There was a M3.6 right on the Mexican Border near the coast south of San Diego. The right-lateral Rose Canyon fault cuts through San Diego. Work by Tom Rockwell and others showed it to be active at 2 mm/yr over the last few thousand years. It is linked with the Newport-Inglewood fault farther north, in one way or another. The offshore Geometry near San Diego features a right-step in a NW-SE fault (Carlsbad fault), and this Geometry may superimpose an extensional component locally. Right-lateral faults cut across northern Baja to link The Rose Canyon and offshore faults to motion in the Gulf of California. I don't know the focal mechanism (slip and slip plane) for this earthquake-would be curious....
Chris