Activation of existing faults represents the principle source of earthquakes in the Los Angeles basin. We use advanced geometric modeling software to integrate geophysical and geologic data in the construction of a three-dimensional structural model of the major faults.

The fault model integrates industry seismic reflection profiles, well data, surface geology, earthquake focal mechanisms and hypocenters, published structural cross-sections, and surficial neotectonic data. The model defines besides other faults an imbricate thrust stack in the northern basin, including the Puente Hills (PHT), Las Cienagas, San Vicente, and Elysian Park faults.

The geometries and tectonic histories of each fault in the stack are evaluated based on the growth strata present in overlying fault-related folds and by the cross-cutting relationships of the faults at depth as suggested by the modeling. Rather than obeying a simple break-forward or break-backward thrusting sequence, faults in the basin exhibit complex Pliocene and Quaternary reactivation histories. Moreover, the model defines areas where coeval thrust, reverse, and strike-slip faults interact at depth. We evaluate the kinematics of these transpressional fault interactions to define permissible fault geometries. The improved quantification of the geometries in the new 3-D fault model provides the basis for earthquake hazard assessment as facilitated by the Southern Californian Earthquake Center (SCEC2).