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Using Vertical Rock Uplift Patterns to Constrain the Three-Dimensional Fault Configuration in the Los Angeles Basin

Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, Oregon 97330

Michele L. Cooke and Scott T. Marshall

University of Massachusetts, 611 North Pleasant Street, Amherst, Massachusetts 01003-9297

Comparison of geologic uplift patterns with results of three-dimensional mechanical models provides constraints on the fault geometry compiled by the Southern California Earthquake Center community fault model in the northern Los Angeles basin, California. The modeled uplift matches well the geologic pattern of uplift associated with the Santa Fe Springs and Coyote Hills segments of the Puente Hills thrust fault but does not match structures to the west of the San Gabriel River. To better match the geologic patterns in this area, alternative fault configurations were tested. The best match to geologic uplift is attained with a model incorporating (1) a steep blind thrust fault at the location of the Los Angeles segment of the Puente Hills thrust system (following interpretations of the Las Cienegas fault geometry at this location), (2) removal of an inferred linking fault between the Raymond and Hollywood faults, and (3) lateral continuation of the Lower Elysian Park fault, a blind low-angle detachment at depth, along strike to the northwest. These geometric revisions alter the connectivity of northern Los Angeles basin faults and significantly improve the match of model uplift pattern to geologic data. Model results suggest that fault connectivity may be more important in governing fault slip rate than are fault dip and fault area. The preferred model alters slip rates by >0.2 mm/yr for the Upper Elysian Park, Hollywood, Lower Elysian Park, Raymond, Sierra Madre West, and Verdugo faults. Additionally, the preferred model alters the surface area of several faults in the northern Los Angeles basin, such as the Puente Hills thrust and the Lower Elysian Park fault, which may have important implications for seismic hazard assessment in the northern Los Angeles basin.

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				S. T. Marshall, M. L. Cooke, and S. E. Owen Effects of Nonplanar Fault Topology and Mechanical Interaction on Fault-Slip Distributions in the Ventura Basin, California Bulletin of the Seismological Society of America, June 1, 2008; 98(3): 1113 - 1127. [Abstract] [Full Text] [PDF]