquake because the epicenter is so close to the dam." (Fan's interview with the South Urban Daily, like other useful documents on the topic, has been translated by Three Gorges Probe.)

Geophysical hazards researcher Christian Klose of Columbia University's Lamont-Doherty Earth Observatory has found that the fault line that triggered the Sichuan quake had not been active for millions of years. Klose presented his research at a meeting of the American Geophysical Union in San Francisco in December. According to Klose, "the ensemble of geophysical observations suggests that the root cause of triggering the M7.9 Wenchuan earthquake may have stemmed from local and rapid mass changes on the surface." As a news article on Klose's findings in the January 16 issue of Science elaborates, "the added weight [of the Zipingpu reservoir] both eased the squeeze on the fault, weakening it, and increased the stress tending to rupture the fault. The effect was 25 times that of a year's worth of natural stress loading from tectonic motions. (…) When the fault did finally rupture, it moved just the way the reservoir loading had encouraged it to."

In a recent paper in the Chinese journal Geology and Seismology, Lei Xinglin, a geophysicist at the China Earthquake Administration in Beijing, and four colleagues produced further evidence for the seismic impacts of the Zipingpu Dam. According to the paper, "some clear correlations were verified between the local seismicity and stress change, thus we concluded that the impoundment of Zipingpu clearly affected the local seismicity and it is worthwhile to further study if the effect played a role in triggering the Wenchuan earthquake."

More research into the cause of the Wenchuan earthquake is needed. What is clear is that the tremor almost broke Zipingpu and other dams. During the quake, the fault line slipped up to seven meters upward. In a story in New Scientist, Fred Pearce describes that "as the tight valley sides juddered, the [Zipingpu Dam] structure was squeezed and ended up to 18 centimeters downstream, and 70 cm lower. The concrete was ripped apart but the core of the dam survived." According to China's Ministry of Water Resources, 69 dams were in danger of collapse after the earthquake in Sichuan Province alone, 310 were at "high risk," and 1,424 posed a "moderate risk."

At the time of the earthquake, the Zipingpu reservoir was half-empty. Pearce suspects that if the tremor had happened two months later during the monsoon season, "Zipingpu and the other dams would probably have failed," inundating Dujiangyan, a city of 600,000, and other downstream areas. The reservoir is now empty, awaiting repair of the dam.