I have done a bit of thinking about earthquakes and fracking.
As I understand it, the Blackpool earthquakes were a direct result of the fracking process. The surface of the well hole was exposed to liquid under high pressure.
In this country, earthquakes in Arkansas and Ohio happened near fracking-liquid disposal pits. Since these pits are extremely deep, there is a high hydrostatic pressure on the pit wall.
In both cases, if a hole is drilled that intersects a geological fault, there is the possibility that liquid will be forced into the fault. Under these conditions, the fault can be unlocked and lubricated by the liquid, particularly if it is a shear fault.
This idea is so simple, that I would be surprised if it hasn't been suggested before.
ADDED 19 Dec 2011 - It has been suggested; see Wikipedia
