Not sure that an official scientist has anything more valuable to say, when most of my information on quake occurence is the same as yours. Northridge was preceded by a much larger swarm of 3s and 4s. I think at the time Lowell Whiteside pointed this swarm out as unusual (we probably had an email back and forth before the Northridge quake). Mark Legg may have pointed out that there may have been a migration of quakes from south to north before Northridge. On Earthwaves, our proud liberal John V suggested that a series of quakes maybe a year or 2 ago in the Gulf of California and also on the Pacific side of Baja was something like a strain transient (??). Maybe he will chime in. So, what might a strain transient be? Maybe a little creep on a deep flat fault? Maybe a little jostling of a crustal block rotating about a vertical axis (these rotations are occuring in areas like the Santa Monica Mountains at least (Argus et al 2005) and the whole Western Transverse Ranges have had over 90 deg of rotation in the last 20 million years.

Of course, for the little lineament of quakes that Penny pointed out, one might wonder if it is even real, or if such alignments are random and our brains are trained to see patterns. BTW, I recall that in the week after Northridge there was a big alignment to the north...up to Bakersfield, with larger quakes than the one Penny pointed out before the 4.3
Chris