HR: 0800h
AN: T31C-1317    [Abstracts]
TI: Mid-Crustal Decollement(s) Beneath the Transverse Ranges of Southern California
AU: * Fuis, G S
EM: fuis@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Team, L
EM: clay@gps.caltech.edu
AF: Southern California Earthquake Center, University of Southern California, Los Angeles, CA 90089 United States
AU: Team, L
EM: clay@gps.caltech.edu
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Team, L
EM: clay@gps.caltech.edu
AF: GeoForschungsZentrum, Telegrafenberg, Potsdam, 14473 Germany
AU: Team, L
EM: clay@gps.caltech.edu
AF: University of Wisconsin, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Team, L
EM: clay@gps.caltech.edu
AF: University of Copenhagen, Geological Institute, Oester Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Team, L
EM: clay@gps.caltech.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968 United States
AB: Composite images of the fault systems associated with the 1971 M 6.7 San Fernando, 1987 M 5.9 Whittier Narrows, and 1994 M 6.7 Northridge earthquakes were constructed using industry reflection and oil-test-well data in the upper few km of the crust, relocated aftershocks in the seismogenic part of the crust, and LARSE reflection and refraction data in the middle and lower crust. In the vicinity of the San Fernando Valley, a newly interpreted San Fernando fault zone dips ~25 degrees northward and extends over a 50-km interval from near the surface in the northern San Fernando Valley to the San Andreas Fault (SAF) in the middle to lower crust. This fault zone, or decollement, follows a prominent, aseismic reflective zone below and northward of the San Fernando hypocenter. The inferred fault zone for the Northridge earthquake extends 20 km southward at a dip of ~33 degrees from the point where it is overridden by the San Fernando decollement, beneath the northern San Fernando Valley, to ~20 km depth beneath the Santa Monica Mountains. It follows a weak, aseismic reflective zone below and southward of the Northridge hypocenter. A prominent mid-crustal reflective zone is also imaged beneath the San Gabriel Mountains 70 km east of the San Fernando Valley. The reversed polarity and large amplitudes of reflections returned from the top of this zone seem to require the presence of fluids in this zone. This zone appears to connect the Puente Hills Fault system, in the northern Los Angeles basin, to the SAF at a gentler angle than the San Fernando decollement. A segment of the Puente Hills Fault system slipped in the 1987 M 5.9 Whittier Narrows earthquake. The Sierra Madre fault, between the Puente Hills Fault system and the SAF, appears to be an upward splay from the decollement. The San Fernando decollement appears to follow a suture between a block of the Peninsular Ranges terrane, underlying the Santa Monica Mountains and San Fernando Valley, and the central Transverse Ranges terrane. The San Gabriel decollement appears to extend across the suture between the Peninsular Ranges terrane, underlying the Los Angeles basin, and the central Transverse Ranges terrane. The similarities and differences between the San Fernando and San Gabriel reflective zones merit further field investigations to determine whether these zones are laterally connected or structurally related.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8102 Continental contractional orogenic belts
DE: 7205 Continental crust (1242)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting