Interaction of a Dynamic Rupture on a Fault Plane with Short Frictionless Fault Branches

Spontaneous bilateral mode II shear ruptures were nucleated on faults in photoelastic Homalite plates loaded in uniaxial compression. Rupture velocities were measured and the interaction between the rupture front and short fault branches was observed using high-speed digital photography. Fault branches were formed by machining slits of varying lengths that intersected the fault plane over a range of angles. These branches were frictionless because they did not close under static loading prior to shear rupture nucleation. Three types of behavior were observed. First, the velocity of both rupture fronts was unaffected when the fault branches were oriented 45 to the main slip surface and the length of the branches were less than or equal to 0.75 R0* (where R0* is the slip-weakening distance in the limit of low rupture speed and an infinitely long slip-pulse). Second, rupture propagation stopped at the branch on the compressive side of the rupture tip but was unaffected by the branch on the tensile side when the branches were 1.5 R0* in length and remained oriented 45 to the principle slip surface. Third, branches on the tensile side of the rupture tip nucleated tensile ‘‘wing tip’’ extensions when the branches were oriented at 70 to the interface. Third, when the branches were oriented at 70 to the interface, branches on the tensile side of the rupture tip nucleated tensile ‘‘wing-crack’’ extensions. We explain these observations using a model in which the initial uniaxial load produces stress concentrations at the tips of the branches, which perturb the initial stress field on the rupture plane. These stress perturbations affect both the resolved shear stress driving the rupture and the fault-normal stress that controls the fault strength, and together they explain the observed changes in rupture speed.