Very, very roughly speaking, you could assume that the recent TX and OK earthquakes near Dallas are an extension of an (assumed very ancient) fault line projecting down through the St Lawrence River valley in Canada, through the Great Lakes, through Memphis TN (the 1812 New Madrid quakes) and down into north Texas.
Over time, there is a very distinct continuous “line” of small (mag 2-3) earthquakes right up and down this line.
I think you are right. Geologically we sit over a fulcrum range of paleozoics or maybe even precambrian, at near the depth of the quake, if one of our geologists could chime in.
I’m not looking forward to the seismic design changes, myself, but we need them. Lateral force is lateral force, and we have it in Texas. We had been in zone “0”.