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Mechanical Validation of the Three-Dimensional Intersection Geometry between the Puente Hills Blind-Thrust System and the Whittier Fault, Los Angeles, California

Department of Geosciences
University of Massachusetts
Amherst, Massachusetts 01003

Manuscript received 13 May 2003.

The sensitivity of fault interaction to alternative, kinematically plausible intersection geometries of the Puente Hills blind-thrust system and the Whittier fault is modeled under geodetically constrained horizontal contraction. Comparisons of modeled slip rates to available geologic rates (1) suggest that the Coyote Hills segment of the Puente Hills system extends to the base of the seismogenic crust and (2) give slight preference for extension of the active Whittier fault to the base of the seismogenic crust rather than limiting active slip to the hanging wall of the Coyote Hills fault. Furthermore, analysis of strain energy density demonstrates that the preferred model has the greatest mechanical efficiency. This correlation of fit to geologic slip rates and mechanical efficiency supports the effectiveness of this method for evaluating among alternative geometries in the absence of geologically constrained slip rates. The methodology implemented in this study may be effectively used in future studies of deformation within fault systems.

Model-generated slip rates along the Puente Hills faults show significant strike slip, implying that seismic hazard of these faults may be underestimated by considering reverse-slip rates alone. Contraction at 036° may not be appropriate in the Puente Hills region because of resulting sinistral slip on the Whittier fault, which disagrees with paleoseisimc observations; however, Whittier strike-slip rates fit paleoseismic rates under 006.5° contraction. Because strike-slip rates are more sensitive to contraction direction than fault intersection geometry, contraction direction should be further constrained in order to accurately assess seismic hazard on these faults.

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