An earth tremor measuring four on the magnitude scale was recorded in Joburg today, the Council for Geoscience said. Manager of the seismology unit Michelle Grobbelaar described the tremor as “quite a big guy”. The preliminary analysis indicated the epicentre was near the University of Johannesburg just before 10am. One Twitter user said: “There was a tremor in JHB about 20mins ago. Building shook for about a minute. I’m still shaking.” Another tweeted: “Oh, so the tremor thing happened all over Joburg? Lol I thought it was just because of the construction in our office park.” Quakes, or tremors, are the result of a stress build-up in the earth’s crust. When the stress becomes too great, it is released in the form of a quake, usually on a geological fault line. Grobbelaar said previously that after an earthquake, it was a rule of thumb that another tremor of similar magnitude could be expected in the same region, but seismologists could not predict when this would happen.