Significance of Reservoir - Induced Seismicity

Bharat Dogra

In view of continuing rush for large dams in many countries, there is need for better appreciation of an important but relatively neglected hazard of Reservoir Induced Seismicity or RIS which is significant in the case of many dams.

A prominent geologist Prof. K. S. Valdia has written,
"Analysis of seismic date from a number of reservoir sites all over the world such as Hsinfengkiang in China (1961, M-6.1), Koyna in Maharashtra (1967, M-6.7), Katiba in Zimbabwe (1963, M-5.8), Gorancarevo in Yugoslavia (M-4.5), Vajont in Italy (1963), Monteynard in France (M-4.5), Mead in California, USA and 300 other cases has established beyond doubt that seismicity is induced soon after the impounding of waters behind the dams, even in essentially aseismic areas. It was noted that the degree of seismic activity increased as the level of the water rose, the strongest shock registered at the peak water level point.”

These earthquake hazards can be severe in the case of very high dams. Dr. Harsh Gupta of the Centre for Earth Sciences, Trivendrum says worldwide statistics have confirmed positive correlation between height of the water column in the reservoir and RIS. For example, 6 out of the 20 reservoirs with heights between 150 and 250 meters have witnessed RIS i.e. 30% whereas for reservoirs having heights between 90 and 120 meters RIS has been observed in only 6% cases.

In the case of dams on Himalayan rivers, this controversy has a special significance. Y.K. Murthy, former Chairman of Central Water Commission, writes, "Earthquakes are known to have accompanied the construction of a number of dams in different parts of the world. While there is no complete agreement among the geologists and seismologists on the exact cause of this seismic activity, there is a consensus that the seepage of water from the impoundments in vulnerable geological formations helps to trigger off seismic activity. The Himalayas being geologically very young and active, the effect of creating lakes on seismic activity in the region would have to e carefully studied."

Valdiya writes, "A majority of the 200 odd hydel projects that have been built or are being constructed or planned in the southern front and the interior are located not far from the seismically and tectonically active zones of boundary thrusts characterized by strongly deformed and weakened rocks. The hundreds of transverse faults that tear the Himalayan terrain into blocks and segments are seismically more active than the boundary thrusts. Many of the rivers emerge into the plains through narrow valleys controlled by these faults. It is established that larger dams are capable of inducing and increasing seismicity of quite larger magnitudes.

The argument regarding the seismicity threat posed by locating large dams in the Himalayan region have been summarised by Prof. Vinod Gaur in the special context of Tehri Dam Project (TDP),

"Concern about the safety of the  Tehri dam arises from an apprehension that the site may be unstable as it is located within the severely deformed Himalayan belt which is the surface expression of one of the most energetic geodynamic processes active today. This mountain belt has been moulded from the northern borderland of the Indian Continental crust into a most dramatic morpho-structural feature of the globe, by persistent crumpling and stacking of its sheared continental slices to accommodate about 500 km of crystal shortening, ever since it collided with the Asian plate over 40 million years ago. The rate of acceleration of the new oceanic crust at the southern edge of the Indian plate points to a continuing convergence of at least 5 cm a year between the far ends of these two plates.

Apparently, about half of this displacement is still being accommodated by deformation within the Himalayan belt, which is marked by prolific seismic activity. Eight major earthquakes of magnitudes 7.5 and larger, and a great many smaller ones have occurred along the Himalayan front since the great Assam earthquake of 1897 and numerous tell tale geomorphic evidences of recent rejuvenation have been reported. So the argument goes : the creation of a large reservoir in a region which may already be critically stressed, might induce rock failure, and if a dislocation should occur near the dam, the 260 meter thick sheet of water supported by it at an elevation of 550 meters above sea level, would turn into a veritable agent of widespread devastation downstream."

Valdiya comments on this risk for TDP and some other dam projects in the Himalayan region, "The primary cause of induced seimicity is the increase in the fluid pore pressure - the pressure within the water filling the empty spaces (cavities and pores) in the rocks at depths. The pore pressure reduces the effective stress (operating perpendicular to the fault plane) and gives edge to the shearing stresses which causes movement along fault planes and the attendant earthquakes. Coupled with the reduced effective stresses due to increased pore pressure, there is addition of vertical stress produced by the load of the reservoir water, plus accumulating sediments, so that the under ground stress regime is considerably changed. This phenomenon causes increased seismicity, particularly in the areas of normal faulting. Recall that the outer and lesser Himalayan belts are riven with tear faults as well as normal faults. As shown by other researchers in parts of the territory of Nepal and adjoining areas high stresses are prevailing. Moreover the zones of thrusting and faulting are characterized by fractured and crushed rocks which may in many cases provide easy path to water infiltrating down and act as zones of permeability capable of transmitting the load of the impounded water to the pore pressure at depth, thus triggering shear failures and resultant earthquakes.

In this perspective has to be reviewed the construction of the Tehri dam in Garhwal and the Panchtheswar and Punyagiri dams on the Kali river in Kumaon border. The 260m high Tehri Dam on the Bhagirathi river draining the region riven with faults will impound 3.2 billion tonnes (3220 million M3) of water spread over 42 sq. km area, besides the enormous load of sediments accumulating at the earlier-estimated rate of 6.8 ha.m/100 sq.km/year. It is located very close to the Tons (Srinagar) Thrust which by and large is inactive. However, microseismicity has been recorded between June 1974 and May 1977 about 20 km north west of the dam site right on the thrust plane between Bhaldiana and Dharasu. Further, the team of the Roorkee University Geophysicists showed that there are small pockets of higher microseismicity in the Uttarkashi area about 40 km northwest of the dam site - Barethi registering 2.4 events per day (in 195 days from 15 January 1975) Bhetiara 2.39 events/days (16 March 1977 to 27 May 1978) Lambgaon 1.35 events/day (20 June 1975) and Tehri 1.9 event/day. Generally the clustering of epicentre is seen along transverse features such as faults and the foci lie in the depth of 5 to 25 km. These facts indicate that the area north west of the Tehri Dam is releasing stresses carrying the implication that the adjoining areas can become potential sites of increased seismic activity when the water is impounded to its maximum level.”

On the basis of the facts and views of experts given above it may be concluded that the risk posed by large and medium dams can be a serious one, particularly in certain specified conditions of dam-construction.

The writer of this document is a journalist and author who has been involved closely with several social movements and initiatives. He is Convener of   Save the Earth Now Campaign and its SED Demand. Web-site www.bharatdogra.in

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Jun 20, 2020