M3.1. I know this is injection, since it is Texas, but I am now 'out of my depth', in thinking of the mechanism. I'm pretty sure that Ok City marks the Grenville Front, and thus, the end of the megathrusts. Nevertheless, the Archean rock is also highly stressed, and soon you get into the mountains. My general thought is that out there you can get injection earthquakes, but they will be limited in size, and you can't grow a large mechanism.
So, whereas, Ok can easily grow to M7's, west of there you probably can't get more than an M5.