Numerical and Analog Models of the Evolution of the San Andreas Fault through the San Gorgonio Knot

While structurally simple in many parts of California, the active San Andreas Fault within the San Gorgonio Pass region slips along several, non-parallel, strike-slip, thrust and oblique faults. The present-day complexity of this region has developed in response to propagation of new active strands, abandonment of old strands and reactivation of other old strands. Our three-dimensional analog and numerical models simulate evolution of the SAF through the San Gorgonio knot over the past 500k years. Within both numerical and analog experiments, we explore the changes in mechanical efficiency associated with abandonment and development of new fault strands. By simulating deformation along the southern SAF, we observe that some fault strands become mechanically inefficient and are abandoned in favor of other, more efficient strands. The models simulate specific fault configurations interpreted to have been active at different times during the evolution sequence of the southern San Andreas Fault. The numerical models incorporate three-dimensional fault surfaces within an elastic-half space and simulate short-term deformation at each ‘snapshot' of fault system evolution. Within the analog models, we cut fault surfaces within a clay cake and computer controlled boundary displacement simulates 100k years of plate boundary deformation. The synergistic integration of elastic numerical and physical models of the SAF in the San Gorgonio Knot provides insights in how short-term and long-term deformation patterns may differ. The mechanical efficiency of the models is assessed by quantifying the work consumed by internal deformation, frictional heating, uplift against gravity and new fault surface development. Within the numerical models, internal work consumes the bulk of the work budget and the abandonment of some San Andreas fault strands follows the minimization of internal work.