Hyderabad: There is an active fault beneath the Tehri dam that enhances the earthquake risk, scientists of the National Geophysical Research Institute (NGRI) here have reported. The Tehri dam is located near Tehri town in Uttarakhand in the Kumaon-Garhwal Himalayas region.
"The tectonic loading on this active fault due to local seismicity, coupled with the reservoir loading and unloading, may generate earthquake(s) and cause additional seismic risk in this critically stressed region," Sandeep Gupta and co-workers in NGRI claim in their report in the latest issue of "Current Science" published by the Indian Academy of Sciences.
They say their conclusion is based on evidence obtained by mapping of the seismicity pattern in the Kumaon-Garhwal Himalayas region in general and around the Tehri dam in particular, using observations from a temporary seismological network they operated for over 39 months during 2005-2008. During this period, over 20 earthquakes of magnitude 1.6-2.8 on the Richter scale were recorded in a 20-km radius of the dam site.
According to the scientists, the dam site was chosen in 1961, "at the time when the plate tectonics theory was taking birth and researchers were not well-educated with the fundamental mechanism responsible for earthquakes in the Himalayas. Since then, much has changed in the context of our theoretical understanding and observations related to the evolution of the Himalaya and the associated earthquake hazards".
The Tehri dam falls in a region of possible future great earthquakes, the researchers say. The last major earthquake of magnitude 7.7 occurred 209 years ago, in 1803, close to Srinagar-Garhwal. Many intermittent earthquakes including the 1991 Uttarkashi earthquake (magnitude 6.8) have rocked the region and released parts of energy stored elastically due to the movement of the Indian plate.
"Still, a large amount of residual stress has to be released by an earthquake of magnitude possibly not less than 8," the NGRI scientists claim. They say that geodetic levelling observations in the region also suggest the possibility of great earthquakes to release the recoverable elastic strain stored in the upper crust of the outer Himalayas in this part of the Himalayas.
According to the NGRI team, the Tehri dam was initially designed to withstand ground acceleration expected from the maximum credible earthquake of magnitude 7.2, whereas the ground acceleration would be twice this if an earthquake like the 1905 Kangra temblor strikes again.
The scientists have also studied the seismicity around the dam in relation to reservoir filing. They observed an increase in seismicity from December 2005 onwards, after its first filling in October 2005. "We, thus, speculate that seismicity in the vicinity of the Tehri dam may have a linkage with instability created by loading and unloading of the reservoir."
This, coupled with the close proximity of the seismically active fault to the dam, has serious implications for the earthquake hazard scenario and calls for "re-assessment, analysis and further examination of our results using dedicated ground and space-based seismic and geodetic networks," the scientists concluded.