These two earthquakes are of exactly the same magnitude, but in different sections. Yet the Precambrian geology is probably similar. At least in Canada, we can map the geology, and down in OK, they haven't a clue. The upper earthquake is in the W. Quebec Zone, a place of frequent, small earthquakes. These earthquakes are activated by a gradient in post-glacial uplift. We can expect a big M6+ earthquake along the rifted edges of this zone every hundred years or so.
The WQ zone is characterized by highly fractured rock with no great linear zones, except for the ancient rifts. Thus, we only expect an M5 maximum in this area, because everything is so mixed up. An M5 only has a fault area of about 300 m, so it doesn't need much coherence.
This is probably the same in OK, in the western section away from their M5.6. These earthquakes are activated by deep injection. Right now everything has been limited to M3, simply because they can't detect anything less, and the rock is probably highly fractured, and mixed up. Thus, we expect a larger number than 10 for the ratio between magnitudes. Nevertheless, as with WQ, there is no physics that limits the size up to M5. If we want to go to M6 and beyond, it has to be on a major structure, such as the nearby megathrust.
It remains to be seen whether OK can last 100 years between a major M6+, since their rate of M3's might be a factor of 10 over WQ. As usual, if you are on swamp, get earthquake insurance. The Clueless Swiss will insure anything!
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