I wonder how many would advocate trying to have a /planned/ earthquake each 100 years? Surely one planned in advance which everyone is prepared for is better than the unknown.
As for the actual method, nuclear bombs every few years for a large magnitude doesn't seem like it's mild enough.
We've seen that fracking often has the side effect of causing faults to get lubricated and slip more easily causing earthquakes.
Maybe research into the ideal chemical cocktail to most effectively lubricate and hook up injection stations along the length of the fault line, when there aren't aquifers in the nearby vicinity. Bonus for some kind of lubricant that degrades into non carcinogenic materials.
You wouldn't want to just start pumping fault lubricant into the system while it's so full of energy, but after the next "big" earthquake you could have a window where the pumping would trigger small size 3 or 4 earthquakes similar to aftershocks in perpetuity.
Heck it could even be fun. Make it a holiday, tell people the time its going to happen and let them ride it out. Personally, I'd be at a bar. :)