Joseph Ellan, Director of Dam Safety, Pennsylvania State said in 1987: “With the exception of nuclear power plants, no man-made structure has a greater potential for killing a larger number of people than a dam.” Tremors were felt around the Koyna Dam — one of the biggest in Maharashtra — and the surrounding areas and even in some parts of Mumbai in November 2009. The epicentre of the earthquake was at Koyna, and it measured 4.8 on the Richter scale. The Koyna dam is claimed to be safe.
However, the phenomenon of reservoir-induced seismicity is once again in evidence. The seismicity associated with the reservoir impoundment remained only of academic interest till the disastrous Koyna earthquake of December 10, 1967, measuring 6.3 on the Richter scale, felt up to 230 km away from its epicentre, destroying the village of Koynanagar, killing about 180 people, injuring 1,500 and rendering thousands homeless besides damaging the dam itself and putting the powerhouse out of action, and thus paralysing the Bombay industry. That was thought to be the most powerful earthquake attributable to the building of a reservoir.
The dam was built in 1962. This earthquake occurred after the reservoir behind the dam was filled up. This dam induced the earthquake due to the enormous pressure of the water, causing the shifting of the underlying earth leading to increased seismicity.
Tremors continued to be felt in the region from time to time. Nineteen earthquakes measuring M 5 and more had occurred till 2005. The area was believed to be seismically stable prior to this event.
The severe earthquake in China last year, measuring 8 on the Richter scale, taking a toll of 87,000 people killed or missing was, also attributed by scientists to the hydro-pressure of tonnes of water in the 156-metre Zipingpu dam reservoir with a storage capacity of up to 1.1 million cubic metres built on a fault line in an already seismic area in the south western province of Sichuan.
Though most of the earthquakes result from natural factors, several have been triggered by large dams which aggravate the intensity of natural faults in the geological make-up of earth phenomena, increasing the danger of reservoir-induced seismicity (RIS) by precipitating two possible mechanisms: (a) stresses generated due to reservoir load; and (b) crustal readjustment as a result of reduction in effective stress caused by increase in water pressure due to the weight of massive amounts of impounded water.
The weight of the reservoir can also force water down micro-cracks and fissures and faults under and near the reservoir till it catalyses an earthquake.
Also, the increase in pressure of water in the rocks acts to lubricate faults which are already under tectonic strain but are prevented from slipping by the friction of the rock surface.
Scientific data obtained from various reservoirs establish the link between the filling up of reservoirs and increase in seismicity. RIS has been observed in about 100 cases all over the world to confirm this phenomenon.
The Koyna in Maharashtra, Lake Karibe in Zambia, Lake Kramista in Greece, Lake Mead in U.S., Vaiont dam in Italy, Murk dam in erstwhile USSR and Kuroba dam in Japan are examples of increased seismicity due to increased hydro-pressure. In fact, in the Indian peninsula, reservoirs are believed to have induced 5 out of 9 earthquakes in the 1980s, which were strong enough to cause damage.
Another devastating dam disaster occurred in the Vajont Dam in Italy where seismic activity started to be registered as soon as the reservoir began to be filled. About 60 shocks were registered in about 15 days and on the night of October 9, 1963, 350 million cubic metres of rock broke off Mt. Toc and plunged into the reservoir.
A huge wave 230-metre high, overtopping the dam by 110 metres resulted and the over-spill drowned the town of Longarone, a kilometre downstream and three other villages, killing 2,600 people. RIS is a reality. We cannot afford to ignore it.
(email: bhatia101@bol.net.in)
