Beth, Puerto Rico is above a subduction zone. Because of the curvature in the plate boundary, the plate motion there must be highly oblique. It is common for oblique subduction plate boundaries to split the motion between pure thrust on the subduction zone and strike slip on steeper faults above it. Let's see, the strike-slip motion should be left-lateral on plate-parallel faults there. (otherwise, I know little about the faults there) As for the 7.4 increasing the stress, if Virgin islands, probably, but I thought it was farher away. As for whether the activity is increasing: sounds like a job for Roger. You could find him some complete catalogues, or maybe he has some. It would be good to have them complete down to M2.5 so we can see if this recent swarm is unusual or not.
Chris