DEFORMATION RATES ACROSS THE PLACERITA (NORTHRIDGE M(W)=6.7 AFTERSHOCK ZONE) AND HOPPER CANYON SEGMENTS OF THE WESTERN TRANSVERSE RANGES DEFORMATION BELT

Aftershocks of the 1994 Northridge (M(w) = 6.7) earthquake provide ins ights into the geometry of the seismic source faults in the San Fernan do Valley and the east Ventura Basin and allow the calculation of defo rmation rates for the region. The Northridge thrust and Santa Susana f aults 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 fo otwall block of that reverse fault. Two balanced cross sections sugges t thick-skinned deformation of the western Transverse Ranges. The west ern section, across the Modelo lobe segment of the north-dipping San C ayetano fault and the easternmost surface trace of the south-dipping O ak Ridge fault, is west of any aftershocks of the Northridge earthquak e and has been termed the Hopper Canyon segment of the deformation bel t. Structural modeling predicts a dip of 46 degrees S on the Oak Ridge fault at seismogenic depths. Horizontal shortening rates are calculat ed by adding the products of the dip-slip displacements and the cosine s of the dips of both faults. The eastern cross section shows the Nort hridge mainshock, with a 45 degrees south-dipping nodal plane at a dep th of 18 km. Aftershocks reach a depth of 20 km, In a thin-skinned par adigm, a hinge should occur at the surface near the Santa Monica Mount ains due to rocks moving from a decollement at the brittle-plastic tra nsition and changing dip as they move up the ramp. No hinge of that ma gnitude occurs there. Calculation of horizontal shortening rates acros s this part of the western Transverse Ranges must take into account th e displacement on both the Northridge thrust (eastern extension of the Oak Ridge fault) and the Santa Susana fault (Placerita segment). Hori zontal shortening rates are 8.2 +/- 2.4 mm/yr across the Modelo lobe s egment 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. Thes e 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 North ridge thrust since 2.3 Ma, 4.1 +/- 0.4 mm/yr for the Oak Ridge fault s ince 500 ka, 5.9(+3.9)/(-3.8) mm/yr for the Santa Susana fault since 6 00 to 2300 ka, and 7.4 +/- 3.0 mm/yr for the Modelo lobe segment of th e San Cayetano fault since 500 ka. This indicates that the slip rates on the north-dipping, the eastern San Cayetano, and the Santa Susana f aults are comparable, but the slip rate on the south-dipping faults de creases eastward; the slip rate on the Oak Ridge fault in the Ventura basin is more than double that of the Northridge thrust.