Roger...with only 45 M4s in the 1990s, did you remove aftershocks? Landers, Northridge, and Hector Mine are all north of your northern limit, but must have had dozens. The large numbers in 1970s may have included aftershocks...wasn't there an Imperial quake in 79? And, Superstition Hills/Elmore Ranch pair was in 87. We have lacked larger quakes in the 2000s that would have had many M4+ aftershocks.

Chris