Brad..does not look like any relation between swarms there and major quakes...on your graph. Maybe an easy way to plot is to have the same graph but one for only quakes > M3, and another for only quakes > M4.
It kind of looks like these quakes could be on NE-SW faults...somewhere recently I saw a paper or figure where there are a bunch of little NE-SW alignments of quakes within the Brawley seismic zone...
Chris