So now we know we have these huge megathrusts forming bands of good and bad rock. Good rock is what we drilled with our deep holes at Darlington -- high stress, extremely tight. The rock came out in 10 foot solid sections that you could use as baseball bats. No fractures, no water.
Bad rock is right at the faults, and it is fractured all the way up. It is what was chosen for the Bruce Black Hole. The fractures go all the way down to the Moho. Recent studies have show there are oceans of billion year old water in those fractures, along with bacteria, probably from the very beginning of life. A whole ecosystem, probably as alien to us as anything on Mars.
All this water lives in perfect harmony with the rock, since it's been there long enough. Once in a while there have been huge earthquakes to accommodate the shifting of the plates, but the whole thing is in chemical equilibrium.
The one thing that disturbs this happiness is a seep of fresh water. This water is aggressive to the strong points of quartz that are holding the little NW stress that remains. Over the millennia, natural seeps have caused local earthquake hotspots along the megathrusts. We are mainly aware of New Madrid, because it happened in historic times, and it still has aftershocks. There could be many other spots.
Seeps happen when lots of surface water accumulates. Lakes Erie and Ontario have them, and there are some in Quebec, wherever that is. :) New Madrid is still sucking in the water.