Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II

Abstract

We have constructed a composite image of the fault systems of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in the upper few kilometers of the crust, relocated aftershocks in the seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust. In this image, the San Fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of ϳ25؇ from near the surface at the Northridge Hills fault, in the northern San Fernando Valley, to the San Andreas fault in the middle to lower crust. It follows a prominent aseismic reflective zone below and northward of the main-shock hypocenter. Interpreted upward splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando earthquake, which appear to divide the hanging wall into shingleor wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II transect, consisting of a fault that extends 20 km southward at a dip of ϳ33؇ from ϳ7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ϳ20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the mainshock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective zone imaged beneath the San Gabriel Mountains along the LARSE I transect, to the east, in that it appears to connect major reverse or thrust faults in the Los Angeles region to the San Andreas fault. However, it differs in having a moderate versus a gentle dip and in containing no mid-crustal bright reflections.

Full‐3‐D tomography for crustal structure in Southern California based on the scattering‐integral and the adjoint‐wavefield methods

“…High-velocity bodies lie on both sides of the low-velocity wedge. The velocity contrasts along the south and the north sides of the low-velocity wedge are aligned with projections of the Northridge Thrust Fault and the San Fernando Thrust Fault on the LARSE-II profile [Carena and Suppe, 2002;Fuis et al, 2003]. …”

A New Perspective on the Geometry of the San Andreas Fault in Southern California and Its Relationship to Lithospheric Structure

“…1, profiles 4 and 5), which were recorded across this oblique section of the SAF, reveal interpreted fluid-lubricated midcrustal decollements originating at the SAF and extending southward into the Pacific plate. Reverse faults that splay upward from these decollements have given rise to destructive earthquakes in the Los Angeles region, including the 1971 M w 6.7 San Fernando and 1987 M w 5.9 Whittier Narrows earthquakes (Fuis et al, 2001(Fuis et al, , 2003. A crustal root is observed beneath the Transverse Ranges centered, or nearly centered, on the SAF (Kohler and Davis, 1997;Fuis et al, 2001;Godfrey et al, 2002;Fuis et al, 2007).…”

Mantle Heterogeneities and the SCEC Reference Three-Dimensional Seismic Velocity Model Version 3

“…Experimental details for these temporary arrays can be found in Kohler et al (1996Kohler et al ( , 2000 and Kohler and Kerr (2002). The temporary experiment station density (2-4 km) was unprecedented in Southern California and has shed light on seismic structures in the lithosphere (Kohler, 1999;Fuis et al, 2003) that could not previously be modeled because of much larger average station spacing (Ն30 km before 1993) resulting in spatial aliasing.…”