Earthquakes caused by the filling of a huge reservoir in the southern African country of Lesotho have terrified local people for more than a year. Houses in seven villages beside the reservoir of Katse Dam have been damaged by tremors, and in the village of Mapeleng, 11 houses were made uninhabitable by the quakes. In late January 1997, twelve families left Mapeleng, abandoning homes which were damaged more than a year ago by earthquakes. Tremors continue to strike the area, according to the World Bank, a project funder.
Reservoir–induced seismicity (RIS) is a widely recognized but little understood phenomenon that has occurred near at least 200 reservoirs; 32 of those cases had quakes larger than 4.0 magnitude on the Richter scale. The biggest quake from RIS is thought to be a 6.3 quake from Koyna Dam in India in 1967, which killed 180 people.
take action to accommodate these people as soon as possible.” The project treaty requires that people affected by the project not be left worse off.
When asked what they thought about the temporary replacement housing, Mapeleng residents told International Rivers in August 1996, “They’re no good. The walls are like paper, and they’re very cold.”
- Lori Pottinger, Director of the Southern Africa Program for International Rivers, says:
- “Mapeleng residents whose homes were dangerously damaged have been living in these inadequate, uninsulated sheds for more than a year now. Given that the project’s highly complex engineering works are proceeding on schedule, there is no reason why the social and environmental aspects of the project are lagging so far behind stated benchmarks.”
In addition to the destroyed houses, another 50 that were cracked by the tremors remain mostly unrepaired to this day. Four springs also dried up because of the tremors, although one has started flowing again. In April 1996, a quake caused a 1.5–kilometer–long crack in the village.
Earthquake damage is only the latest in a long list of problems the dam has brought local people problems which project authorities have been mostly unable to resolve in the ten years of construction.
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BACKGROUND ON RESERVOIR–INDUCED SEISMICITY
Excerpted from Silenced Rivers: The Ecology and Politics of Large Dams by Patrick McCully (Zed Books, UK, 1996)
As with most aspects of seismology, the actual mechanisms of RIS are not well understood, and it is impossible to predict accurately which dams will induce earthquakes or how strong the tremors are likely to be. Most of the strongest cases of RIS have been observed for dams over 100 metres high –– but dams just half this height are also believed to have induced quakes. Reservoirs can both increase the frequency of earthquakes in areas of already high seismic activity and cause earthquakes to happen in areas previously thought to be seismically inactive. The latter effect is the most dangerous as structures in areas thought to be quiescent are not built to withstand even minor earthquakes.
The most widely accepted explanation of how dams cause earthquakes is related to the extra water pressure created in the microcracks and fissures in the ground under and near a reservoir. When the pressure of the water in the rocks increases, it acts to lubricate faults which are already under tectonic strain, but are prevented from slipping by the friction of the rock surfaces.
For most well–studied cases of RIS, the intensity of seismic activity increased within around 25 kilometres of the reservoir as it was filled. The strongest shocks normally occured relatively soon –– often within days but sometimes within several years –– after the reservoir reached its greatest depth. After the initial filling of the reservoir, RIS events normally continued as the water level rose and fell but usually with less frequency and strength than before. The pattern of RIS is, however, unique for every reservoir.
Also available: Excerpts from “Seismic Phenomena at Mapeleng Village – Katse Reservoir – Report by Experts,” February 1996