With a series of droughts and floods in recent years, the large-scale Three Gorges Dam in China has filled up and drained out faster than it was designed to do. In the process, it may have helped to prevent floods downstream, though not all observers agree on this. At the same time, the rapid changes in water level have showed that the biggest concern about the dam, that it could trigger earthquakes, was not such an exaggeration as the central government had suggested beforehand.
It is not that the weight of the water in the reservoir has triggered a damaging earthquake — possibly it has, but it’s hard to say so with any confidence. However, Chinese government scientists studying the dam issued a report last month counting some 3,000 small earthquakes that most likely are reservoir-related. An environmental group that translated the study said this in a press release:
"This represents a 30-fold increase in frequency over the pre-dam period," according to Patricia Adams, executive director of Toronto-based Probe International and English editor of the translated study. "The earthquake activity especially increases when the dam operators rapidly increase or decrease the level of water in the reservoir."
Most of the microquakes mentioned in the study have been less than 2.9 on the Richter scale, but the largest was 4.1, large enough to notice. Where small earthquakes occur, large ones are also likely, if not as frequent, and scientists believe the dam-related quakes have probably contributed to the frequent mudslides that have killed thousands of people in the surrounding area in recent years. The reservoir water itself, though, caused more of the landslides.
The worst-case scenario is if the reservoir triggers an earthquake that damages the dam. The dam is engineered to withstand a magnitude 7.0 earthquake, in an area that has regularly produced earthquakes with a magnitude up to 6.0. Those quakes occurred without the added pressure of a reservoir, so there is not much margin for error. But the dam operator and the central government are financially committed to the Three Gorges Dam now, so until there is a much more obvious sign of trouble than what we have seen so far, they will have to keep the dam operating.