3.4.7 Induced earthquakes
Some reservoirs cause earthquakes to occur. This is perhaps not so surprising, as earthquakes are caused by stress in rocks, and the addition of a large mass of water in a reservoir on top of the rocks at the Earth's surface stresses the rocks and can trigger an earthquake. Not all reservoirs induce earthquakes: it is in general only the larger reservoirs, or the deeper ones (over 100m deep), and only if the reservoir is built in an earthquake area, releasing stress already stored in the rocks. Induced earthquakes mainly occur during changes in water level in a reservoir, particularly during initial filling or during seasonal changes of water level.
Reservoirs that have experienced induced earthquakes include Marathon, Greece (1931), Lake Mead, USA (from 1938), Kariba, Zimbabwe (1963), Lake Nasser, Egypt (from 1965), Koyna, India (1967) and Thomson, Australia (1996). So far (2004), none of these induced earthquakes have caused total dam failure, but the largest, at Koyna, was of considerable size (Richter magnitude 6.5, the equivalent of 100 Hiroshima atomic bombs), and led to damage of the dam.
Many of these environmental effects are illustrated by the Narmada Project (Box 4).
Box 4 The Narmada Valley Development Project, India: salvation or environmental disaster?
India suffers from widespread water scarcity, mainly because precipitation comes in one seasonal period, the monsoon, which lasts from about June to September (Figure 20). Reservoirs can be used to store the seasonal rain so it can be used year-round or transferred to an area of need. The Narmada River (the name means 'one who endows with bliss') is 1400 km long and is one of India's most sacred rivers. It originates in the centre of India and flows westwards through three states - Madhya Pradesh, Gujarat and Maharastra — and then into the Arabian Sea. These three states are particularly prone to prolonged drought.
A project is under way (the Narmada Valley Development Project, NVDP) to build 30 major dams, along with smaller ones, on the river over the next 50 years, to supply drinking water for the region, for irrigation, to protect the area from flooding and to generate hydroelectric power. Its estimated cost is around $11,400 million, the most expensive construction project undertaken in India. In 1987, work began on the largest dam for the NVDP, Sardar Sarovar. The NVDP, and particularly the building of the giant dam, has caused huge controversy, both in India (Figure 21) and globally. The Sardar Sarovar dam has a planned final height of 139 m and the reservoir length is 214 km. It will displace around half a million people, and destroy forest as well as some of India's most fertile agricultural land.
The main Indian opposition organization to the NVDP, the Narmarda Bachao Andolan (NBA), campaigned strongly against the dam, gaining support from both Indian and world opposition. In the 1980s NBA mobilized the local population, with marches, sit-ins and fasts (resulting in imprisonment for the leaders); caused the World Bank to review its funding of the project and to withdraw funding in 1993; and petitioned the Indian supreme court in 1994, which halted construction in 1995. However, in 2000 the supreme court ruled that the construction should go ahead, and at the time of writing (2004) the dam was near completion.
The scheme is enormously popular with the dam contractors and with political parties in those local Indian states which will benefit from the extra water and electricity. It is also popular with those who have had to walk miles to collect water each day and who do not have enough water to irrigate their crops. It is not popular, however, with those forced to move from villages that will be submerged, or environmental organizations that fear for the ecology of the Narmada.
Opposition to the NVDP is on social, environmental, economic and safety grounds:
Social: huge numbers of people are evicted from their lands, homes, and traditional way of life, to make way for reservoirs. Past resettlement schemes in India have had a very poor record, offering poor land and a reduced standard of living.
Environmental: the loss of agricultural land and forests, the loss of fisheries, contamination of water, a reduction in fertility of the land, and an increase in waterborne diseases.
Economic: the water will be very expensive. Independent analyses by the World Bank, the Central Government of India and the NBA all find that the costs have been underestimated (and the benefits exaggerated).
Safety: the Narmada valley is an earthquake area and has had substantial earthquakes in the past, up to magnitude 6.25. The Sardar Sarovar dam has also been built on a geological fault that may move in an earthquake. Any failure of the dam through an earthquake, unsound foundations, poor construction, failure of an upstream dam or terrorism could wipe out towns and villages downstream, killing hundreds of thousands of people.
The opponents' case is that the same benefits could be gained by a less grandiose scheme of local water projects, which could do the same job at a fraction of the cost, and with minimal environmental damage. This would involve decentralized, small rainwater harvesting schemes, more equitable sharing of water, water conservation, improved efficiency of water supply and irrigation, the restoration of degraded watershed vegetation and less water-intensive farming.