The energy released by an earthquake of magnitude 8.0 on SA is approximately 6.31×10^16 joules. When converted to an equivalent in terms of TNT, this energy amounts to about 15,080 kilotons of TNT. (About the same as the Castle Bravo nuclear test)

Now, to use such a weapon to cause a stress-relief earthquake, some factors need to come into play: Depth at which the device should be detonated to optimize stress relief without causing surface damage. detailed seismic imaging and rock mechanics studies. Geometry of the fault zone.

Even w/ a Castle Bravo type test, massive radioactive fallout would occur in groundwater as well as air. There's also the chance that such an event would cause local volcanic activity.