Abstract: Comparison of the Seismicity and Associated Stress Systems to the GPS Strain Rates for the Ventura Basin and Northridge Regions

SIMILA, G. W., Department of Geological Sciences, California State University, Northridge, CA

A comparison of seismic moment release and geodetic rates before and after the 1994 Northridge earthquake is presented for the Ventura Basin - San Fernando Valley region. Seismicity (1981-1993) for the region of the GPS profile produced only a total seismic moment of 5-21 x 10^23 dyne-cm, equivalent to a magnitude of 5.25.6, depending on the area (35 x 95 km; 90 x 115 km, respectively) which is two orders of magnitude smaller than the geodetic moment determined from the N-S shortening of 7-10 mm/yr (1987-93). Cross-sections of seismicity show that most of the activity is associated with the San Cayetano fault, north of the Ventura Basin where the strain gradients are high.

Following the 1994 Northridge earthquake, the aftershock moment release during the past three years represents only 20% of the total geodetic postseismic release which is about 30% of the mainshock moment release. Modeling of the GPS data indicate that considerate aseismic deformation is occurring, possibly resulting from folding of sediments in the upper 5 km which may relieve stress rather than loading nearby faults. The spatial distribution of shallow seismicity and faulting mechanisms support this interpretation. Continued activity along the northwestern corner of the 1994 aftershock zone like the recent April, 1997 (M=5.0) event indicated near-vertical reverse faulting possibly along the Oak Ridge fault. Analysis of focal mechanisms indicate a general compressive stress direction of N12E.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California