Dams and Earthquakes

Clean energy’s dark side: safety of “green” dams in China called into question

(May 11, 2010) Being a geophysicist in China these days is a stressful occupation, particularly if you suspect a $750 million hydroelectric project of killing more than 86,000 people. The assertion of a handful of uncharacteristically vocal Chinese scientists that includes Fan Xiao of the Sichuan Geology and Mineral Bureau is that reservoir-induced seismicity (RIS) associated with the nearby Zipingpu Dam was at least partially responsible for the killer 7.9 magnitude earthquake along the Beichuan fault on May 12th 2008. Adding to the angst is the knowledge that there might be as many as 1,000 other dams in the identified earthquake zone.

Given the current trend towards clean energy solutions for the world’s hottest economy, it is not surprising that the claim has proven to be largely unpopular in Beijing—despite receiving cautious support from members of the geoscientific community elsewhere. Christian Klose, now a senior research scientist at the US private firm Think GeoHazards, argued that “local and abnormal mass imbalances” on the surface contributed to the disaster. Notably, he stopped short of directly naming the Zipingpu Reservoir in his presentation to the American Geophysical Union last December.

Since then, others have expressed concern that RIS may have triggered the “natural” disaster. A study by Professor Lei Xinglin, a geophysicist at the China Earthquake Administration in Beijing, showed that the Zipingpu Reservoir “clearly affected the local seismicity” and called for further study to see if the dam acted as a trigger to the quake. Professor Shemin Ge from the University of Colorado (Boulder) and co-authors said in a report in the Geophysical Research Letters of the American Geophysical Union paper last October that the dam may have played a role in the earthquake, arguing that water infiltration from Zipingpu “potentially hastened the occurrence of the Wenchuan earthquake by tens to hundreds of years.”

To date, Chinese officials have argued that the dam at Zipingpu is innocent, while refusing calls from scientists for access to key seismic monitoring data.

How filling a lake can start an earthquake has been the subject of esoteric research since the 1960’s and it appears that seismicity is more likely to be triggered by larger dams, although structural and chemical features of bedrock geology that determine groundwater flow rates are also important determining factors. Having said that, the usual RIS suspects are voluminous reservoirs that impound anywhere between 13 million and 175 billion cubic metres of water. Tall dams that average 113 metres in height in steep-sloped river valleys are also more likely to shake things up.

The thinking is that the relatively sudden accumulation of massive amounts of water in inauspicious places can have the effect of inducing rapid changes in both mechanical stress (tipping) and pore pressure (lubricating) regimes along susceptible fault lines, thus modifying how the Earth handles stress in its outermost layer. Conceptually, large reservoir impoundment is akin to over-loading the wrong side of a teeter-totter and greasing the joints between blocks of bedrock, effectively neutralising the stabilising forces of gravity and friction in the extensive area of influence of large dams like Zipingpu.

At 156 metres-high and with a storage capacity of 1 billion cubic metres of the Min River—itself a tributary of the Yangtze—the Zipingpu Reservoir fits the description of a potential earthquake trigger.

However, as earth scientists who use rock hammers instead of computer models are fond of pointing out, a myriad of individual geologic factors like lithology, degree of cohesiveness and permeability to water of the underlying bedrock and valley sides also play a role in determining a reservoir’s proclivity for trouble-making. The highly fractured and severely weathered carbonate rocks of southwest China, being virtual super-highways for fluid flow, have hosted 15 of 19 lower magnitude RIS events in the People’s Republic since 1962. The interval it takes for rising waters to find new ways of reaching critical segments of even deeply-buried “hard rock” faults via the subterranean fracture networks accounts for the average time delay of 4.5 years seen in most reservoir-triggered quakes.

Interestingly, the artificial lake created by the Zipingpu dam near the crowded city of Chengdu began filling in December 2004.

In response to global warming concerns, there has been a mad rush to fund and build ever more hydroelectric dams as an alternative to sooty coal-fired generating plants. But dams are far from environmentally friendly, as they stir up naturally-occurring and stack-strewn metals like mercury and lead in soils, inundate vast swathes of productive agricultural land and disrupt the river’s hydrology. Nonetheless, dam enthusiasts are quick to cite, often erroneously, their assumed benefits—power for economic growth, environmental stewardship, down-river flood control, agricultural irrigation and respiratory health protection. And this is perhaps what the Chairman had in mind when he envisioned the damming of the Yangtze River at Three Gorges.

Though Three Gorges dwarfs the installation at Zipingpu by almost any measure that can be named, very little attention was devoted to RIS assessment in the 1988 feasibility study by the Canadian engineers. Fan Xiao and his colleagues have expressed concern over the potential seismic activity from Three Gorges which began filling in 2003 and has already triggered hundreds of seismic tremors, one as large as magnitude 4.7, and has yet to reach maximum water level.

Whether Three Gorges causes a Great Leap Forward or side-to-side, up, down and finally backwards, only time will tell.

Heather Gingerich, Probe International, May 11, 2010

Heather Gingerich, B.Sc. (Human Biology) and M.Sc. (Earth Science), is Probe’s Medical Geologist-in-Residence and a Ph.D. candidate in Population Health at the University of Queensland (Brisbane). She is the current Director of the International Medical Geology Association in Canada and has been tracking geophysical and geochemical health hazards for almost two decades. IMGA’s general web-site is www.medicalgeology.org.

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