Geodetic studies have produced different interpretations of the relative vertical thickening versus escape tectonics occurring within the Los Angeles basin based on calculated horizontal strain rates ranging from 56 × 10−9/yr contraction at N36°E to 85 × 10−9/yr north–south contraction and 60 × 10−9/yr east–west extension. Three-dimensional forward mechanical models of faults in the Los Angeles basin are used to compare modeled slip rates and slip styles, represented as the rakes of average net slip vectors, under four representative horizontal strain conditions with geologically determined late-Quaternary slip rates and observed slip styles. The comparisons strive to identify the most mechanically viable tectonic boundary conditions for the Los Angeles basin. The three-dimensional fault surfaces used within the mechanical models have been defined by Southern California Earthquake Center (scec) workers based on surface geology and subsurface data. The model results suggest that northeast–southwest principal contraction is inappropriate for the Los Angeles basin because this tectonic boundary condition produces left-lateral slip along the Whittier and San Gabriel faults in contrast with abundant geologic evidence of right-lateral slip. Of the four tectonic boundary conditions evaluated, north–south contraction accompanied by either zero or slow east–west contraction gives the lowest mismatch of modeled and observed fault slip style. These results suggest that vertical thickening may be more predominant than escape tectonics in this region.