Dams and Earthquakes

Was China’s deadly earthquake man-made or an act of God?

(March 4, 2009) Chinese government must learn the lessons of Zipingpu.

After weeks of conjecture from some of the world’s most prominent geological scientists, it is still unclear what role the Zipingpu dam reservoir played in causing China’s deadly earthquake last spring. While many experts agree that further study is needed, others argue that a dam reservoir can’t “make” a quake: earthquakes are the result of powerful tectonic forces already at play, they say, and are beyond man’s influence. Lawyers would call an earthquake an “act of God.”

But the phenomenon of reservoir-induced seismicity (RIS) has been observed worldwide for decades: seismic activity and earthquakes occur as large reservoirs are impounded, sometimes with deadly effect. In China alone, RIS is thought to explain the occurrence of 19 earthquakes across the country.

Up until now, the most powerful reservoir-induced seismic event to occur in China happened in 1962 when a magnitude 6.1 earthquake was triggered just 1.1 kilometres upstream of the Xinfengjiang dam. The quake killed 6 people and destroyed 1800 homes [PDF].

Never before, though, have scientists thought that RIS could trigger an earthquake of such destructive magnitude as China’s deadly M7.9 quake that struck last May, which was close to 1000 times more powerful than the Xinfengjiang event. The possibility threatens Chinese dam builders seeking to forge ahead with a rash of new projects. Meanwhile, it is of huge importance to the scientific community and, needless to say, to the countless communities living near large dams.

The idea that the devastating Sichuan earthquake was triggered by human activity, specifically the building of the Zipingpu dam, has inflamed the RIS debate among geological scientists the world over.

On one side, experts argue that physical processes are triggered by fluctuations in the reservoir level and stress from the weight of the water, and by the infiltration of water into fissures and fractures. The additional pressure then crushes the underlying rock and triggers earthquakes. These processes are generally thought to be more potent in areas with pre-existing seismic stresses. While the reservoirs don’t necessarily “make” the earthquake, the experts say, reservoirs can change the timing, the magnitude or the location of a quake that may be waiting to happen, like the proverbial straw that broke the camel’s back.

Christian Klose, geophysical hazards expert at Columbia University, thinks that a combination of these forces eased the squeeze on the fault running under the dam, weakening it, and increasing the stress at the same time, effectively creating 25 times the amount of natural stress loading from tectonic motions.

Others, including experts from the government-run Chinese Academy of Sciences, strongly deny that the Zipingpu dam had any role in the earthquake. They argue instead that the quake’s magnitude proves that massive crustal movement was in play and that the quake was going to happen eventually.

Roger Musson, a seismologist with the British Geological Survey, downplays the import of Zipingpu too, telling the Associated Press that “the scale of the Wenchuan earthquake (185 miles of rupture) indicates that it was a true tectonic event which would have occurred with or without the Zipingpu dam … It is thus only a question as to whether stresses from the reservoir advanced the timing of the earthquake.”

But from the point of view of last year’s quake victims, if Zipingpu advanced the earthquake by 200 years (as Christian Klose predicts), then the Zipingpu dam “made” China’s deadly earthquake. To a geologist, 200 years may be inconsequential, but for the affected population it meant the difference between life and death.

Chinese authorities seem to be hiding behind the geological argument. They claim that the quake was going to happen anyway; with or without Zipingpu, and that no additional seismic concerns for future dam construction projects need to be considered. But in so doing they show their readiness to discount the lives of the people they are supposed to protect.

Geological authorities in other countries cannot take the same risks with public safety. Though reservoirs are often built on or near fault lines, before construction can begin, extensive geological study is first required to ensure that any fault lines in the immediate vicinity are not seismically active.

In a recent Wired interview about Zipingpu, U.S. Geological Survey geologist David Schwartz said, “I’m sure we wouldn’t have built a dam [in the United States] on or so close to an active fault.” The Zipingpu reservoir lies 3.4 miles from the epicentre and a mere 550 yards from the Beichuan-Yingxiu fault belt that triggered last May’s lethal seismic event.

Rhetorically, Schwartz asks, “Would you build a dam across the San Andreas or the Hayward fault?” Answering his own question, he immediately said, “no.”China’s feverish campaign to build even more mega-dams throughout the geologically unstable southwest region of China, while downplaying the risk of reservoir induced seismicity; where Chen Guojie of the Chinese Academy of Sciences says, “no valley is being left undisturbed, and no river left undammed,” exposes just how far Chinese authorities are prepared to place the safety of their own citizens at risk.

Jameson Berkow and Patricia Adams, Probe International, March 4, 2009

Categories: Dams and Earthquakes, RIS

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