Dynamic path selection along branched faults: Experiments involving sub-Rayleigh and supershear ruptures

Building upon previous laboratory earthquake experiments of dynamic shear rupture growth taking place along faults with simple kinks, new and complex fault geometries involving cohesively held fault branches are studied. Asymmetric impact at the specimen boundaries controls the incoming shear ruptures, which are manipulated to propagate at either sub-Rayleigh or supershear velocities. High-speed photography and dynamic photoelasticity are used with a model material, Homalite-100, to monitor incoming and outgoing rupture propagation, acceleration, deceleration, or arrest at the vicinity of the branch location. Differences and similarities of rupture velocity history between cases involving faults with either simple kinks or branches, on the one hand, and sub-Rayleigh and supershear incoming ruptures, on the other, are highlighted and explained. Results of the experiments show a clear general bias toward large branch inclination, smaller branch angles appearing to be overshadowed and suppressed by the stress field associated with the main fault. Of great interest, also, is the sustenance of rupture propagation along a branch by the Mach cone, when the initial rupture is supershear driven. Generally, higher rupture speeds favors larger arrays of branching angles to be triggered. A companion analysis by Templeton et al. (2009) featuring detailed numerical simulations of these experiments provides further insight into the observed phenomena.