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Experts dispute NBA theory on causes of quake
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Experts dispute NBA theory on causes of quake

MUMBAI, JAN 27: The devastation due to the earthquake in Gujarat has again raised doubts about whether the Narmada Valley Project is environmentally sound. Although experts say there is no possiblity of the quake being linked to the dam's construction, NBA activists insist that a project of such magnitude can induce seismic activity and trigger and accentuate earthquakes.

Sunjay Sanghavi, author of River and Life and a NBA activist, says, "The Narmada Valley Dam falls in the area of the triple junction of the fault zone, which means it is seismologically very sensitive and a geologically disturbed area."

"In such a situation the Reservoir Induced Seismicity (RIS) becomes extremely significant." RIS is a tectonic activity which is accelerated by a large mass or large body of water. As the Narmada Valley lies in the fault zone and is where 30 dams including the massive Sardar Sarovar are located, the RIS factor cannot be ignored.

Simply explained, RIS means water that is stored and impounded seeps into the earth and changes the stress-regime. This pressure facilitates tectonic movements and can trigger an earthquakes.

However, Dr V Subramanian, visiting faculty of earth sciences at IIT, Mumbai and Project Director with the Maharashtra Siesmic Study, argues that though the project lies in a fault zone, there is no possibilty of an earthquake being triggered by the dam.

"For an earthquake to occur, the stress regime has to be very high, ie. the stress should be of around several thousand kilograms per square centimeter.The pressure exerted by the weight of water in a reservoir is just not enough, because the reservoir is spread over very large areas. Even the largest dam is just not significant enough to trigger an earthquake."

Senior geologist with the National Geo-Physical Research Institute, Harsh Gupta, also subscribes to the NBA theory. In a paper he authored on RIS, he says that the best known site of a reservoir-induced earthquake in the world is at Koyna in Maharashtra where the largest known reservoir-induced quake of a magnitude of 6.3 on the Richter scale occured, and where tremors occur regularly. According to him, there are a dozen other sites vulnerable to reservoir-induced earthquakes in India.

Renowned geologist Dr Arun Kumar Singh concurs with this view. Singh who made a presentation to the World Commission of Dams recently, said there are atleast 100 instances of RIS occurrences in the world, twenty of which have been reported in India.

Explains Mr Sanghvi, "Various scientific studies have shown that RIS has caused a change in the geology of the area. Places not seismologically sensitive earlier have become so, because of large dams."

Activists point out that the 1998 earthquake of Jabalpur was also suspected to be RIS-related. The epicentre of the 1998 earthquake was very close to the Bhagi Dam, a part of the Narmada project.

Dr V Subramanian strongly diagrees and points out that there are two schools of thought regarding RIS. "Though RIS can trigger of an earthqauke this is only a very remote possiblity and only possible if other geological factors are perfectly conducive to an earthquake which is again a very remote possibility."

Copyright © 2001 Indian Express Newspapers (Bombay) Ltd.


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