4 December 1809
On 4 December 1809 Cape Town was hit by an earthquake measuring 6.5 on the Richter scale. It is the biggest recorded earthquake to have hit the area. The quake was caused by the Milnerton fault, running across the  Milnerton area, the Cape Flats and  right through to the city center. In 1809 there wasn’t much infrastructure for the quake to destroy, leaving only fissures on the ground. A quake of that magnitude would be devastating if it hit the area today due to increased population and infrastructure.
References

4x4 Community Cape Town 1809 earthquake anniversary [online] Available at: www.4x4community.co.za [Accessed on 15 November 2012]| Storm Chasing 4 December 1809 - Milnerton Earthquake [online] Available at: www.stormchasing.co.za [Accessed on 15 November 2012]