▲ Map of Earthquakes 1620–2008 contained in the South African National Seismological Database (SANSD). Known clusters relating to natural and mining-induced seismicity are highlighted. The seismic recording stations are represented by triangles.
… The event occurred on 29 September, 1969 in the Ceres-Tulbagh region of the Western Cape Province about 100 km northeast of Cape Town. Serious damage occurred to buildings in the area (valued at a total of U.S. $24 million) (Singh et al., 2009). The structural damage to buildings varied from almost total destruction of old and poorly constructed buildings to large cracks in better designed and built structures (Figure 1). …
… The structural damage to buildings varied from almost total destruction of old and poorly constructed buildings to large cracks in better designed and built structures (Figure 1). Twelve people were killed and many more injured (Singh et al., 2009).
Other large events of tectonic origin felt in South Africa include the 1912 Koffiefontein earthquake of magnitude 6.2, the 31 December 1932 Cape St Lucia earthquake of local magnitude 6. Paleoseismic studies show that a large earthquake with an estimated magnitude of about 8 occurred in the area of Koffiefontein some 50,000 years ago (Visser and Joubert 1990; Joubert et al., 1991). …
Southern Africa is considered a stable continental region in spite of several reported medium size earthquakes, some of which caused considerable damage and casualties. The 1969 Ceres 6.3 magnitude earthquake is considered the most destructive and caused serious damage estimated at US$24 million, with 12 mortalities and many more injured. Others include six mining related tremors which caused significant damage i.e. Welkom 1976, Klerksdorp 1977, Welkom 1989 and Carletonville 1992 seismic events. Notable for their damage to infrastructure was the 9th March 2005 Stilfontein event near Klerksdorp and 5th August 2014 event near Orkney.
Most buildings and structures in South Africa are not designed to resist even relatively low intensity earthquake. Most architects, engineers and builders in South Africa do not consider seismic resistance as a design requirement. In this work, potential damage caused by strong earthquake was estimated for three classes of buildings situated in Sandton, Cape Town, Durban and Port Elizabeth. The effect of earthquakes causing damage was studied by considering the “worst case-scenario”, i.e. the occurrence of an earthquake with the maximum possible magnitude for an area. In four studied urban areas, expected damage was estimated for three classes of buildings: unreinforced masonry, bearing wall, low rise, reinforced concrete shear wall, without moment resisting frame, medium rise, and reinforced concrete shear wall, without moment resisting frame, high rise. The results of the analysis showed that in case of occurrence of a strong earthquake, the most damage is expected for the building classified as ‘unreinforced masonry, bearing wall, low rise, and reinforced concrete shear wall
The primary effects of earthquakes are ground shaking, ground rupture, landslides, tsunamis, and liquefaction.
Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. … When the rocks break, the earthquake occurs.
Track down our quakes
The few that do happen are overwhelmingly man-made. This is unusual, earthquakes are usually caused by seismological events related to continental plate movement.
On the rare occasions they happen in South Africa it is because of gold-mining weakening the structure of the crust or bore-holes pumping too much water out of underground aquifers (the latter is also the primary cause of the Pretoria region’s frequent sinkhole problem).
But a greater cause of the very few earth quakes we have is underground erosion and drought.
Places like Pretoria and other places as well, have a large amount of ground water, the effect of drought on seasonal rainfall, and poor water infrastructure planning in SA over the last quarter century, we are using more ground water, pumping it up through boreholes.
The ground water dissolves soluble minerals in the dolomite bed rock, leaving the rock porous and able to collapse and shift more easily after the saturated water is removed, and is replaced by less saturated water from further up the aquifers, this less mineral saturated water then dissolves more minerals and is pumped out, effectively like slowly pouring a drink onto ice cubes, just in the opposite direction. Trees and their roots are also using water.
Mining activities causes a slip on the fault line which triggers the earthquakes as well as fracking.
Although fracturing-related earthquakes are chronic, they were thought to be minor. But new research is showing that they can be quite large and damaging. The focus of the study, a 5.7 magnitude quake near Prague, Oklahoma, damaged 14 homes and other structures in the area.
TEXAS – FRACKING
Fracking and Earthquakes – West Texas
Hydraulic fracturing injects millions of gallons of water into oil and gas containing geologic formations deep underground.
Scientific and government research indicates that fracking can cause earthquakes in two ways:
- Primarily, during the fracking process: “[Earthquakes] were caused by fluid injection during hydraulic fracturing in proximity to pre-existing faults.”
- Secondarily, via the disposal of fracking wastewater via underground injection.
Our report Shaky Ground explores the risks of fracking triggered earthquakes in California. And increased earthquake activity in shale plays with active injection wells, like Texas, Oklahoma and Ohio show the risks are real.
Earthquakes caused by fracking
- British Columbia Oil & Gas Commission: Investigation of Observed Seismicity in the Horn River Basin, August 2012
- CBC Fracking causes minor earthquakes, B.C. regulator says
- British Geological Survey: Fracking and Earthquake Hazard
- Cuadrilla Resources: Geomechanical Study of Bowland Shale Seismicity, 2011.
- Schlumberger Seismicity in the Oil Field
- Kansas Agland What’s known – and suspected – about induced earthquakes
- Washington County (PA) Observer Reporter DEP links quakes to fracking in 2016 Lawrence Co. event
Earthquakes caused by fracking wastewater injection
- Science Injection-Induced Earthquakes 2013.
- National Academy of Sciences: Induced Seismicity Potential in Energy Technologies, 2012.
- The Earth Institute Columbia University Wastewater Injection Spurred Biggest Earthquake Yet, Says Study
- Journal of Geology Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake Abstract Article
- Capitol Confidential Tkaczyk, Avella question DEC’s seismic work
- Mother Jones Fracking’s Latest Scandal? Earthquake Swarms
- Southern Methodist University Causal factors for seismicity near Azle, Texas