Abstract

Aftershocks of the 1994 Northridge (Mw = 6.7) earthquake provide insights into the geometry of the seismic source faults in the San Fernando Valley and the east Ventura Basin and allow the calculation of deformation rates for the region. The Northridge thrust and Santa Susana faults dip in opposite directions, with the Northridge thrust entirely beneath the Santa Susana fault. These opposing reverse faults interact, resulting in a folded active Santa Susana fault and uplift in the footwall block of that reverse fault.

Two balanced cross sections suggest thick-skinned deformation of the western Transverse Ranges. The western section, across the Modelo lobe segment of the north-dipping San Cayetano fault and the easternmost surface trace of the south-dipping Oak Ridge fault, is west of any aftershocks of the Northridge earthquake and has been termed the Hopper Canyon segment of the deformation belt. Structural modeling predicts a dip of 46° S on the Oak Ridge fault at seismogenic depths. Horizontal shortening rates are calculated by adding the products of the dip-slip displacements and the cosines of the dips of both faults. The eastern cross section shows the Northridge mainshock, with a 45° south-dipping nodal plane at a depth of 18 km. Aftershocks reach a depth of 20 km. In a thin-skinned paradigm, a hinge should occur at the surface near the Santa Monica Mountains due to rocks moving from a decollement at the brittle-plastic transition and changing dip as they move up the ramp. No hinge of that magnitude occurs there. Calculation of horizontal short-ening rates across this part of the western Transverse Ranges must take into account the displacement on both the Northridge thrust (eastern extension of the Oak Ridge fault) and the Santa Susana fault (Placerita segment).

Horizontal shortening rates are 8.2 ± 2.4 mm/yr across the Modelo lobe segment of the San Cayetano fault and the Oak Ridge fault and 5.7 ± 2.5 mm/yr across the Northridge thrust and the Santa Susana fault. These rates are consistent with those based on tectonic geodesy using GPS. Dip-slip displacement rates on the faults are 1.7 mm/yr for the Northridge thrust since 2.3 Ma, 4.1 ± 0.4 mm/yr for the Oak Ridge fault since 500 ka, 5.9+3.9/−3.8 mm/yr for the Santa Susana fault since 600 to 2300 ka, and 7.4 ± 3.0 mm/yr for the Modelo lobe segment of the San Cayetano fault since 500 ka. This indicates that the slip rates on the north-dipping, the eastern San Cayetano, and the Santa Susana faults are comparable, but the slip rate on the south-dipping faults decreases eastward; the slip rate on the Oak Ridge fault in the Ventura basin is more than double that of the Northridge thrust.

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