(November 6, 2009) A recent article [Ge, S., M. Liu, N. Lu, J. W. Godt, and G. Luo (2009), Did the Zipingpu Reservoir trigger the 2008 Wenchuan earthquake?, Geophys. Res. Lett., 36, L20315, doi:10.1029/2009GL040349] by scientists in the U.S. provides further evidence that the Zipingpu dam’s reservoir may have triggered the devastating May 2008 Wenchuan earthquake in China. The Wenchuan earthquake killed nearly 90,000 people and unleashed a chorus of speculation that the Zipingpu reservoir may have contributed to the severity of the earthquake, or helped to trigger it.
According to this latest analysis, just published in the current issue of Geophysical Research Letters of the American Geophysical Union, the Zipingpu dam’s reservoir may have “hastened the occurrence of the Wenchuan earthquake by tens to hundreds of years.” The authors argue that the dam’s reservoir could have altered stress levels on the fault plane, and that this heightened level of stress has been shown to be significant in the triggering of earthquakes.
In “Did the Zipingpu Reservoir trigger the 2008 Wenchuan earthquake?” the authors point out that when a dam’s reservoir is built on or near a fault, the rise or fall of the water level may affect the stability of the fault plane through two physical processes: first through the sheer weight of the water, and second through the diffusion of water to rocks below the reservoir. These processes can trigger earthquakes on critically stressed faults, the authors say.
The authors argue that their data shows an increase in stress levels on the fault plane since the reservoir was filled more than two years prior to the earthquake.
“At 2.7 years after the impoundment (filling) of the Zipingpu Reservoir, changes of more than ± 0.01 MPa (megapascal) are observed in the region extending 20 km west and 20km below the reservoir,” they write. “Such changes are comparable in magnitude to those induced by large earthquakes and that have been shown to trigger or delay subsequent earthquakes and to the coseismic Coulomb stress changes by the Wenchuan Earthquake.”
The authors also point out that a Coulomb stress increase of 0.01 MPa is considered the point at which seismic activity will be affected, and possibly increased. The Zipingpu reservoir, by elevating the Coulomb stress level by 0.01 to 0.05 MPa, could, the authors say, have hastened the rupture of the underlying Yingxiu-Beichuan fault.
The authors also note anecdotal evidence from residents living in the area, who say there was an increase in the amount of seismic activity since the reservoir was filled.
The idea of Reservoir-Induced Seismicity (RIS) is a particularly sensitive one for the Chinese government which is overseeing frenetic dam-building in seismically-active south-west China. Evidence that Zipingpu triggered the devastating earthquake last year could increase public pressure to stop those plans. In fact, Chinese officials have adamantly denied that Zipingpu could have triggered the earthquake and appear keen to keep potentially damaging data away from interested scientists.
This suspicion seems to have been confirmed by the scientists of this recent study who noted that “systematic microseismicity data near the reservoir…are lacking.” It seems that neither the China Earthquake Administration, nor the reservoir authority, have microseismicity data gathered near the reservoir, and prior to 2004, “or these data are currently unavailable to the public.”
Despite missing some of the vital microseismic data, the authors of this latest report say enough is available to determine that the dam’s reservoir elevated the Coulomb stress and could have hastened the Wenchuan earthquake.
Patricia Adams and Brady Yauch, Probe International, November 6, 2009