The great OK fault model - thrust

Here we have a few from above.  Don't forget that the main lines are trending NE towards Toronto.  All the stress is down this trend, so you can't have a good earthquake without a break.  Although the last movement of the megathrust was downwards, the plane of the megathrust just acts as a weakness.

The weakness that gives us a New Madrid is a transform, or adjustment fault.  The old mountain ranges weren't perfectly straight.  Where there was a curve, or a change in strength, you got a crack.  Wasn't much movement, but it became a weakness.  The intersection of the two faults is an essential thing to get a growing fault mechanism.