When full, the Katse reservoir will be 185 metres deep. Charles Merry of the department of geodetic engineering at the University of Cape Town estimates that its 2 billion tonnes of water will depress the reservoir bed by some 10 centimetres. The Katse reservoir began to fill in October 1995, and within weeks the quakes began. Gary Gibson, director of the Seismology Research Centre at the Royal Melbourne Institute of Technology, Australia, who was appointed as a seismology expert by the Lesotho Highland Development Authority, confirms that the tremors are caused by the filling reservoir. They are 'most frequent after heavy rainfall', he says.
Reservoirs often induce small local tremors.
They are triggered either by the weight of water or by leaks lubricating fault lines. The worst acknowledged case was in India in 1967, when the Koyna reservoir near Bombay triggered a quake with magnitude 6.5 in a previously nonseismic region. The dam broke and drowned 177 people downstream. In Lesotho, a seismic study conducted for the development authority in 1988 concluded that there would be 'no significant tremors'.