June 1, 2011 - A study by seismologists at the China Earthquake Administration, a government agency, has confirmed that the massive Three Gorges dam on the Yangtze River has "significantly increased" seismic activity along the dam's reservoir. The study, recently obtained by Toronto-based Probe International, supports Chinese press reports and anecdotal evidence from Chinese citizens living in the vicinity of the Three Gorges Dam, and helps to explain the numerous landslides that have caused havoc in the region, necessitating the evacuation of 300,000 people. Part of the study in Probe International's possession had earlier been published in a Chinese journal.
According to the Chinese study, which Probe International scientists have translated, seismic monitors around the reservoir and in Hubei Province registered 3,429 earthquakes between June of 2003 (when inundation of the reservoir began) and December 31, 2009.
"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 quakes fell under 2.9 magnitude on the Richter scale, classifying them as "microseismic" tremors. One earthquake reached magnitude 4.1 on the Richter scale. It occurred as the dam authorities were attempting to fill the reservoir to its maximum height of 175 metres above sea level. Fan Xiao, chief engineer of the Sichuan Geology and Mineral Bureau, warns that "strong earthquakes could occur in the future as the reservoir fills because the microfractures, caused by the large number of microearthquakes, could make the area dangerously prone to a strong earthquake."
Large reservoirs are known to trigger earthquakes in a phenomenon called "Reservoir-induced Seismicity (RIS)." In a report of 19 dams in China that have suffered from RIS, 15 have geological conditions similar to Three Gorges.
Interest in RIS has grown since geoscientists began suggesting that the Zipingpu dam in Sichuan province may have triggered the deadly 2008 earthquake that killed close to 90,000 people. Researchers and citizens alike are now alert to the risk of Three Gorges, the world's largest dam, triggering an earthquake that could topple buildings. In a worst-case scenario, an earthquake could also damage the dam itself, with catastrophic consequence for the millions of people who live downstream of the project.
Chinese authorities have long dismissed such concerns, saying that a low-level seismic response to filling the reservoir is to be expected. Because the Three Gorges area is only moderately seismic, the largest earthquake that might occur would not cause harm, they say. This official position may be under revision, however. The Chinese State Council, the country's federal cabinet, acknowledged last month that preventing geological disasters is one of the problems with the dam that "should be solved urgently."
By submitting a comment you consent to our rules. You must use your real first and last name, not a nickname or alias. A comment here is just like a letter to the editor or a post on Facebook. Thank you.