The Minister of Minerals and Energy Lefoko Moagi has denied that the cracking of houses in the mining town of Selibe Phikwe occurred as a result of mining induced seismic activity.
The Minister claimed in parliament that the cracking of houses may be partially attributed to structural defects which town planners would be better placed to explain, absolving mining activity as a primary cause.
“A number of houses in Selebi-Phikwe were observed to have been cracked, however upon examination some of the cracks were clearly too old to be attributable to the tremors that only started in December 2018. For the houses which showed relatively new cracks, it was difficult to conclusively say if these were directly linked to the tremors,” Moagi told parliament.
He said this in response to a question by Member of Parliament (MP) for Selebi Phikwe East Kgoberego Nkawana recently.
The MP had sought to know when Moagi’s Ministry will conclude the long promised assessment of houses damaged by mine induced tremors in Selebi Phikwe.
“Earth tremors, in Selebi Phikwe, were first experienced on the 17th December 2018. Following reports of tremors, Botswana Geoscience Institute (BGI) and the Department of Mines (DoM) carried out investigations from 19th to 25th January 2019. The investigations involved detailed inspections of the Mine underground workings (Macro-Seismic Survey) as well as interviews of the Selebi Phikwe town residents to assess the scale and extent of the problem. Macro-seismic surveys are a standard practice of investigating earthquake/tremor to estimate their damage based on the Modified Mercalli Intensity Scale (MMI). This qualitative approach was used to determine the most affected areas to guide instrument-based monitoring plans,” said Moagi.
Moagi said the assessments showed that further scientific investigations and monitoring were required to establish causes, location, magnitude, frequency and risk of the micro-tremors.
The minister said a scoping study was conducted on the 13th February 2019 by a consultant, Open House Management Solutions (OHMS) from South Africa, led by Botswana Geoscience Institute and the Department of Mines.
“The purpose of the study was to familiarise the Consultant with the incidents and discuss the intention of the Government to determine the way forward. Subsequent to the scoping study, OHMS (in partnership with a citizen company), Aqualogic Pty Ltd. were engaged to carry out studies under the leadership of BGI. The first phase of these studies was concluded in October 2019, and a report was presented to the key stakeholders that included the Selebi-Phikwe Town Council, the Office of the District Commissioner (Selebi-Phikwe) and the Selebi-Phikwe community. The study was to detect and locate the events (in terms of locations, distributions and magnitudes) and clarify the nature of the sources of the local earth-tremors in the region in order to assess the seismic hazard these vibrations pose to people, property and the Mine infrastructure.,” he said.
He said that the study concluded that the earth-tremors originated from at least 1000m below surface coincident with BCL SE Extension shaft and their magnitudes were NOT large enough to cause any damage to structures based on the Modified Mercalli Intensity Scale (MMI). Observations also showed that pumping water to a stagnated level was required to reduce the frequency and magnitude of tremors. Pumping has been ongoing since September 2019 to maintain the water at 1000ml and this has proved to be effective in managing the severity and frequency of the tremors.
The maximum recorded magnitude was 2.0 on a Richter scale, which was recorded in July 2019. This was before pumping of water resumed. From the trend of recorded tremors, the study further predicted that the maximum magnitude that could be expected from this activity should be 2.3, which is still considered safe and not likely to cause damage to property constructed to an appropriate standard.
“However, the possibility of high magnitude tremors from other triggers that can cause future damage to property and injuries cannot be ruled out and BGI has extended the monitoring stations from the initial study that comprised six stations with an additional station, three underground and four on surface, with capability of automatic reporting to closely monitor any changes in the known areas of the tremors. Furthermore, BGI continues to monitor both macro and micro tremors in the region,” said Moagi.
