Influence of Tehri reservoir impoundment on local seismicity of NW Himalaya

KalpnaGahalaut, Sandeep Gupta, Vineet Kumar Gahalaut, P. Mahesh

The 260.5 meter high Tehri dam in the NW Himalaya is the tallest dam in India. Impoundment of the reservoir started in October 2005 and a local network of 40 broadband stations in the Garhwal-Kumaun Himalaya was established in April 2005, which operated until June 2008. First earthquake in the vicinity of the reservoir was recorded by this network in December 2005. Majority of the earthquakes recorded by this network were located farther north of the Tehri dam in the Main Central Thrust (MCT) zone, where the earthquakes of Himalayan Seismic belt are concentrated. But a cluster of earthquakes was located immediately south of the dam with no earthquakes immediately north of it. The fault stability analysis, incorporating the influence of water load and pore pressure, clearly shows that almost all the earthquakes in the surrounding region of the Tehrireservoir recorded from the local seismic network operating occurred in the region of destabilization due to the reservoir impoundment. Absence of pre-impoundment (1967-2014) seismicity in the ISC catalogue in this region where these post-impoundment earthquakes occurred implies that the reservoir triggered earthquakes to the south of the Tehri reservoir.However, immediately north of the dam a few earthquakes are reported by some Himalayan networks and in the ISC catalogue in the pre-impoundment period, which suggests that either the reservoir inhibited the occurrence of earthquakes in this region or the pre-impoundment earthquakes were mislocated and they actually belonged to the Himalayan seismic belt, farther north of the reservoir.

Figure caption:Earthquakes from the local network are shown by filled circles. Earthquakes (magnitude varying from 3.2 to 6.4) from global catalogue (ISC) for the pre-impoundment period (1967-2004) and post-impoundment period (2005-2014) are shown by open and filled stars, respectively. Temporal variation of water level in the Tehri reservoir (height above mean sea level) and earthquakes from local network within 20 km radius circle centered at Tehri dam are also shown (histogram in bottom left panel). The Coulomb stress change (△ S in kPa) on the Main Himalayan thrust (MHT) due to the reservoir load and pore pressure diffusion is superimposed as colour image with red and green colours denoting increase and decrease in △S, respectively, with the dash line showing zero △S, separating the two zones.
Kalpna Gahalaut, Sandeep Gupta, Vineet Kumar Gahalaut, P. Mahesh (2018) Influence of Tehri reservoir impoundment on local seismicity of NW Himalaya, BSSA,108, 3119–3125, doi: 10.1785/0120180077