THE 17 January 1994 Northridge earthquake (moment magnitude M(W) = 6.7) in the San Fernando Valley in southern California illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip(1). This fault lies beneath the active north-dipping Santa Susana fault system and uplifted both the footwall and the hanging wall of the Santa Susana fault during the earthquake(1). Here we argue that footwall uplift on the Santa Susana fault before the earthquake could have been used to identify the Northridge blind thrust as an active fault. Moreover, we propose that the Northridge earthquake occurred on a continuation of the Oak Ridge fault system, which reaches the surface in the Ventura basin to the west. Slip rates on the western part of this fault system(2,3) are nearly three times larger than on the Northridge blind thrust(4), increasing the probability of a Northridge-sized earthquake in the heavily populated Ventura basin.