Tectonic setting of the 1971 Sylmar and 1994 Northridge earthquakes in theSan Fernando Valley, California

The San Fernando Valley lies above the north-dipping 1971 Sylmar and south- dipping 1994 Northridge earthquake faults. To understand the tectonic setti ng of these two earthquakes, we mapped subsurface geology of the San Fernan do Valley down to a depth of similar to 3 km, using industry oil-well and s eismic data. The 1994 Northridge earthquake did not rupture the surface, an d the south-dipping aftershock zone terminated against the north-dipping 19 71 aftershock zone at a depth of 5-8 km. However, the blind Northridge faul t has a near surface geologic expression; fault-propagation folding related to the Northridge fault has preserved a thick forelimb sequence of Plio-Pl eistocene Saugus Formation in the Sylmar basin and Merrick syncline, which are located on the hanging wall side of north-dipping reverse faults. The n orth-dipping Mission Hills, Verdugo, and Northridge Hills reverse faults ar e interpreted to be potential seismic sources because fault-propagation fol ds above these faults have tectonic geomorphic expression. These north-dipp ing reverse faults were initiated during the deposition of the Saugus Forma tion between 2.3 and 0.5 Ma. and have minimum dip-slip rates of 0.35 to 1.1 mm/yr based on the oldest possible age of the initiation of faulting. The Northridge Hills and Mission Hills faults are interpreted to merge at depth and are located at the updip extension of the 1971 aftershock zone, even t hough these faults did not rupture during the 1971 earthquake. Surface brea ks appeared north of these faults mostly along north-dipping bedding planes and are interpreted as secondary features related to flexural-slip folding rather than a direct extension of the 1971 seismogenic fault. Surface and subsurface geology, together with seismological data of the 1971 and 1994 e arthquakes, suggests that the north- and south-dipping deformation zones in the San Fernando Valley are divided into multiple segments separated by no rtheast-trending structural discontinuities.