Issued by: Department of Water Affairs and Forestry
FEARS ALLAYED OVER KATSE DAM QUAKES 22 FEBRUARY 1996
Prof Kader Asmal, MP, Minister of Water Affairs and Forestry, and senior officials of the Department of Water Affairs and Forestry have evaluated the findings of a technical investigation into the causes and effects of the recent tremors experienced at Mapeleng Village in the vicinity of the Katse Dam in Lesotho.
The technical assessment was commissioned by the Lesotho Highlands Development Authority, (LHDA) the implementing authority for the Lesotho Highlands Water Project (LHWP), in response to the public's concern about tremors experienced at Mapeleng since the commencement of impounding of the Katse Dam late last year.
The assessment was conducted by independent technical experts, Dr Alfred Hendron Jnr and Mr Gary Gibson, who both have wide experience in this field. Dr Henderon is Professor Emeritus of Civil Engineering at the University of Illinois in the United States. Mr Gibson is the Director of the Seismology Research Centre of the Royal Melbourne Institute of Technology in Australia.
The experts have found that the magnitude of the earthquakes induced by the Katse impoundment is not unusual compared with similar events recorded at almost 200 reservoirs worldwide. These events are small and shallow, which is why they have only been felt at Mapeleng. In their view, the opening of a crack along an existing weak zone is only a near surface phenomena. The experts visited the village for on site inspection.
The recent events, according to the two experts, give no cause to change the design of the Katse Dam which is planned to withstand an earthquake measuring 6.5 on the Richter scale. They noted that while the occurrence of a magnitude 4.0 to 4.5 earthquake during the filling of the reservoir cannot be ruled out, since the dam is designed to withstand more than this, there is no technical reason to restrict the filling of the reservoir.
Considering the impact on the villagers of Mapeleng, they have advised the LHDA to provide appropriate temporary housing, or to reinforce existing structures as required because the most likely hazard is local collapse of packed stone and mud walls due to shaking. This recommendation, they say, is not made because the shaking is so intense but because some of the stone walls are only marginally stable.
It is expected that the reservoir-induced micro-earthquake activity will continue as the reservoir level is raised and may continue for some time. The experts have found no evidence to suggest that the whole Mapeleng village or any large rock mass near the reservoir can be expected to slide into the reservoir as happened at Vaiont, Italy, in October 1963.
In the course of the evaluation of the experts' report. Minister Kader Asmal had discussions with Prof Chris Hartnady of the Department of Geological Sciences at the University of Cape Town. Prof Hartnady is of the opinion that the recent findings of seismologists ought to be considered when assessing the seismic hazard of the area around the Katse Dam. Minister Asmal invited Prof Hartnady to submit his opinion in writing for consideration by the Department of Water Affairs and Forestry and the relevant project authorities.
The recommendations of the report by Dr Hendron and Mr Gibson will be implemented by the LHDA.
Enquiries: Mr W S Croucamp (012) 299-2404