Roger has it right. The definition of aftershock does have to do with the 'background seismicity' of a region - LA has definitely returned to its 'normal background seismicity' and this is no Northridge aftershock.

Northridge occured on the Pico Thrust at 34° 12.80' N, 118° 32.22'W - last night's quakes were closer to the Santa Susanna thrust fault at 34 deg. 21.6 min. N (34.361N), 118 deg. 40.3 min. W (118.671W) - they are close but still about 13 miles away from eachother.

There's a very good article linked below about definitions of foreshock, aftershock and mainshock.

Briefly an aftershock:
We call an earthquake an aftershock as long as the rate of earthquakes in the area is greater than it was before the mainshock. How long this lasts depends on the size of the mainshock (bigger earthquakes have more aftershocks) and how active the region was before the mainshock (if it was quiet, the aftershocks continue to occur above the previous rate for a longer time).
It can be weeks or decades.

• FORESHOCKS, MAINSHOCKS,AND AFTERSHOCKS