EVOLUTION OF THE PUENTE HILLS THRUST SYSTEM, LOS ANGELES

The Puente Hills thrust (PHT) system represents a significant seismic hazard to the city of Los Angeles. The three blind thrust fault segments that comprise this system, the Coyote Hills, Santa Fe Springs, and Los Angeles faults, evolved from east to west over the past 6 million years. A tectonic shift in the late Miocene, is believed to have both altered slip on the existing Whittier and Chino faults and spurred development of the PHT. Four sequential three-dimensional Boundary Element Method models simulate the evolution of the PHT at different stages of development. These models use three factors, 1) energy release rate , 2) strain energy density and 3) Navier-Coulomb stress, to assess the potential for propagation of the fault system. In each model, energy release rate along the western-most (leading) tip of the PHT drops with growth of the next neighboring fault. The strain energy density is most concentrated at the western tip of the youngest fault and a region of high strain energy density envelopes at least a portion of the next fault to develop. In each model, one of the two predicted planes of maximum Navier-Coulomb stress matches strike of the incipient fault plane. Together these three factors recreate the location, orientation and geometry of the PHT faults as they developed. Therefore by examining these factors in a fault system that is still evolving we may be able to extrapolate the location and orientation of future faulting.