Abstract

The San Fernando Valley lies above the north-dipping 1971 Sylmar and south-dipping 1994 Northridge earthquake faults. To understand the tectonic setting of these two earthquakes, we mapped subsurface geology of the San Fernando Valley down to a depth of ∼3 km, using industry oil-well and seismic data. The 1994 Northridge earthquake did not rupture the surface, and the south-dipping aftershock zone terminated against the north-dipping 1971 aftershock zone at a depth of 5-8 km. However, the blind Northridge fault has a near surface geologic expression; fault-propagation folding related to the Northridge fault has preserved a thick forelimb sequence of Plio-Pleistocene Saugus Formation in the Sylmar basin and Merrick syncline, which are located on the hanging wall side of north-dipping reverse faults. The north-dipping Mission Hills, Verdugo, and Northridge Hills reverse faults are interpreted to be potential seismic sources because fault-propagation folds above these faults have tectonic geomorphic expression. These north-dipping reverse faults were initiated during the deposition of the Saugus Formation 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 though these faults did not rupture during the 1971 earthquake. Surface breaks 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 earthquakes, suggests that the north- and south-dipping deformation zones in the San Fernando Valley are divided into multiple segments separated by northeast-trending structural discontinuities.

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