Numerical Modeling of Deformation in the Los Angeles Basin, Southern California

Deformation in the Los Angeles Basin is characterized by a narrow band of shortening between downtown Los Angeles and the San Gabriel Mountains. Geodetic measurements indicate that there is ∼4.5 mm/yr shortening across this band; however, the mechanism by which this shortening is accommodated is poorly characterized and understood. It is unclear whether shortening is accommodated by elastic strain accumulation and release along a series of sub-parallel thrust faults, including the Sierra Madre fault (the frontal fault of the San Gabriel Mountains) or whether it is accommodated by anelastic processes taking place in the low rigidity sediments within the basin. We will present the results of numerical modeling of the region using both 2D and 3D finite element models from the GeoFEST (Geophysical Finite Element Simulation Tool) code and the QuakeSim Computational Portal. These results will be compared with geodetically observed deformation being recorded by the Southern California Integrated GPS Network (SCIGN). Previous modeling indicates that the low rigidity basin sediments play a key role in accommodating the observed shortening, but those models were unable to predict the observed deformation. We have further refined our 2D models by testing model sensitivity to changes in the modeling domain (i.e. creating a box with greater depth) and to the degree of mesh refinement in areas characterized by large material contrasts. The results of these model sensitivity tests indicate that a modeling domain that is 400 km x 600 km with a mesh refinement of ∼1.25 km node spacing near the transition between basin sediments and bedrock (and 5 km node spacing elsewhere) produce velocities that do not reflect errors caused by numerical artifacts within the model. We will compare the newly refined 2D results with kinematic models constructed by other workers. We will also construct a simple 3D model of the Los Angeles Basin in order to characterize the problem and to see if we are better able to predict observed deformation with a more realistic geometric and mechanical model.