Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II
- Gary S. Fuis1,
- Robert W. Clayton2,
- Paul M. Davis3,
- Trond Ryberg4,
- William J. Lutter5,
- David A. Okaya6,
- Egill Hauksson7,
- Claus Prodehl8,
- Janice M. Murphy9,
- Mark L. Benthien10,
- Shirley A. Baher11,
- Monica D. Kohler11,
- Kristina Thygesen12,
- Gerry Simila13 and
- G. Randy Keller14
- 1Earthquakes Hazards Team, U.S. Geological Survey, Menlo Park, California 94025, USA
- 2Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA
- 3Department of Earth and Space Sciences, University of California, Los Angeles, California 90024, USA
- 4Division of Physics of the Earth and Disaster Research, GeoForschungsZentrum, Potsdam D-14473, Germany
- 5Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53711, USA
- 6Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
- 7Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA
- 8Geophysical Institute, University of Karlsruhe, Karlsruhe D-76187, Germany
- 9Earthquake Hazards Team, U.S. Geological Survey, Menlo Park, California 94025, USA
- 10Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
- 11Department of Earth and Space Sciences, University of California, Los Angeles, California 90024, USA
- 12Geological Institute, University of Copenhagen, Copenhagen 1350, Denmark
- 13Department of Geological Sciences, California State University, Northridge, California 91330, USA
- 14Department of Geological Sciences, University of Texas, El Paso, Texas 79968, USA
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 ∼25° 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 shingle- or 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 ∼33° from ∼7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ∼20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the main-shock 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.
Footnotes
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↵Loose insert: Figure 2, Reflection data for line 2 and corresponding migrated industry reflection data, Figure 3, Cross section of line 2 and velocity models, and Figure 4, Similar to Figure 3, with expanded depth and distance frame.
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- Accepted October 3, 2002.
- Received March 25, 2002.
- Revision received October 2, 2002.
- Geological Society of America
