[abkfenris]

The total seismic moment energy for a 8.1 magnitude quake is roughly 1.7 * 10^21 Joules.

For comparison, Tsar Bomba (the largest nuke ever built) as tested was around 50 megatons of TNT or 210 Petajoules 2.1 * 10^17.

So you would roughly need 8000 Tsar Bombas triggered in just the right pattern to make their force additive in order to release that much energy into the plate.

I have a sneaking suspicion that if you could mine all the material for those Tsar Bombas in the same place, isostasy would give you equal or greater response from the plate, but I'm not going to do those equations right now.

[blacksmith_tb]

Do you need to put as much energy into an already-loaded system as will be released, though? Compare the energy released by a stack of blocks falling to the amount it takes to topple it. I think it's unlikely we will ever be able to trigger quakes intentionally, but there's some implication fracking to extract natural gas [1] is causing seismic activity, for example.

1: http://time.com/84225/fracking-and-earthquake-link/