Although earthquakes are not common in Namibia - or on the southern African subcontinent in general- they do occur with a frequency of one to two events per year - many of them however are so weak that they remain unnoticed, and are only detected with the aid of sophisticated instrumentation. Some 160 ground tremors between magnitude 1.0 and 5.6 have been recorded in the last hundred years, occurring all over the country but favouring certain zones of structural weakness in the Earth's crust, such as the Waterberg Thrust, the Windhoek Graben or the Kuboos-Bremen Line of intrusives crossing the border to South Africa. The map below shows a correlation between present day earthquakes and the distribution of ancient granitic bodies, indicating the rejuvenation of these zones of crustal weakness.