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Walter Nyamukondiwa Kariba Bureau
THE 3,9 magnitude earthquake that hit Kariba and parts of Zambia recently did not affect key infrastructure, including the Kariba Dam wall, investigations by the Zambezi River Authority (ZRA) have revealed.

ZRA chief executive officer Engineer Munyaradzi Munodawafa said the rattling effect of the tremor, which lasted for about 30 seconds, on June 1, did not trigger seismic risk analysis and diagnosis machines that monitor and assess risk on the Kariba Dam wall.

Investigations show that the epicentre of the earthquake was about 28,5 kilometres south of Kariba Dam in the Matusadonha National Park.

“As part of the standing operation procedures for Kariba Dam, the dam and its appurtenances are inspected after an earthquake event,” said Eng Munodawafa.

“To this end, the dam was inspected and the instrumentation data analysed to establish if the tremor had any effect on the structural integrity of the dam.

“The inspection and the results of the data analysis show that the tremor did not affect the dam and related structures.”

Eng Munodawafa said occurrence of tremors in Kariba Dam and reservoir area was induced by reservoir earthquakes and date back to 1963 after the filling of the dam.

He said the frequency of the tremors were initially high, but gradually reduced with the passage of time.

The largest earthquake of a magnitude of 6,1 on the Richter Scale was recorded on September 23, 1963, a few weeks after Lake Kariba was full, and ranks among the world’s five largest reservoir induced earthquakes.

Statistics show that Kariba averaged about 200 tremors of more than 4.0 magnitude per year after the lake filled between 1966 and 1973, which decreased to almost non from 1998 to 2008.

The frequency marginally increased to about 48 tremors per year between 2009 and 2013.

“The earthquakes are of a small magnitude and do not impact the structural integrity of the dam. The earthquakes will continue to occur in the future and could also be influenced by the variation in level of the reservoir,” he said.

The Kariba tremor occurred hardly a fortnight after tremors hit Chipinge in the Eastern Highlands on May 18, with no reported damages.

Seismologists and the Meteorological Services Department have variously recorded more than 50 mild to moderate earth movements which are mostly attributed to plate tectonic movements on the East African Rift System.

The US Geographical Survey notes that earthquakes with magnitude of up to 2,0 on the Richter Scale (a unit of measurement of earthquakes) are normally known as micro-earthquakes and are generally undetected, only picked up by localised seismographs.

Those of magnitudes of 4.5 or greater are strong enough to be recorded by sensitive seismographs.