Dams and Earthquakes

Zipingpu and the 2008 Sichuan earthquake: The debate continues

(April 1, 2010) More scientists are joining the debate over whether China’s Wenchuan May 2008 earthquake was triggered by the Zipingpu dam. A recent report by Chinese experts insists that the dam can now be ruled out as a trigger in the disaster, saying it was so unlike the four largest reservoir-induced seismicity (RIS) events on record, that there can be no connection between the Zipingpu dam and the quake. A follow-up report by Science magazine says otherwise.

Scientists think reservoir-induced seismicity (sometimes called reservoir-triggered seismicity) occurs when the water impounded in large reservoirs shifts land masses and infiltrates underlying fault lines, weakening them. It’s possible for these changes to unleash a chain of ruptures leading to an earthquake. Globally, there are over 90 identified incidences of earthquakes triggered by water reservoirs.

But, say the Chinese scientists writing in the January issue of International Water Power and Dam Construction, there were “no characteristic features of reservoir-triggered earthquakes” apparent at the Wenchuan earthquake.  In fact, they argue, there are a number of qualities about the Zipingpu-Wenchuan area and dam that, if considered, rule out RIS.

For one, they say, the Wenchuan quake, occurred on a thrust fault—where one block of crust is pushed over the inclined fault of another block—while the other four RIS events with magnitudes larger than 6.0 were all “normal or strike slip faults.” The Zipingpu reservoir also sits, they argue, on a “relatively stable region” of the fault’s footwall (the underlying block).

The authors—all three of whom hail from the China Institute of Water Resources and Hydropower Research and are led by structural engineer Chen Houqun—also claim that new data from a team of Chinese researchers led by geophysicist Liu Qiyuan of the China Earthquake Administration’s Institute of Geology in Beijing shows the rupture began 18.8 kilometres beneath the surface and that reservoir pressure would not have driven water through cracks and pores to such a depth.

They also argue that, in other RIS events, the filling of the reservoirs caused an increase in the number of temblors (seismic disturbances) prior to the advent of a major quake. But this did not happen at Zipingpu, they say: according to figures from a seismic-monitoring network that was established around the reservoir 13 months before impoundment began in 2004, only a “normal variation” of seismic activity occurred prior to the 7.9 quake on May 12, 2008.

“All of these factors rule out triggering,” says Chen.

Not so fast, say Richard A. Kerr and Richard Stone who have been following the issue for Science magazine. In their latest article, they quote earth scientists who don’t buy Chen and company’s analysis.

Arthur McGarr, of the U.S. Geological Survey (USGS), and co-author of a 2002 review of reservoir-triggered seismicity (RTS) says that although RTS is often shallow, there are certainly exceptions to this rule, such as the Aswan Dam in Egypt which triggered a 5.3-magnitude quake in 1981 at a depth of 18 kilometres, just shy of Wenchuan’s depth.

And a magnitude-4.5 RTS quake in Tajikistan also struck in a region of thrusting—similar to the Wenchuan quake.

For Ross Stein, of the USGS, the argument that the Wenchuan quake didn’t  fit the pattern of other cases of RIS is problematic because there is no such thing as “consensus” or a pattern of reservoir induced seismic activity. “The [four] examples are all over the map as to how seismicity has responded to dam impoundment. It shows just how little we know about this process.”

“Wenchuan doesn’t fit the pattern? What pattern?” Stein asks.

His colleague, Arthur McGarr, concurs: “I don’t think [triggering] has been put to rest yet,” he says.

Indeed, even Martin Wieland, chair of the International Commission on Large Dams’ committee on seismic aspects of dam design, feels that though Chen’s report makes “a persuasive case,” there are still far too many uncertainties to be sure the dam didn’t trigger the earthquake.

Other scientists, such as Fan Xiao of the Sichuan Bureau of Geology and Mineral Resources in Chengdu, also believe the dam played a major role in the Wenchuan quake and argues, for example, that a swarm of small quakes, or foreshocks, struck the region near Zipingpu three months before the Wenchuan earthquake. Mr. Fan has been calling for government officials to release data of seismic activity smaller than magnitude 0.5 collected by monitoring stations in the Zipingpu dam area. To date, this data has not been made public.

Hydrogeologist Shemin Ge of the University of Colorado, Boulder, who co-authored a report in the Geophysical Research Letters of the American Geophysical Union paper last October, also concluded that water infiltration from Zipingpu “potentially hastened the occurrence of the Wenchuan earthquake by tens to hundreds of years.”

Professor Ge would like to see the missing data too: in an e-mail to Probe International last November she said, “the location and timing of the seismic events in the vicinity of the reservoir would be useful for further studying the impact of reservoir water on earthquake occurrence. Specifically, these data would help examine how fluid pressure/stress changes through time may correlate to seismicity location and timing.”

“Unfortunately, I do not know whether or when these data would be available to public,” she added.

Richard A. Kerr and Richard Stone of Science magazine suggest the debate as to whether the Zipingpu dam triggered the 2008 Sichuan earthquake is still very much open—and will remain so, at least, until data from the reservoir area is made available to scientific experts in the field. “What researchers still want almost 2 years after the earthquake is wide dissemination of the raw data from the Zipingpu monitoring,” they say. Geophysicist Evelyn Roeloffs of USGS in Vancouver, Washington, says that until such data sets become commonplace, “it’s always going to be this kind of story.”

Brady Yauch, Probe International, April 1, 2010

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