Don, you probably did not get the answer you were looking for because no one knows. I suppose it would be good to take a look at whether, excluding aftershocks, the lack of M5s is actually unusual...there are not a whole lot on southern California (excluding aftershocks) either, in the last 18 years that I have been paying a little attention....There should be 100 M5s for each M7...I don't know if that includes aftershocks. With Hector Mines (1999), Landers (1992), and Northridge (1994), you would expect maybe several hundred M5s....excluding aftershocks I would guess there have only been a few dozen...
(all my statements here are impressions and probably need to be checked).
Chris