More fodder for the geology geeks (such as myself, obviously). Note that Northridge (6.7 mag) is considered a "moderate" sized earthquake and the kind of fault that generated it is more likely to trigger a later "moderate" earthquake than is a longer fault.

Very interesting graphics in this report:


http://www.whoi.edu/science/GG/people/jlin/papers/Stein_Lin_OFR_2006.pdf

"A fundamental feature of blind thrust faulting is the stress transfer along strike that leads to successive earthquake sequences or large off-fault aftershocks. M≤6.8 blind thrust faults tend to be square in aspect ratio; in other words, their length is similar to their width [Lettis et al., 1997]. Such short faults are highly efficient at increasing the stress beyond the fault ends along strike, whereas long faults increase the stress beyond the ends only slightly more [Lin and Stein, 2004] (Fig. 14). In contrast, the region of stress drop is much larger for the long fault. This means that M≤6.8 blind fault earthquakes are more prone to trigger successive earthquakes on adjacent fault segments than are M>7 events, which tend to be much longer along strike in comparison to their width (Fig. 14).

"This suggests that the moderate size events like the 1994 Northridge earthquake are capable of promoting further moderate size events in the surrounding crust."


..