Reservoirs Found to Trigger Quakes Larry O'Hanlon, Discovery News
July 16, 2007 - A new study of seismic activity at a reservoir in India strengthens yet another case of a manmade lake triggering nearby earthquake faults. A comparison of water levels in the Govind Ballav Pant reservoir with nearby tremors revealed more quakes when the water was at the highest stand, with about a month lag time, say a team of Indian and Nepalese geologists. The month lag suggests that the water is percolating down to the nearby faults, increasing the grain-to-grain pore pressure and thereby reducing friction there so the faults can more easily slip. "There are many other worldwide cases where reservoir operation is correlated with the seismicity," explained Vineet Gahalaut if India's National Geophysical Research Institute. "Very few of them are unambiguous." Gahalaut is the co-author of a paper regarding the Govind Ballav Pant reservoir's quakes which appears in the latest issue of the journal Tectonophysics. Some other known or suspected cases of reservoir-triggered quakes are at the Aswan Dam on the Nile in Egypt, Koyna in India, the Kariba Dam on the Zambezi River in Africa and, in the 1930s, near Hoover Dam on the Colorado River. Because only a few of these had good seismological data collected on their quakes, the reservoir-quake relationship was hard to argue scientifically. Few of the reservoir-triggered quakes in the world have been dangerous, but that's not to say they can't be. "Since the impoundment of the (Govind Ballav Pant) reservoir, which began in 1962, no major (Magnitude greater than 7) or even moderate earthquake (Magnitude greater than 5) has occurred in the region," Gahalaut told Discovery News. "However, in the past the region has witnessed a major earthquake in 1927 (Magnitude 6.5)." It's also located in what's called the Narmada-Son-Tapti failed rift zone, which is the most earthquake-prone zone of peninsular India. "The occurrence of the 1927 earthquake and several other earthquakes elsewhere along the failed rift, and the presence of weak zones in the form of faults in the failed rift region, may suggest that the region can produce a moderate or major earthquake," Gahalaut explained. To other geologists, the Govind Ballav Pant case is yet another chance to refine their understanding of these unanticipated side effects of damming large rivers. "Although there are a fair number of cases, each has something new to offer," said geophysicist Art McGarr of the United States Geological Survey. "I think ideally every time a new large dam is constructed they should install a good seismic network before (filling the reservoir)." Perhaps the best case of all will be the world's largest reservoir: behind the Three Gorges Dam, now being filled in China. "I'd be surprised if there weren't a fair number of earthquakes (at Three Gorges)," said McGarr. Despite some early discussions with the Chinese about the matter, McGarr does not know whether the Chinese government decided to install a seismic network at Three Gorges to monitor the almost certain reservoir-triggered quakes there. 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