We determined a high-resolution 3D P-wave velocity model for an 80 km × 80 km area around the San Fernando Valley extending 22 km below sea level. We used events recorded from 1981 to 2000 as well as 799 aftershocks of the 1971 San Fernando earthquake recorded by a portable network. The total number of events and stations used were 13,455 and 101, respectively. Most of the events are aftershocks of the 1994 Northridge earthquake. The inversion software includes a raytracing subroutine that can handle the sharp lateral velocity variations that exist in the area. The resulting model shows an overall good agreement with the Southern California Earthquake Center (scec) 3D velocity model, but images the deeper structure of the San Fernando valley in more detail. In addition, our results are supported by the inversion of synthetic data and by the remarkable agreement with a density model derived from gravity data along a 55-km-long profile. An approximately 10- km-wide and sharply defined zone of Northridge aftershocks occurred in basement rocks between about 8 and 20 km depth and identifies the fault that slipped during the mainshock. Horizontal slices at deep (8–18 km) levels show that the Northridge earthquake occurred within a high-velocity basement block that may have controlled the size of the earthquake. The fault plane inferred from the seismicity is in good agreement with the geodetic fault plane determined by Hudnut et al. (1996) assuming uniform slip. Most of the aftershocks of the Northridge earthquake occurred within the sedimentary rocks of the San Fernando basin, with a large concentration of events located to the east of the eastern boundary of the mainshock fault plane. The latter events form a zone that dips about 45° to the southwest at depths less than about 14 km. These may represent a lateral ramp. This zone terminates at the steeply inclined eastern edge of the San Fernando basin. These events do not lie on the mainshock fault plane, and because the San Fernando aftershocks also lie east of the Northridge fault plane, the San Fernando and the Northridge mainshocks must have occurred on conjugate en echelon faults (i.e., they do not abut each other). In addition, an 8-km-wide cross section along the density model referred to previously suggests the Northridge aftershocks east of the mainshock and most of the aftershocks of the San Fernando earthquake illuminate two thrust faults that have a common high- velocity basement block in their footwalls. The velocity model produced by our analysis suggests that thrust faults formed by modern contraction in this region are strongly influenced by prior basin and basement block geometry.

Online material: 3D P-wave velocity models without seismicity.

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